From: Robert Haas Date: Fri, 16 Oct 2015 18:06:22 +0000 (-0400) Subject: Remove cautions about using volatile from spin.h. X-Git-Tag: REL9_6_BETA1~1196 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=78652a3332128c89ae369e03698f7212ca73d022;p=postgresql Remove cautions about using volatile from spin.h. Commit 0709b7ee72e4bc71ad07b7120acd117265ab51d0 obsoleted this comment but neglected to update it. Thomas Munro --- diff --git a/src/include/storage/spin.h b/src/include/storage/spin.h index dc6856e4ac..4674f48164 100644 --- a/src/include/storage/spin.h +++ b/src/include/storage/spin.h @@ -25,15 +25,10 @@ * Callers must beware that the macro argument may be evaluated multiple * times! * - * CAUTION: Care must be taken to ensure that loads and stores of - * shared memory values are not rearranged around spinlock acquire - * and release. This is done using the "volatile" qualifier: the C - * standard states that loads and stores of volatile objects cannot - * be rearranged *with respect to other volatile objects*. The - * spinlock is always written through a volatile pointer by the - * spinlock macros, but this is not sufficient by itself: code that - * protects shared data with a spinlock MUST reference that shared - * data through a volatile pointer. + * Load and store operations in calling code are guaranteed not to be + * reordered with respect to these operations, because they include a + * compiler barrier. (Before PostgreSQL 9.5, callers needed to use a + * volatile qualifier to access data protected by spinlocks.) * * Keep in mind the coding rule that spinlocks must not be held for more * than a few instructions. In particular, we assume it is not possible