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From: Attilio Rao <attilio@FreeBSD.org>
Date: Mon, 29 Oct 2012 01:35:17 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r242274 - head/sys/sys
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Author: attilio
Date: Mon Oct 29 01:35:17 2012
New Revision: 242274
URL: http://svn.freebsd.org/changeset/base/242274

Log:
  Compiler have a precise knowledge of the content of sched_pin() and
  sched_unpin() as they are functions static and inline.  This way it
  can do two dangerous things:
  - Reorder instructions around both of them, taking out from the safe
    path operations that are supposed to be (ie. per-cpu accesses)
  - Cache the value of td_pinned in CPU registers not making visible
    in kernel context to the scheduler once it is scanning the runqueue,
    as td_pinned is not marked volatile.
  
  In order to avoid both possible bugs explicitly, protect the safe path
  with compiler memory barriers. This will prevent reordering and caching
  by the compiler about td_pinned operations.
  
  Generally this could lead to suboptimal code traversing the pinnings
  but this is not the case as can be easilly verified:
  http://lists.freebsd.org/pipermail/svn-src-projects/2012-October/005797.html
  
  Discussed with:	jeff, jhb
  MFC after:	2 weeks

Modified:
  head/sys/sys/sched.h

Modified: head/sys/sys/sched.h
==============================================================================
--- head/sys/sys/sched.h	Mon Oct 29 00:51:53 2012	(r242273)
+++ head/sys/sys/sched.h	Mon Oct 29 01:35:17 2012	(r242274)
@@ -151,11 +151,13 @@ static __inline void
 sched_pin(void)
 {
 	curthread->td_pinned++;
+	__compiler_membar();
 }
 
 static __inline void
 sched_unpin(void)
 {
+	__compiler_membar();
 	curthread->td_pinned--;
 }