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Re: [PATCH 1/2] [PING] Optimize generic spinlock code and use C11 like atomic macros.
- From: Stefan Liebler <stli at linux dot vnet dot ibm dot com>
- To: libc-alpha at sourceware dot org
- Cc: Torvald Riegel <triegel at redhat dot com>
- Date: Wed, 31 May 2017 10:29:23 +0200
- Subject: Re: [PATCH 1/2] [PING] Optimize generic spinlock code and use C11 like atomic macros.
- Authentication-results: sourceware.org; auth=none
- References: <1481905917-15654-1-git-send-email-stli@linux.vnet.ibm.com> <5857CF10.1060100@arm.com> <628f6311-239c-5eea-572c-c2acae6fcbee@linux.vnet.ibm.com> <1487017743.16322.80.camel@redhat.com> <60a34645-17e4-6693-1343-03c55b0c47ad@linux.vnet.ibm.com> <1487437038.20203.68.camel@redhat.com> <25ad863b-6f20-bfb7-95e6-3b04a2b3eee8@linux.vnet.ibm.com> <1487598702.20203.138.camel@redhat.com> <b57d3477-a041-7b06-82ac-6d2b6c6bb08c@linux.vnet.ibm.com> <1491487245.5374.161.camel@redhat.com> <09ae8ea7-4119-76c1-cd58-36cbdf587390@linux.vnet.ibm.com> <1491745903.5374.199.camel@redhat.com> <9c6a4c3a-4de9-1cd0-46e6-3a43151aed7d@linux.vnet.ibm.com> <f7fe729c-6dc0-7327-deba-01c685c65a3f@linux.vnet.ibm.com> <2607f4f9-768e-e1e6-2e2e-17696991813d@linux.vnet.ibm.com> <b6339fef-a7bd-8b40-c73f-99d0f21731f6@linux.vnet.ibm.com> <6d90fe07-19e1-8c05-d283-dd21f6902db3@linux.vnet.ibm.com> <1496128691.5890.973.camel@redhat.com>
On 05/30/2017 09:18 AM, Torvald Riegel wrote:
On Wed, 2017-05-10 at 15:00 +0200, Stefan Liebler wrote:
On 05/03/2017 01:38 PM, Stefan Liebler wrote:
On 04/25/2017 08:46 AM, Stefan Liebler wrote:
On 04/18/2017 03:09 PM, Stefan Liebler wrote:
On 04/10/2017 01:59 PM, Stefan Liebler wrote:
On 04/09/2017 03:51 PM, Torvald Riegel wrote:
On Fri, 2017-04-07 at 18:22 +0200, Stefan Liebler wrote:
@architecture maintainers:
I've added defines of ATOMIC_EXCHANGE_USES_CAS in the architecture
specific atomic-machine.h files.
See comment in include/atomic.h:
/* ATOMIC_EXCHANGE_USES_CAS is equal to 1 if atomic_exchange
operations
are implemented based on a CAS loop; otherwise, this is 0 and we
assume
that the atomic_exchange operations could provide better
performance
than a CAS loop. */
Can review the definition to 0 or 1 in the atomic-machine.h file of
your
architecture, please?
PING
PING
PING
PING
@Torvald:
I'm not sure if we will get answers from everybody.
What do you propose how to proceed with the definitions of
ATOMIC_EXCHANGE_USES_CAS?
Which archs are still missing? If maintainers don't reply, I suggest we
do our best to figure out what's the case for each missing arch, and add
a note that it should be checked eventually (eg, /* XXX Is this actually
correct? */) to the code, and then commit. Please CC: me on such a
patch, so I can have a last look at it.
We are sure for these archs:
-aarch64: #define ATOMIC_EXCHANGE_USES_CAS 0
-i386/x86: #define ATOMIC_EXCHANGE_USES_CAS 0
-mips: #define ATOMIC_EXCHANGE_USES_CAS 0/1
-powerpc: #define ATOMIC_EXCHANGE_USES_CAS 1
-s390: #define ATOMIC_EXCHANGE_USES_CAS 1
-tile: #define ATOMIC_EXCHANGE_USES_CAS 0
Nobody answered for these archs:
-alpha: #define ATOMIC_EXCHANGE_USES_CAS 1
-arm: #define ATOMIC_EXCHANGE_USES_CAS 1
-hppa: #define ATOMIC_EXCHANGE_USES_CAS 1
-ia64: #define ATOMIC_EXCHANGE_USES_CAS 0
-m68k: #define ATOMIC_EXCHANGE_USES_CAS 1
-microblaze: #define ATOMIC_EXCHANGE_USES_CAS 1
-nios2: #define ATOMIC_EXCHANGE_USES_CAS 1
-sh: #define ATOMIC_EXCHANGE_USES_CAS 1
-sparc: #define ATOMIC_EXCHANGE_USES_CAS 1
In the two of three sparc files ...
./sysdeps/sparc/sparc64/atomic-machine.h
./sysdeps/sparc/sparc32/sparcv9/atomic-machine.h
... there is the swap instruction in macro atomic_exchange_acq
for 4 bytes. Other sizes are aborted or done by CAS.
Shall we use #define ATOMIC_EXCHANGE_USES_CAS 0 here?
I've attached the current version with
/* XXX Is this actually correct? */
for all architecture for which we didn't get an answer.
The second patch "S390: Use generic spinlock code." remains the same.
Bye
Stefan
>From eb3d559d07a93508d62e9a1ff08dede5b69da1a8 Mon Sep 17 00:00:00 2001
From: Stefan Liebler <stli@linux.vnet.ibm.com>
Date: Wed, 31 May 2017 10:06:36 +0200
Subject: [PATCH 1/2] Optimize generic spinlock code and use C11 like atomic
macros.
This patch optimizes the generic spinlock code.
The type pthread_spinlock_t is a typedef to volatile int on all archs.
Passing a volatile pointer to the atomic macros which are not mapped to the
C11 atomic builtins can lead to extra stores and loads to stack if such
a macro creates a temporary variable by using "__typeof (*(mem)) tmp;".
Thus, those macros which are used by spinlock code - atomic_exchange_acquire,
atomic_load_relaxed, atomic_compare_exchange_weak - have to be adjusted.
According to the comment from Szabolcs Nagy, the type of a cast expression is
unqualified (see http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_423.htm):
__typeof ((__typeof (*(mem)) *(mem)) tmp;
Thus from spinlock perspective the variable tmp is of type int instead of
type volatile int. This patch adjusts those macros in include/atomic.h.
With this construct GCC >= 5 omits the extra stores and loads.
The atomic macros are replaced by the C11 like atomic macros and thus
the code is aligned to it. The pthread_spin_unlock implementation is now
using release memory order instead of sequentially consistent memory order.
The issue with passed volatile int pointers applies to the C11 like atomic
macros as well as the ones used before.
I've added a glibc_likely hint to the first atomic exchange in
pthread_spin_lock in order to return immediately to the caller if the lock is
free. Without the hint, there is an additional jump if the lock is free.
I've added the atomic_spin_nop macro within the loop of plain reads.
The plain reads are also realized by C11 like atomic_load_relaxed macro.
The new define ATOMIC_EXCHANGE_USES_CAS determines if the first try to acquire
the spinlock in pthread_spin_lock or pthread_spin_trylock is an exchange
or a CAS. This is defined in atomic-machine.h for all architectures.
The define SPIN_LOCK_READS_BETWEEN_CMPXCHG is now removed.
There is no technical reason for throwing in a CAS every now and then,
and so far we have no evidence that it can improve performance.
If that would be the case, we have to adjust other spin-waiting loops
elsewhere, too! Using a CAS loop without plain reads is not a good idea
on many targets and wasn't used by one. Thus there is now no option to
do so.
Architectures are now using the generic spinlock automatically if they
do not provide an own implementation. Thus the pthread_spin_lock.c files
in sysdeps folder are deleted.
ChangeLog:
* NEWS: Mention new spinlock implementation.
* include/atomic.h:
(__atomic_val_bysize): Cast type to omit volatile qualifier.
(atomic_exchange_acq): Likewise.
(atomic_load_relaxed): Likewise.
(ATOMIC_EXCHANGE_USES_CAS): Check definition.
* nptl/pthread_spin_init.c (pthread_spin_init):
Use atomic_store_relaxed.
* nptl/pthread_spin_lock.c (pthread_spin_lock):
Use C11-like atomic macros.
* nptl/pthread_spin_trylock.c (pthread_spin_trylock):
Likewise.
* nptl/pthread_spin_unlock.c (pthread_spin_unlock):
Use atomic_store_release.
* sysdeps/aarch64/nptl/pthread_spin_lock.c: Delete File.
* sysdeps/arm/nptl/pthread_spin_lock.c: Likewise.
* sysdeps/hppa/nptl/pthread_spin_lock.c: Likewise.
* sysdeps/m68k/nptl/pthread_spin_lock.c: Likewise.
* sysdeps/microblaze/nptl/pthread_spin_lock.c: Likewise.
* sysdeps/mips/nptl/pthread_spin_lock.c: Likewise.
* sysdeps/nios2/nptl/pthread_spin_lock.c: Likewise.
* sysdeps/aarch64/atomic-machine.h (ATOMIC_EXCHANGE_USES_CAS): Define.
* sysdeps/alpha/atomic-machine.h: Likewise.
* sysdeps/arm/atomic-machine.h: Likewise.
* sysdeps/i386/atomic-machine.h: Likewise.
* sysdeps/ia64/atomic-machine.h: Likewise.
* sysdeps/m68k/coldfire/atomic-machine.h: Likewise.
* sysdeps/m68k/m680x0/m68020/atomic-machine.h: Likewise.
* sysdeps/microblaze/atomic-machine.h: Likewise.
* sysdeps/mips/atomic-machine.h: Likewise.
* sysdeps/powerpc/powerpc32/atomic-machine.h: Likewise.
* sysdeps/powerpc/powerpc64/atomic-machine.h: Likewise.
* sysdeps/s390/atomic-machine.h: Likewise.
* sysdeps/sparc/sparc32/atomic-machine.h: Likewise.
* sysdeps/sparc/sparc32/sparcv9/atomic-machine.h: Likewise.
* sysdeps/sparc/sparc64/atomic-machine.h: Likewise.
* sysdeps/tile/tilegx/atomic-machine.h: Likewise.
* sysdeps/tile/tilepro/atomic-machine.h: Likewise.
* sysdeps/unix/sysv/linux/hppa/atomic-machine.h: Likewise.
* sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h: Likewise.
* sysdeps/unix/sysv/linux/nios2/atomic-machine.h: Likewise.
* sysdeps/unix/sysv/linux/sh/atomic-machine.h: Likewise.
* sysdeps/x86_64/atomic-machine.h: Likewise.
---
NEWS | 11 ++++
include/atomic.h | 16 +++--
nptl/pthread_spin_init.c | 3 +-
nptl/pthread_spin_lock.c | 71 +++++++++++++---------
nptl/pthread_spin_trylock.c | 54 +++++++++++++++-
nptl/pthread_spin_unlock.c | 6 +-
sysdeps/aarch64/atomic-machine.h | 1 +
sysdeps/aarch64/nptl/pthread_spin_lock.c | 24 --------
sysdeps/alpha/atomic-machine.h | 3 +
sysdeps/arm/atomic-machine.h | 3 +
sysdeps/arm/nptl/pthread_spin_lock.c | 23 -------
sysdeps/hppa/nptl/pthread_spin_lock.c | 23 -------
sysdeps/i386/atomic-machine.h | 1 +
sysdeps/ia64/atomic-machine.h | 3 +
sysdeps/m68k/coldfire/atomic-machine.h | 3 +
sysdeps/m68k/m680x0/m68020/atomic-machine.h | 3 +
sysdeps/m68k/nptl/pthread_spin_lock.c | 24 --------
sysdeps/microblaze/atomic-machine.h | 3 +
sysdeps/microblaze/nptl/pthread_spin_lock.c | 24 --------
sysdeps/mips/atomic-machine.h | 13 +++-
sysdeps/mips/nptl/pthread_spin_lock.c | 23 -------
sysdeps/nios2/nptl/pthread_spin_lock.c | 24 --------
sysdeps/powerpc/powerpc32/atomic-machine.h | 1 +
sysdeps/powerpc/powerpc64/atomic-machine.h | 1 +
sysdeps/s390/atomic-machine.h | 2 +
sysdeps/sparc/sparc32/atomic-machine.h | 3 +
sysdeps/sparc/sparc32/sparcv9/atomic-machine.h | 3 +
sysdeps/sparc/sparc64/atomic-machine.h | 3 +
sysdeps/tile/tilegx/atomic-machine.h | 1 +
sysdeps/tile/tilepro/atomic-machine.h | 1 +
sysdeps/unix/sysv/linux/hppa/atomic-machine.h | 3 +
.../unix/sysv/linux/m68k/coldfire/atomic-machine.h | 3 +
sysdeps/unix/sysv/linux/nios2/atomic-machine.h | 3 +
sysdeps/unix/sysv/linux/sh/atomic-machine.h | 3 +
sysdeps/x86_64/atomic-machine.h | 1 +
35 files changed, 182 insertions(+), 205 deletions(-)
delete mode 100644 sysdeps/aarch64/nptl/pthread_spin_lock.c
delete mode 100644 sysdeps/arm/nptl/pthread_spin_lock.c
delete mode 100644 sysdeps/hppa/nptl/pthread_spin_lock.c
delete mode 100644 sysdeps/m68k/nptl/pthread_spin_lock.c
delete mode 100644 sysdeps/microblaze/nptl/pthread_spin_lock.c
delete mode 100644 sysdeps/mips/nptl/pthread_spin_lock.c
delete mode 100644 sysdeps/nios2/nptl/pthread_spin_lock.c
diff --git a/NEWS b/NEWS
index b4ecd62..b3cbdb8 100644
--- a/NEWS
+++ b/NEWS
@@ -66,6 +66,17 @@ Version 2.26
* The port to Native Client running on ARMv7-A (--host=arm-nacl) has been
removed.
+* The synchronization that pthread_spin_unlock performs has been changed
+ to now be equivalent to a C11 atomic store with release memory order to
+ the spin lock's memory location. This ensures correct synchronization
+ for the spin lock's operations and critical sections protected by a spin
+ lock. Previously, several (but not all) architectures used stronger
+ synchronization (e.g., containing what is often called a full barrier).
+ This change can improve performance, but may affect odd fringe uses of
+ spin locks that depend on the previous behavior (e.g., using spin locks
+ as atomic variables to try to implement Dekker's mutual exclusion
+ algorithm).
+
Security related changes:
* The DNS stub resolver limits the advertised UDP buffer size to 1200 bytes,
diff --git a/include/atomic.h b/include/atomic.h
index 7f32640..d002b11 100644
--- a/include/atomic.h
+++ b/include/atomic.h
@@ -54,7 +54,7 @@
and following args. */
#define __atomic_val_bysize(pre, post, mem, ...) \
({ \
- __typeof (*mem) __atg1_result; \
+ __typeof ((__typeof (*(mem))) *(mem)) __atg1_result; \
if (sizeof (*mem) == 1) \
__atg1_result = pre##_8_##post (mem, __VA_ARGS__); \
else if (sizeof (*mem) == 2) \
@@ -162,9 +162,9 @@
/* Store NEWVALUE in *MEM and return the old value. */
#ifndef atomic_exchange_acq
# define atomic_exchange_acq(mem, newvalue) \
- ({ __typeof (*(mem)) __atg5_oldval; \
+ ({ __typeof ((__typeof (*(mem))) *(mem)) __atg5_oldval; \
__typeof (mem) __atg5_memp = (mem); \
- __typeof (*(mem)) __atg5_value = (newvalue); \
+ __typeof ((__typeof (*(mem))) *(mem)) __atg5_value = (newvalue); \
\
do \
__atg5_oldval = *__atg5_memp; \
@@ -668,7 +668,7 @@ void __atomic_link_error (void);
# ifndef atomic_load_relaxed
# define atomic_load_relaxed(mem) \
- ({ __typeof (*(mem)) __atg100_val; \
+ ({ __typeof ((__typeof (*(mem))) *(mem)) __atg100_val; \
__asm ("" : "=r" (__atg100_val) : "0" (*(mem))); \
__atg100_val; })
# endif
@@ -818,4 +818,12 @@ void __atomic_link_error (void);
# define atomic_spin_nop() do { /* nothing */ } while (0)
#endif
+/* ATOMIC_EXCHANGE_USES_CAS is non-zero if atomic_exchange operations
+ are implemented based on a CAS loop; otherwise, this is zero and we assume
+ that the atomic_exchange operations could provide better performance
+ than a CAS loop. */
+#ifndef ATOMIC_EXCHANGE_USES_CAS
+# error ATOMIC_EXCHANGE_USES_CAS has to be defined.
+#endif
+
#endif /* atomic.h */
diff --git a/nptl/pthread_spin_init.c b/nptl/pthread_spin_init.c
index 01dec5e..fe30913 100644
--- a/nptl/pthread_spin_init.c
+++ b/nptl/pthread_spin_init.c
@@ -22,6 +22,7 @@
int
pthread_spin_init (pthread_spinlock_t *lock, int pshared)
{
- *lock = 0;
+ /* Relaxed MO is fine because this is an initializing store. */
+ atomic_store_relaxed (lock, 0);
return 0;
}
diff --git a/nptl/pthread_spin_lock.c b/nptl/pthread_spin_lock.c
index 4d03b78..682af80 100644
--- a/nptl/pthread_spin_lock.c
+++ b/nptl/pthread_spin_lock.c
@@ -19,27 +19,35 @@
#include <atomic.h>
#include "pthreadP.h"
-/* A machine-specific version can define SPIN_LOCK_READS_BETWEEN_CMPXCHG
- to the number of plain reads that it's optimal to spin on between uses
- of atomic_compare_and_exchange_val_acq. If spinning forever is optimal
- then use -1. If no plain reads here would ever be optimal, use 0. */
-#ifndef SPIN_LOCK_READS_BETWEEN_CMPXCHG
-# warning machine-dependent file should define SPIN_LOCK_READS_BETWEEN_CMPXCHG
-# define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-#endif
-
int
pthread_spin_lock (pthread_spinlock_t *lock)
{
- /* atomic_exchange usually takes less instructions than
- atomic_compare_and_exchange. On the other hand,
- atomic_compare_and_exchange potentially generates less bus traffic
- when the lock is locked.
- We assume that the first try mostly will be successful, and we use
- atomic_exchange. For the subsequent tries we use
- atomic_compare_and_exchange. */
- if (atomic_exchange_acq (lock, 1) == 0)
+ int val = 0;
+
+ /* We assume that the first try mostly will be successful, thus we use
+ atomic_exchange if it is not implemented by a CAS loop (we also assume
+ that atomic_exchange can be faster if it succeeds, see
+ ATOMIC_EXCHANGE_USES_CAS). Otherwise, we use a weak CAS and not an
+ exchange so we bail out after the first failed attempt to change the
+ state. For the subsequent attempts we use atomic_compare_and_exchange
+ after we observe that the lock is not acquired.
+ See also comment in pthread_spin_trylock.
+ We use acquire MO to synchronize-with the release MO store in
+ pthread_spin_unlock, and thus ensure that prior critical sections
+ happen-before this critical section. */
+#if ! ATOMIC_EXCHANGE_USES_CAS
+ /* Try to acquire the lock with an exchange instruction as this architecture
+ has such an instruction and we assume it is faster than a CAS.
+ The acquisition succeeds if the lock is not in an acquired state. */
+ if (__glibc_likely (atomic_exchange_acquire (lock, 1) == 0))
return 0;
+#else
+ /* Try to acquire the lock with a CAS instruction as this architecture
+ has no exchange instruction. The acquisition succeeds if the lock is not
+ acquired. */
+ if (__glibc_likely (atomic_compare_exchange_weak_acquire (lock, &val, 1)))
+ return 0;
+#endif
do
{
@@ -47,23 +55,26 @@ pthread_spin_lock (pthread_spinlock_t *lock)
to cmpxchg is not a good idea on many targets as that will force
expensive memory synchronizations among processors and penalize other
running threads.
- On the other hand, we do want to update memory state on the local core
- once in a while to avoid spinning indefinitely until some event that
- will happen to update local memory as a side-effect. */
- if (SPIN_LOCK_READS_BETWEEN_CMPXCHG >= 0)
+ There is no technical reason for throwing in a CAS every now and then,
+ and so far we have no evidence that it can improve performance.
+ If that would be the case, we have to adjust other spin-waiting loops
+ elsewhere, too!
+ Thus we use relaxed MO reads until we observe the lock to not be
+ acquired anymore. */
+ do
{
- int wait = SPIN_LOCK_READS_BETWEEN_CMPXCHG;
+ /* TODO Back-off. */
- while (*lock != 0 && wait > 0)
- --wait;
- }
- else
- {
- while (*lock != 0)
- ;
+ atomic_spin_nop ();
+
+ val = atomic_load_relaxed (lock);
}
+ while (val != 0);
+
+ /* We need acquire memory order here for the same reason as mentioned
+ for the first try to lock the spinlock. */
}
- while (atomic_compare_and_exchange_val_acq (lock, 1, 0) != 0);
+ while (!atomic_compare_exchange_weak_acquire (lock, &val, 1));
return 0;
}
diff --git a/nptl/pthread_spin_trylock.c b/nptl/pthread_spin_trylock.c
index 593bba3..83921b0 100644
--- a/nptl/pthread_spin_trylock.c
+++ b/nptl/pthread_spin_trylock.c
@@ -23,5 +23,57 @@
int
pthread_spin_trylock (pthread_spinlock_t *lock)
{
- return atomic_exchange_acq (lock, 1) ? EBUSY : 0;
+ /* For the spin try lock, we have the following possibilities:
+
+ 1) If we assume that trylock will most likely succeed in practice:
+ * We just do an exchange.
+
+ 2) If we want to bias towards cases where trylock succeeds, but don't
+ rule out contention:
+ * If exchange is not implemented by a CAS loop, and exchange is faster
+ than CAS, do an exchange.
+ * If exchange is implemented by a CAS loop, use a weak CAS and not an
+ exchange so we bail out after the first failed attempt to change the state.
+
+ 3) If we expect contention to be likely:
+ * If CAS always brings the cache line into an exclusive state even if the
+ spinlock is already acquired, then load the value first with
+ atomic_load_relaxed and test if lock is not acquired. Then do 2).
+
+ We assume that 2) is the common case, and that this won't be slower than
+ 1) in the common case.
+
+ We use acquire MO to synchronize-with the release MO store in
+ pthread_spin_unlock, and thus ensure that prior critical sections
+ happen-before this critical section. */
+#if ! ATOMIC_EXCHANGE_USES_CAS
+ /* Try to acquire the lock with an exchange instruction as this architecture
+ has such an instruction and we assume it is faster than a CAS.
+ The acquisition succeeds if the lock is not in an acquired state. */
+ if (atomic_exchange_acquire (lock, 1) == 0)
+ return 0;
+#else
+ /* Try to acquire the lock with a CAS instruction as this architecture
+ has no exchange instruction. The acquisition succeeds if the lock is not
+ acquired. */
+ do
+ {
+ int val = 0;
+ if (atomic_compare_exchange_weak_acquire (lock, &val, 1))
+ return 0;
+ }
+ /* atomic_compare_exchange_weak_acquire can fail spuriously. Whereas
+ C++11 and C11 make it clear that trylock operations can fail spuriously,
+ POSIX does not explicitly specify this; it only specifies that failing
+ synchronization operations do not need to have synchronization effects
+ themselves, but a spurious failure is something that could contradict a
+ happens-before established earlier (e.g., that we need to observe that
+ the lock is acquired). Therefore, we emulate a strong CAS by simply
+ checking with a relaxed MO load that the lock is really acquired before
+ returning EBUSY; the additional overhead this may cause is on the slow
+ path. */
+ while (atomic_load_relaxed (lock) == 0);
+#endif
+
+ return EBUSY;
}
diff --git a/nptl/pthread_spin_unlock.c b/nptl/pthread_spin_unlock.c
index 5fd73e5..f83b696 100644
--- a/nptl/pthread_spin_unlock.c
+++ b/nptl/pthread_spin_unlock.c
@@ -23,7 +23,9 @@
int
pthread_spin_unlock (pthread_spinlock_t *lock)
{
- atomic_full_barrier ();
- *lock = 0;
+ /* The atomic_store_release synchronizes-with the atomic_exchange_acquire
+ or atomic_compare_exchange_weak_acquire in pthread_spin_lock /
+ pthread_spin_trylock. */
+ atomic_store_release (lock, 0);
return 0;
}
diff --git a/sysdeps/aarch64/atomic-machine.h b/sysdeps/aarch64/atomic-machine.h
index a5d2213..eb59a5b 100644
--- a/sysdeps/aarch64/atomic-machine.h
+++ b/sysdeps/aarch64/atomic-machine.h
@@ -38,6 +38,7 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 1
#define USE_ATOMIC_COMPILER_BUILTINS 1
+#define ATOMIC_EXCHANGE_USES_CAS 0
/* Compare and exchange.
For all "bool" routines, we return FALSE if exchange succesful. */
diff --git a/sysdeps/aarch64/nptl/pthread_spin_lock.c b/sysdeps/aarch64/nptl/pthread_spin_lock.c
deleted file mode 100644
index fcfcb40..0000000
--- a/sysdeps/aarch64/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,24 +0,0 @@
-/* Copyright (C) 2008-2017 Free Software Foundation, Inc.
-
- This file is part of the GNU C Library.
-
- The GNU C Library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public License as
- published by the Free Software Foundation; either version 2.1 of the
- License, or (at your option) any later version.
-
- The GNU C Library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with the GNU C Library; if not, see
- <http://www.gnu.org/licenses/>. */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
- it will resolve to this very file. Using "sysdeps/.." as reference to the
- top level directory does the job. */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/alpha/atomic-machine.h b/sysdeps/alpha/atomic-machine.h
index 06e93f2..2cb2290 100644
--- a/sysdeps/alpha/atomic-machine.h
+++ b/sysdeps/alpha/atomic-machine.h
@@ -45,6 +45,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 1
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
#ifdef UP
# define __MB /* nothing */
diff --git a/sysdeps/arm/atomic-machine.h b/sysdeps/arm/atomic-machine.h
index eeac7f0..cfa5bf3 100644
--- a/sysdeps/arm/atomic-machine.h
+++ b/sysdeps/arm/atomic-machine.h
@@ -36,6 +36,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
void __arm_link_error (void);
#ifdef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4
diff --git a/sysdeps/arm/nptl/pthread_spin_lock.c b/sysdeps/arm/nptl/pthread_spin_lock.c
deleted file mode 100644
index 037b3b8..0000000
--- a/sysdeps/arm/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,23 +0,0 @@
-/* Copyright (C) 2008-2017 Free Software Foundation, Inc.
- This file is part of the GNU C Library.
-
- The GNU C Library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public
- License as published by the Free Software Foundation; either
- version 2.1 of the License, or (at your option) any later version.
-
- The GNU C Library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with the GNU C Library. If not, see
- <http://www.gnu.org/licenses/>. */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
- it will resolve to this very file. Using "sysdeps/.." as reference to the
- top level directory does the job. */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/hppa/nptl/pthread_spin_lock.c b/sysdeps/hppa/nptl/pthread_spin_lock.c
deleted file mode 100644
index 14f36a6..0000000
--- a/sysdeps/hppa/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,23 +0,0 @@
-/* Copyright (C) 2005-2017 Free Software Foundation, Inc.
- This file is part of the GNU C Library.
-
- The GNU C Library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public
- License as published by the Free Software Foundation; either
- version 2.1 of the License, or (at your option) any later version.
-
- The GNU C Library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with the GNU C Library. If not, see
- <http://www.gnu.org/licenses/>. */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
- it will resolve to this very file. Using "sysdeps/.." as reference to the
- top level directory does the job. */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/i386/atomic-machine.h b/sysdeps/i386/atomic-machine.h
index 77759f7..0e24200 100644
--- a/sysdeps/i386/atomic-machine.h
+++ b/sysdeps/i386/atomic-machine.h
@@ -56,6 +56,7 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 0
#define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \
diff --git a/sysdeps/ia64/atomic-machine.h b/sysdeps/ia64/atomic-machine.h
index ecf9750..f04ac05 100644
--- a/sysdeps/ia64/atomic-machine.h
+++ b/sysdeps/ia64/atomic-machine.h
@@ -46,6 +46,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 1
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 0
+
#define __arch_compare_and_exchange_bool_8_acq(mem, newval, oldval) \
(abort (), 0)
diff --git a/sysdeps/m68k/coldfire/atomic-machine.h b/sysdeps/m68k/coldfire/atomic-machine.h
index 9aeb993..d2e4a1f 100644
--- a/sysdeps/m68k/coldfire/atomic-machine.h
+++ b/sysdeps/m68k/coldfire/atomic-machine.h
@@ -53,6 +53,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 1
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
/* The only basic operation needed is compare and exchange. */
#define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \
({ __typeof (mem) __gmemp = (mem); \
diff --git a/sysdeps/m68k/m680x0/m68020/atomic-machine.h b/sysdeps/m68k/m680x0/m68020/atomic-machine.h
index 00dc22d..83238f5 100644
--- a/sysdeps/m68k/m680x0/m68020/atomic-machine.h
+++ b/sysdeps/m68k/m680x0/m68020/atomic-machine.h
@@ -47,6 +47,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 1
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
#define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval) \
({ __typeof (*(mem)) __ret; \
__asm __volatile ("cas%.b %0,%2,%1" \
diff --git a/sysdeps/m68k/nptl/pthread_spin_lock.c b/sysdeps/m68k/nptl/pthread_spin_lock.c
deleted file mode 100644
index 62795f4..0000000
--- a/sysdeps/m68k/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,24 +0,0 @@
-/* Copyright (C) 2010-2017 Free Software Foundation, Inc.
- This file is part of the GNU C Library.
- Contributed by Maxim Kuvyrkov <maxim@codesourcery.com>, 2010.
-
- The GNU C Library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public
- License as published by the Free Software Foundation; either
- version 2.1 of the License, or (at your option) any later version.
-
- The GNU C Library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with the GNU C Library. If not, see
- <http://www.gnu.org/licenses/>. */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
- it will resolve to this very file. Using "sysdeps/.." as reference to the
- top level directory does the job. */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/microblaze/atomic-machine.h b/sysdeps/microblaze/atomic-machine.h
index dc5309c..d73c69d 100644
--- a/sysdeps/microblaze/atomic-machine.h
+++ b/sysdeps/microblaze/atomic-machine.h
@@ -38,6 +38,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
/* Microblaze does not have byte and halfword forms of load and reserve and
store conditional. So for microblaze we stub out the 8- and 16-bit forms. */
diff --git a/sysdeps/microblaze/nptl/pthread_spin_lock.c b/sysdeps/microblaze/nptl/pthread_spin_lock.c
deleted file mode 100644
index fcfcb40..0000000
--- a/sysdeps/microblaze/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,24 +0,0 @@
-/* Copyright (C) 2008-2017 Free Software Foundation, Inc.
-
- This file is part of the GNU C Library.
-
- The GNU C Library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public License as
- published by the Free Software Foundation; either version 2.1 of the
- License, or (at your option) any later version.
-
- The GNU C Library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with the GNU C Library; if not, see
- <http://www.gnu.org/licenses/>. */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
- it will resolve to this very file. Using "sysdeps/.." as reference to the
- top level directory does the job. */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/mips/atomic-machine.h b/sysdeps/mips/atomic-machine.h
index 54c182b..16fef6b 100644
--- a/sysdeps/mips/atomic-machine.h
+++ b/sysdeps/mips/atomic-machine.h
@@ -92,7 +92,15 @@ typedef uintmax_t uatomic_max_t;
have no assembly alternative available and want to avoid the __sync_*
builtins if at all possible. */
-#define USE_ATOMIC_COMPILER_BUILTINS 1
+# define USE_ATOMIC_COMPILER_BUILTINS 1
+
+/* MIPS is an LL/SC machine. However, XLP has a direct atomic exchange
+ instruction which will be used by __atomic_exchange_n. */
+# ifdef _MIPS_ARCH_XLP
+# define ATOMIC_EXCHANGE_USES_CAS 0
+# else
+# define ATOMIC_EXCHANGE_USES_CAS 1
+# endif
/* Compare and exchange.
For all "bool" routines, we return FALSE if exchange succesful. */
@@ -213,7 +221,8 @@ typedef uintmax_t uatomic_max_t;
/* This implementation using inline assembly will be removed once glibc
requires GCC 4.8 or later to build. */
-#define USE_ATOMIC_COMPILER_BUILTINS 0
+# define USE_ATOMIC_COMPILER_BUILTINS 0
+# define ATOMIC_EXCHANGE_USES_CAS 1
/* Compare and exchange. For all of the "xxx" routines, we expect a
"__prev" and a "__cmp" variable to be provided by the enclosing scope,
diff --git a/sysdeps/mips/nptl/pthread_spin_lock.c b/sysdeps/mips/nptl/pthread_spin_lock.c
deleted file mode 100644
index 19d87a5..0000000
--- a/sysdeps/mips/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,23 +0,0 @@
-/* Copyright (C) 2012-2017 Free Software Foundation, Inc.
- This file is part of the GNU C Library.
-
- The GNU C Library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public
- License as published by the Free Software Foundation; either
- version 2.1 of the License, or (at your option) any later version.
-
- The GNU C Library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with the GNU C Library. If not, see
- <http://www.gnu.org/licenses/>. */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
- it will resolve to this very file. Using "sysdeps/.." as reference to the
- top level directory does the job. */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/nios2/nptl/pthread_spin_lock.c b/sysdeps/nios2/nptl/pthread_spin_lock.c
deleted file mode 100644
index b203469..0000000
--- a/sysdeps/nios2/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,24 +0,0 @@
-/* pthread spin-lock implementation for Nios II.
- Copyright (C) 2005-2017 Free Software Foundation, Inc.
- This file is part of the GNU C Library.
-
- The GNU C Library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public
- License as published by the Free Software Foundation; either
- version 2.1 of the License, or (at your option) any later version.
-
- The GNU C Library is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public
- License along with the GNU C Library. If not, see
- <http://www.gnu.org/licenses/>. */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
- it will resolve to this very file. Using "sysdeps/.." as reference to the
- top level directory does the job. */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/powerpc/powerpc32/atomic-machine.h b/sysdeps/powerpc/powerpc32/atomic-machine.h
index 20d5e85..96c7d81 100644
--- a/sysdeps/powerpc/powerpc32/atomic-machine.h
+++ b/sysdeps/powerpc/powerpc32/atomic-machine.h
@@ -35,6 +35,7 @@
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 1
/*
* The 32-bit exchange_bool is different on powerpc64 because the subf
diff --git a/sysdeps/powerpc/powerpc64/atomic-machine.h b/sysdeps/powerpc/powerpc64/atomic-machine.h
index 40c308e..46df488 100644
--- a/sysdeps/powerpc/powerpc64/atomic-machine.h
+++ b/sysdeps/powerpc/powerpc64/atomic-machine.h
@@ -35,6 +35,7 @@
#define __HAVE_64B_ATOMICS 1
#define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 1
/* The 32-bit exchange_bool is different on powerpc64 because the subf
does signed 64-bit arithmetic while the lwarx is 32-bit unsigned
diff --git a/sysdeps/s390/atomic-machine.h b/sysdeps/s390/atomic-machine.h
index 690d2e3..adaca40 100644
--- a/sysdeps/s390/atomic-machine.h
+++ b/sysdeps/s390/atomic-machine.h
@@ -67,6 +67,8 @@ typedef uintmax_t uatomic_max_t;
# define __HAVE_64B_ATOMICS 0
#endif
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
/* Implement some of the non-C11 atomic macros from include/atomic.h
with help of the C11 atomic builtins. The other non-C11 atomic macros
are using the macros defined here. */
diff --git a/sysdeps/sparc/sparc32/atomic-machine.h b/sysdeps/sparc/sparc32/atomic-machine.h
index acd029e..a2fe848 100644
--- a/sysdeps/sparc/sparc32/atomic-machine.h
+++ b/sysdeps/sparc/sparc32/atomic-machine.h
@@ -50,6 +50,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
/* We have no compare and swap, just test and set.
The following implementation contends on 64 global locks
diff --git a/sysdeps/sparc/sparc32/sparcv9/atomic-machine.h b/sysdeps/sparc/sparc32/sparcv9/atomic-machine.h
index 7f7895e..6a19721 100644
--- a/sysdeps/sparc/sparc32/sparcv9/atomic-machine.h
+++ b/sysdeps/sparc/sparc32/sparcv9/atomic-machine.h
@@ -47,6 +47,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
#define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval) \
(abort (), (__typeof (*mem)) 0)
diff --git a/sysdeps/sparc/sparc64/atomic-machine.h b/sysdeps/sparc/sparc64/atomic-machine.h
index 44a43ff..7200932 100644
--- a/sysdeps/sparc/sparc64/atomic-machine.h
+++ b/sysdeps/sparc/sparc64/atomic-machine.h
@@ -47,6 +47,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 1
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
#define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval) \
(abort (), (__typeof (*mem)) 0)
diff --git a/sysdeps/tile/tilegx/atomic-machine.h b/sysdeps/tile/tilegx/atomic-machine.h
index 6345251..e77f670 100644
--- a/sysdeps/tile/tilegx/atomic-machine.h
+++ b/sysdeps/tile/tilegx/atomic-machine.h
@@ -31,6 +31,7 @@
#endif
#define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 0
/* Pick appropriate 8- or 4-byte instruction. */
#define __atomic_update(mem, v, op) \
diff --git a/sysdeps/tile/tilepro/atomic-machine.h b/sysdeps/tile/tilepro/atomic-machine.h
index 33a8b85..45e36de 100644
--- a/sysdeps/tile/tilepro/atomic-machine.h
+++ b/sysdeps/tile/tilepro/atomic-machine.h
@@ -23,6 +23,7 @@
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 0
/* 32-bit integer compare-and-exchange. */
static __inline __attribute__ ((always_inline))
diff --git a/sysdeps/unix/sysv/linux/hppa/atomic-machine.h b/sysdeps/unix/sysv/linux/hppa/atomic-machine.h
index 2cd2235..59581bd 100644
--- a/sysdeps/unix/sysv/linux/hppa/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/hppa/atomic-machine.h
@@ -39,6 +39,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
/* prev = *addr;
if (prev == old)
*addr = new;
diff --git a/sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h b/sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h
index 6755a05..1daac51 100644
--- a/sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h
@@ -39,6 +39,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
/* The only basic operation needed is compare and exchange. */
/* For ColdFire we'll have to trap into the kernel mode anyway,
so trap from the library rather then from the kernel wrapper. */
diff --git a/sysdeps/unix/sysv/linux/nios2/atomic-machine.h b/sysdeps/unix/sysv/linux/nios2/atomic-machine.h
index 6111ccf..d4dd025 100644
--- a/sysdeps/unix/sysv/linux/nios2/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/nios2/atomic-machine.h
@@ -34,6 +34,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
#define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval) \
(abort (), (__typeof (*mem)) 0)
#define __arch_compare_and_exchange_val_16_acq(mem, newval, oldval) \
diff --git a/sysdeps/unix/sysv/linux/sh/atomic-machine.h b/sysdeps/unix/sysv/linux/sh/atomic-machine.h
index 3c58b70..5a7e953 100644
--- a/sysdeps/unix/sysv/linux/sh/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/sh/atomic-machine.h
@@ -47,6 +47,9 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 0
#define USE_ATOMIC_COMPILER_BUILTINS 0
+/* XXX Is this actually correct? */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
/* SH kernel has implemented a gUSA ("g" User Space Atomicity) support
for the user space atomicity. The atomicity macros use this scheme.
diff --git a/sysdeps/x86_64/atomic-machine.h b/sysdeps/x86_64/atomic-machine.h
index 2e8a9aa..c454734 100644
--- a/sysdeps/x86_64/atomic-machine.h
+++ b/sysdeps/x86_64/atomic-machine.h
@@ -59,6 +59,7 @@ typedef uintmax_t uatomic_max_t;
#define __HAVE_64B_ATOMICS 1
#define USE_ATOMIC_COMPILER_BUILTINS 1
+#define ATOMIC_EXCHANGE_USES_CAS 0
#define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \
__sync_val_compare_and_swap (mem, oldval, newval)
--
2.7.4