@@ -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);
@@ -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);
}
}
@@ -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;
}
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(-)