Date: Sat, 15 Jun 2013 14:56:51 +0200 From: Marius Strobl <marius@alchemy.franken.de> To: Ed Schouten <ed@FreeBSD.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r251782 - head/sys/sparc64/sparc64 Message-ID: <20130615125651.GH91573@alchemy.franken.de> In-Reply-To: <201306150821.r5F8Lst5089231@svn.freebsd.org> References: <201306150821.r5F8Lst5089231@svn.freebsd.org>
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On Sat, Jun 15, 2013 at 08:21:54AM +0000, Ed Schouten wrote: > Author: ed > Date: Sat Jun 15 08:21:54 2013 > New Revision: 251782 > URL: http://svnweb.freebsd.org/changeset/base/251782 > > Log: > Stick to using the documented atomic(9) API. > > The atomic_store_ptr() function is not part of the atomic(9) API. We > only provide a version with a release barrier. > > Modified: > head/sys/sparc64/sparc64/pmap.c > > Modified: head/sys/sparc64/sparc64/pmap.c > ============================================================================== > --- head/sys/sparc64/sparc64/pmap.c Sat Jun 15 08:15:22 2013 (r251781) > +++ head/sys/sparc64/sparc64/pmap.c Sat Jun 15 08:21:54 2013 (r251782) > @@ -2246,7 +2246,7 @@ pmap_activate(struct thread *td) > pm->pm_context[curcpu] = context; > #ifdef SMP > CPU_SET_ATOMIC(PCPU_GET(cpuid), &pm->pm_active); > - atomic_store_ptr((uintptr_t *)PCPU_PTR(pmap), (uintptr_t)pm); > + atomic_store_rel_ptr((uintptr_t *)PCPU_PTR(pmap), (uintptr_t)pm); > #else > CPU_SET(PCPU_GET(cpuid), &pm->pm_active); > PCPU_SET(pmap, pm); Semantically, this change is wrong; what we really need here is an acquire variant. Using the release variant instead happens to also work - albeit additionally wastes CPU cycles for the write memory barrier - because in total store order, atomic operations implicitly include the read memory barrier necessary for acquire semantics. In other words, atomic(9) is in dare need of atomic_store_acq_<type>(). Marius
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