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refcount: update comments about fencing when releasing counts after r357989

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Requested by:	kib
Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D23719
This commit is contained in:
Mateusz Guzik 2020-02-16 18:20:09 +00:00
parent db41308a28
commit d8a84f08e8
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=358007
2 changed files with 8 additions and 2 deletions
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4
sys/kern/kern_synch.c
View File

@ -368,8 +368,8 @@ refcount_release_last(volatile u_int *count, u_int n, u_int old)
/*
* Last reference. Signal the user to call the destructor.
*
* Ensure that the destructor sees all updates. The fence_rel
* at the start of refcount_releasen synchronizes with this fence.
* Ensure that the destructor sees all updates. This synchronizes
* with release fences from all routines which drop the count.
*/
atomic_thread_fence_acq();
return (true);

6
sys/sys/refcount.h
View File

@ -119,6 +119,9 @@ refcount_releasen(volatile u_int *count, u_int n)
KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
("refcount_releasen: n=%u too large", n));
/*
* Paired with acquire fence in refcount_release_last.
*/
atomic_thread_fence_rel();
old = atomic_fetchadd_int(count, -n);
if (__predict_false(n >= REFCOUNT_COUNT(old) ||
@ -198,6 +201,9 @@ refcount_release_if_gt(volatile u_int *count, u_int n)
return (false);
if (__predict_false(REFCOUNT_SATURATED(old)))
return (true);
/*
* Paired with acquire fence in refcount_release_last.
*/
if (atomic_fcmpset_rel_int(count, &old, old - 1))
return (true);
}