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refcount: add missing release fence to refcount_release_if_gt

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The CPU succeeding in releasing the not last reference can still have pending
stores to the object protected by the affected counter. This opens a time
window where another CPU can release the last reference and free the object,
resulting in use-after-free. On top of that this prevents the compiler from
generating more accesses to the object regardless of how atomic_fcmpset_rel_int
is implemented (of course as long as it provides the release semantic).

Reviewed by:	markj
This commit is contained in:
Mateusz Guzik 2020-02-16 03:14:55 +00:00
parent 6d88d784f8
commit 890611286e
1 changed files with 1 additions and 1 deletions
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2
sys/sys/refcount.h
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@ -198,7 +198,7 @@ refcount_release_if_gt(volatile u_int *count, u_int n)
return (false);
if (__predict_false(REFCOUNT_SATURATED(old)))
return (true);
if (atomic_fcmpset_int(count, &old, old - 1))
if (atomic_fcmpset_rel_int(count, &old, old - 1))
return (true);
}
}