Author: kib Date: Fri Jun 30 16:16:21 2017 New Revision: 320501 URL: https://svnweb.freebsd.org/changeset/base/320501
Log: Correct fences for sys/refcount.h. The acq barrier in refcount_acquire() has no use, constructor must ensure that the changes are visible before publication by other means. Last release must sync/with the constructor and all updaters. This is based on the refcount/shared_ptr analysis I heard at the Hans Boehm and Herb Sutter talks about C++ atomics. Reviewed by: alc, jhb, markj Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Differential revision: https://reviews.freebsd.org/D11270 Modified: head/sys/sys/refcount.h Modified: head/sys/sys/refcount.h ============================================================================== --- head/sys/sys/refcount.h Fri Jun 30 16:12:57 2017 (r320500) +++ head/sys/sys/refcount.h Fri Jun 30 16:16:21 2017 (r320501) @@ -50,7 +50,7 @@ refcount_acquire(volatile u_int *count) { KASSERT(*count < UINT_MAX, ("refcount %p overflowed", count)); - atomic_add_acq_int(count, 1); + atomic_add_int(count, 1); } static __inline int @@ -58,10 +58,20 @@ refcount_release(volatile u_int *count) { u_int old; - /* XXX: Should this have a rel membar? */ + atomic_thread_fence_rel(); old = atomic_fetchadd_int(count, -1); KASSERT(old > 0, ("negative refcount %p", count)); - return (old == 1); + if (old > 1) + return (0); + + /* + * Last reference. Signal the user to call the destructor. + * + * Ensure that the destructor sees all updates. The fence_rel + * at the start of the function synchronized with this fence. + */ + atomic_thread_fence_acq(); + return (1); } #endif /* ! __SYS_REFCOUNT_H__ */ _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
