From 1b468a131bd260c9041484f78b8580c7f232d580 Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Fri, 26 Jun 2015 17:00:01 +0200 Subject: [PATCH] Fix the fallback memory barrier implementation to be reentrant. This was essentially "broken" since 0c8eda62; but until more recently (14e8803f) barriers usage in signal handlers was infrequent. The failure to be reentrant was noticed because the test_shm_mq, which uses memory barriers at a high frequency, occasionally got stuck on some solaris buildfarm animals. Turns out, those machines use sun studio 12.1, which doesn't yet have efficient memory barrier support. A machine with a newer sun studio did not fail. Forcing the barrier fallback to be used on x86 allows to reproduce the problem. The new fallback is to use kill(PostmasterPid, 0) based on the theory that that'll always imply a barrier due to checking the liveliness of PostmasterPid on systems old enough to need fallback support. It's hard to come up with a good and performant fallback. I'm not backpatching this for now - the problem isn't active in the back branches, and we haven't backpatched barrier changes for now. Additionally master looks entirely different than the back branches due to the new atomics abstraction. It seems better to let this rest in master, where the non-reentrancy actively causes a problem, and then consider backpatching. Found-By: Robert Haas Discussion: 55626265.3060800@dunslane.net --- src/backend/port/atomics.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/src/backend/port/atomics.c b/src/backend/port/atomics.c index 3350fb7df3..439b3c18c7 100644 --- a/src/backend/port/atomics.c +++ b/src/backend/port/atomics.c @@ -20,15 +20,32 @@ */ #define ATOMICS_INCLUDE_DEFINITIONS +#include "miscadmin.h" #include "port/atomics.h" #include "storage/spin.h" +#ifdef PG_HAVE_MEMORY_BARRIER_EMULATION +#ifdef WIN32 +#error "barriers are required (and provided) on WIN32 platforms" +#endif +#include +#include +#endif + #ifdef PG_HAVE_MEMORY_BARRIER_EMULATION void pg_spinlock_barrier(void) { - S_LOCK(&dummy_spinlock); - S_UNLOCK(&dummy_spinlock); + /* + * NB: we have to be reentrant here, some barriers are placed in signal + * handlers. + * + * We use kill(0) for the fallback barrier as we assume that kernels on + * systems old enough to require fallback barrier support will include an + * appropriate barrier while checking the existence of the postmaster + * pid. + */ + (void) kill(PostmasterPid, 0); } #endif