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powerpc: Fix write-after-destroy in lock elision [BZ #20822]

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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.
This commit is contained in:
Tulio Magno Quites Machado Filho
2017-01-03 17:16:02 -02:00
parent daaff5cc79
commit e9a96ea1ac
4 changed files with 33 additions and 12 deletions
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15
sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
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@ -28,13 +28,16 @@ __lll_unlock_elision (int *lock, short *adapt_count, int pshared)
__libc_tend (0);
else
{
lll_unlock ((*lock), pshared);
/* 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 has to be contained within
the critical region of the fall-back lock in order to not violate
the mutex destruction requirements. */
short __tmp = atomic_load_relaxed (adapt_count);
if (__tmp > 0)
atomic_store_relaxed (adapt_count, __tmp--);
/* 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. */
if (*adapt_count > 0)
(*adapt_count)--;
lll_unlock ((*lock), pshared);
}
return 0;
}