svn commit: r242274 - head/sys/sys
Gleb Smirnoff
glebius at FreeBSD.org
Mon Oct 29 05:02:31 UTC 2012
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.
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