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[PATCHv2] powerpc: Fix write-after-destroy in lock elision
- From: "Tulio Magno Quites Machado Filho" <tuliom at linux dot vnet dot ibm dot com>
- To: libc-alpha at sourceware dot org
- Cc: raji at linux dot vnet dot ibm dot com, triegel at redhat dot com, munroesj at linux dot vnet dot ibm dot com
- Date: Fri, 9 Dec 2016 17:37:39 -0200
- Subject: [PATCHv2] powerpc: Fix write-after-destroy in lock elision
- Authentication-results: sourceware.org; auth=none
- References: <1479394283.7146.1158.camel@localhost.localdomain>
From: Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>
Changes since v1:
- Removed references to data race by the actual error: write-after-destroy.
- Enclosed adapt_count accesses in C11 atomics.
--- 8< ---
The update of *adapt_count after the release of the lock causes a race
condition when thread A unlocks, thread B continues and destroys the
mutex, and thread A writes to *adapt_count.
2016-12-09 Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>
Steven Munroe <sjmunroe@us.ibm.com>
Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com>
[BZ #20822]
* sysdeps/unix/sysv/linux/powerpc/elision-lock.c
(__lll_lock_elision): Access adapt_count via C11 atomics.
* sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
(__lll_trylock_elision): Likewise.
* sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
(__lll_unlock_elision): Update adapt_count variable inside the
critical section.
---
sysdeps/unix/sysv/linux/powerpc/elision-lock.c | 8 +++++---
sysdeps/unix/sysv/linux/powerpc/elision-trylock.c | 7 ++++---
sysdeps/unix/sysv/linux/powerpc/elision-unlock.c | 14 +++++++++-----
3 files changed, 18 insertions(+), 11 deletions(-)
diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-lock.c b/sysdeps/unix/sysv/linux/powerpc/elision-lock.c
index dd1e4c3..c1ad1dd 100644
--- a/sysdeps/unix/sysv/linux/powerpc/elision-lock.c
+++ b/sysdeps/unix/sysv/linux/powerpc/elision-lock.c
@@ -45,7 +45,7 @@
int
__lll_lock_elision (int *lock, short *adapt_count, EXTRAARG int pshared)
{
- if (*adapt_count > 0)
+ if (atomic_load_relaxed(adapt_count) > 0)
{
goto use_lock;
}
@@ -67,7 +67,8 @@ __lll_lock_elision (int *lock, short *adapt_count, EXTRAARG int pshared)
if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
{
if (aconf.skip_lock_internal_abort > 0)
- *adapt_count = aconf.skip_lock_internal_abort;
+ atomic_store_relaxed (adapt_count,
+ aconf.skip_lock_internal_abort);
goto use_lock;
}
}
@@ -75,7 +76,8 @@ __lll_lock_elision (int *lock, short *adapt_count, EXTRAARG int pshared)
/* Fall back to locks for a bit if retries have been exhausted */
if (aconf.try_tbegin > 0 && aconf.skip_lock_out_of_tbegin_retries > 0)
- *adapt_count = aconf.skip_lock_out_of_tbegin_retries;
+ atomic_store_relaxed (adapt_count,
+ aconf.skip_lock_out_of_tbegin_retries);
use_lock:
return LLL_LOCK ((*lock), pshared);
diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-trylock.c b/sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
index 0807a6a..6248e32 100644
--- a/sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
+++ b/sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
@@ -34,7 +34,7 @@ __lll_trylock_elision (int *futex, short *adapt_count)
__libc_tabort (_ABORT_NESTED_TRYLOCK);
/* Only try a transaction if it's worth it. */
- if (*adapt_count > 0)
+ if (atomic_load_relaxed(adapt_count) > 0)
{
goto use_lock;
}
@@ -49,7 +49,7 @@ __lll_trylock_elision (int *futex, short *adapt_count)
__libc_tend (0);
if (aconf.skip_lock_busy > 0)
- *adapt_count = aconf.skip_lock_busy;
+ atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
}
else
{
@@ -59,7 +59,8 @@ __lll_trylock_elision (int *futex, short *adapt_count)
result in another failure. Use normal locking now and
for the next couple of calls. */
if (aconf.skip_trylock_internal_abort > 0)
- *adapt_count = aconf.skip_trylock_internal_abort;
+ atomic_store_relaxed (adapt_count,
+ aconf.skip_trylock_internal_abort);
}
}
diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c b/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
index 43c5a67..0e0baf5 100644
--- a/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
+++ b/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
@@ -28,13 +28,17 @@ __lll_unlock_elision (int *lock, short *adapt_count, int pshared)
__libc_tend (0);
else
{
- lll_unlock ((*lock), pshared);
-
- /* Update the adapt count AFTER completing the critical section.
- Doing this here prevents unneeded stalling when entering
- a critical section. Saving about 8% runtime on P8. */
+ /* Update adapt_count in the critical section to prevent a
+ write-after-destroy error as mentioned in BZ 20822. The
+ following update of adapt_count is clearly contained within
+ the critical region of the fall-back lock as it immediately
+ precedes the unlock. Attempting to use C11 atomics to access
+ adapt_count would be (at minimum) misleading and (at worse) a
+ serious error forcing a larx-hit-stcx flush. */
if (*adapt_count > 0)
(*adapt_count)--;
+
+ lll_unlock ((*lock), pshared);
}
return 0;
}
--
2.1.0