On Tuesday 17 November 2009 1:25:01 pm Jilles Tjoelker wrote: > On Tue, Nov 17, 2009 at 01:50:32AM +0200, Dmitry Pryanishnikov wrote: > > > Author: edwin > > > Date: Tue Jun 23 22:28:44 2009 > > > New Revision: 194783 > > > URL: http://svn.freebsd.org/changeset/base/194783 > > > > Log: > > > Remove duplicate if-statement on gmt_is_set in gmtsub(). > > > > MFC after: 1 week > > > > Modified: > > > head/lib/libc/stdtime/localtime.c > > > This change looks like a (small?) pessimization to me: before it, > > _MUTEX_LOCK/_MUTEX_UNLOCK pair would be skipped for the case gmt_is_set > > == TRUE (all invocations except the first one), now it won't. I'm not > > sure whether this is critical here though... > > It is certainly less efficient, but the old code was (most likely) > wrong. It used an idiom known as "double checked locking", which is > incorrect in most memory models. The problem is that the store to > gmt_is_set may become visible without stores to other memory (gmtptr and > what it points to) becoming visible.
That is easily fixed with a memory barrier. Just use atomic_store_rel() to set gmt_is_set at the end of the setup phase. -- John Baldwin _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
