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Re: [PATCH] misc/tst-clone3: Fix waiting for exited thread.
Attached the updated patch.
See also notes below.
On 02/08/2019 07:37 PM, Adhemerval Zanella wrote:
On 08/02/2019 13:12, Stefan Liebler wrote:
Hi,
from time to time the test misc/tst-clone3 fails with an timeout. Then futex_wait is blocking. Usually ctid should be set to zero due to CLONE_CHILD_CLEARTID and the futex should be waken up. But the fail occures if the thread has already exited before ctid is set to the return value of clone(). Then futex_wait() will block as there will be nobody who wakes the futex up again.
This patch initializes ctid to a known value before calling clone and the kernel is the only one who updates the value to zero after clone. If futex_wait is called then it is either waked up due to the exited thread or the futex syscall fails as *ctid_ptr is already zero instead of the specified value 1.
Okay to commit?
Bye
Stefan
ChangeLog:
* sysdeps/unix/sysv/linux/tst-clone3.c (do_test):
Initialize ctid with a known value and remove update of ctid
after clone.
(wait_tid): Adjust arguments and call futex_wait with ctid_val
as assumed current value of ctid_ptr.
20190208_misc_tst-clone3.patch
commit 5a8f80973dbbf06a0eebc2064d2a14521c6a6131
Author: Stefan Liebler <stli@linux.ibm.com>
Date: Fri Feb 8 12:42:52 2019 +0100
misc/tst-clone3: Fix waiting for exited thread.
From time to time the test misc/tst-clone3 fails with an timeout.
Then futex_wait is blocking. Usually ctid should be set to zero
due to CLONE_CHILD_CLEARTID and the futex should be waken up.
But the fail occures if the thread has already exited before
ctid is set to the return value of clone(). Then futex_wait() will
block as there will be nobody who wakes the futex up again.
This patch initializes ctid to a known value before calling clone
and the kernel is the only one who updates the value to zero after clone.
If futex_wait is called then it is either waked up due to the exited thread
or the futex syscall fails as *ctid_ptr is already zero instead of the
specified value 1.
ChangeLog:
* sysdeps/unix/sysv/linux/tst-clone3.c (do_test):
Initialize ctid with a known value and remove update of ctid
after clone.
(wait_tid): Adjust arguments and call futex_wait with ctid_val
as assumed current value of ctid_ptr.
Thanks for catching it.
diff --git a/sysdeps/unix/sysv/linux/tst-clone3.c b/sysdeps/unix/sysv/linux/tst-clone3.c
index aa8e718afe..ffa2056eb6 100644
--- a/sysdeps/unix/sysv/linux/tst-clone3.c
+++ b/sysdeps/unix/sysv/linux/tst-clone3.c
@@ -42,11 +42,11 @@ f (void *a)
/* Futex wait for TID argument, similar to pthread_join internal
implementation. */
-#define wait_tid(tid) \
+#define wait_tid(ctid_ptr, ctid_val) \
do { \
- __typeof (tid) __tid; \
- while ((__tid = (tid)) != 0) \
- futex_wait (&(tid), __tid); \
+ __typeof (*(ctid_ptr)) __tid; \
+ while ((__tid = *(ctid_ptr)) != 0) \
+ futex_wait (ctid_ptr, ctid_val); \
} while (0)
lll_wait_tid uses an atomic load with acquire semantic, I think we should
use it as well. We can either include the atomic header or use c11 atomic.
I've first tried to include atomic.h, but it failed building on x86_64.
Thus I'm using the c11 atomic load in the updated patch.
Okay to commit?
As information, I've observed those gcc errors on x86_64:
In file included from ../sysdeps/unix/sysv/linux/x86_64/sysdep.h:30,
from ../sysdeps/x86_64/nptl/tls.h:28,
from ../sysdeps/x86/atomic-machine.h:23,
from ../include/atomic.h:50,
from ../sysdeps/unix/sysv/linux/tst-clone3.c:29:
../sysdeps/unix/sysv/linux/dl-sysdep.h: In function
‘_dl_discover_osversion’:
../sysdeps/unix/sysv/linux/dl-sysdep.h:31:42: error: expected
declaration specifiers before ‘attribute_hidden’
extern int _dl_discover_osversion (void) attribute_hidden;
^~~~~~~~~~~~~~~~
In file included from ../sysdeps/x86_64/nptl/tls.h:31,
from ../sysdeps/x86/atomic-machine.h:23,
from ../include/atomic.h:50,
from ../sysdeps/unix/sysv/linux/tst-clone3.c:29:
../sysdeps/generic/dl-dtv.h:22:1: error: empty declaration [-Werror]
struct dtv_pointer
^~~~~~
../sysdeps/generic/dl-dtv.h:33:3: error: storage class specified for
parameter ‘dtv_t’
} dtv_t;
^~~~~
...
many many more errors
...
I've also tried to add tst-clone3 to tests_internal instead of tests in
the Makefile, but then I've got:
In file included from ../sysdeps/x86_64/nptl/tls.h:130,
from ../sysdeps/x86/atomic-machine.h:23,
from ../include/atomic.h:50,
from ../sysdeps/unix/sysv/linux/tst-clone3.c:29:
../nptl/descr.h:104:8: error: redefinition of ‘struct robust_list_head’
struct robust_list_head
^~~~~~~~~~~~~~~~
In file included from ../sysdeps/unix/sysv/linux/tst-clone3.c:26:
/usr/include/linux/futex.h:70:8: note: originally defined here
struct robust_list_head {
^~~~~~~~~~~~~~~~
static inline int
@@ -64,7 +64,11 @@ do_test (void)
clone_flags |= CLONE_VM | CLONE_SIGHAND;
/* We will used ctid to call on futex to wait for thread exit. */
clone_flags |= CLONE_CHILD_CLEARTID;
- pid_t ctid, tid;
+ /* Initialize with a known value. ctid is set to zero by the kernel after the
+ cloned thread has exited. */
+#define CTID_INIT_VAL 1
+ pid_t ctid = CTID_INIT_VAL;
+ pid_t tid;
#ifdef __ia64__
extern int __clone2 (int (*__fn) (void *__arg), void *__child_stack_base,
@@ -86,8 +90,7 @@ do_test (void)
if (tid == -1)
FAIL_EXIT1 ("clone failed: %m");
- ctid = tid;
- wait_tid (ctid);
+ wait_tid (&ctid, CTID_INIT_VAL);
return 2;
}
commit 3672d05bd3c5c51831ef8b91ee2b0ff491fd2986
Author: Stefan Liebler <stli@linux.ibm.com>
Date: Fri Feb 8 12:42:52 2019 +0100
misc/tst-clone3: Fix waiting for exited thread.
From time to time the test misc/tst-clone3 fails with a timeout.
Then futex_wait is blocking. Usually ctid should be set to zero
due to CLONE_CHILD_CLEARTID and the futex should be waken up.
But the fail occures if the thread has already exited before
ctid is set to the return value of clone(). Then futex_wait() will
block as there will be nobody who wakes the futex up again.
This patch initializes ctid to a known value before calling clone
and the kernel is the only one who updates the value to zero after clone.
If futex_wait is called then it is either waked up due to the exited thread
or the futex syscall fails as *ctid_ptr is already zero instead of the
specified value 1.
ChangeLog:
* sysdeps/unix/sysv/linux/tst-clone3.c (do_test):
Initialize ctid with a known value and remove update of ctid
after clone.
(wait_tid): Adjust arguments and call futex_wait with ctid_val
as assumed current value of ctid_ptr.
diff --git a/sysdeps/unix/sysv/linux/tst-clone3.c b/sysdeps/unix/sysv/linux/tst-clone3.c
index aa8e718afe..b345d04b4d 100644
--- a/sysdeps/unix/sysv/linux/tst-clone3.c
+++ b/sysdeps/unix/sysv/linux/tst-clone3.c
@@ -42,11 +42,13 @@ f (void *a)
/* Futex wait for TID argument, similar to pthread_join internal
implementation. */
-#define wait_tid(tid) \
- do { \
- __typeof (tid) __tid; \
- while ((__tid = (tid)) != 0) \
- futex_wait (&(tid), __tid); \
+#define wait_tid(ctid_ptr, ctid_val) \
+ do { \
+ __typeof (*(ctid_ptr)) __tid; \
+ /* We need acquire MO here so that we synchronize with the \
+ kernel's store to 0 when the clone terminates. */ \
+ while ((__tid = __atomic_load_n (ctid_ptr, __ATOMIC_ACQUIRE)) != 0) \
+ futex_wait (ctid_ptr, ctid_val); \
} while (0)
static inline int
@@ -64,7 +66,11 @@ do_test (void)
clone_flags |= CLONE_VM | CLONE_SIGHAND;
/* We will used ctid to call on futex to wait for thread exit. */
clone_flags |= CLONE_CHILD_CLEARTID;
- pid_t ctid, tid;
+ /* Initialize with a known value. ctid is set to zero by the kernel after the
+ cloned thread has exited. */
+#define CTID_INIT_VAL 1
+ pid_t ctid = CTID_INIT_VAL;
+ pid_t tid;
#ifdef __ia64__
extern int __clone2 (int (*__fn) (void *__arg), void *__child_stack_base,
@@ -86,8 +92,7 @@ do_test (void)
if (tid == -1)
FAIL_EXIT1 ("clone failed: %m");
- ctid = tid;
- wait_tid (ctid);
+ wait_tid (&ctid, CTID_INIT_VAL);
return 2;
}