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Date: Mon, 29 Oct 2012 09:02:21 +0400
From: Gleb Smirnoff <glebius@FreeBSD.org>
To: Attilio Rao <attilio@FreeBSD.org>
Subject: Re: svn commit: r242274 - head/sys/sys
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On Mon, Oct 29, 2012 at 01:35:17AM +0000, Attilio Rao wrote:
A> Author: attilio
A> Date: Mon Oct 29 01:35:17 2012
A> New Revision: 242274
A> URL: http://svn.freebsd.org/changeset/base/242274
A> 
A> Log:
A>   Compiler have a precise knowledge of the content of sched_pin() and
A>   sched_unpin() as they are functions static and inline.  This way it
A>   can do two dangerous things:
A>   - Reorder instructions around both of them, taking out from the safe
A>     path operations that are supposed to be (ie. per-cpu accesses)
A>   - Cache the value of td_pinned in CPU registers not making visible
A>     in kernel context to the scheduler once it is scanning the runqueue,
A>     as td_pinned is not marked volatile.
A>   
A>   In order to avoid both possible bugs explicitly, protect the safe path
A>   with compiler memory barriers. This will prevent reordering and caching
A>   by the compiler about td_pinned operations.
A>   
A>   Generally this could lead to suboptimal code traversing the pinnings
A>   but this is not the case as can be easilly verified:
A>   http://lists.freebsd.org/pipermail/svn-src-projects/2012-October/005797.html

Now __compiler_membar() can be removed from kern_rmlock.c:360

-- 
Totus tuus, Glebius.