Subject: Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for
 !PREEMPT_COUNT
From: Steven Rostedt <srostedt@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Vineet Gupta <Vineet.Gupta1@synopsys.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Christian Ruppert <christian.ruppert@abilis.com>,
        Pierrick Hascoet <pierrick.hascoet@abilis.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Organization: Red Hat
Date: Mon, 08 Apr 2013 10:31:10 -0400
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On Mon, 2013-04-08 at 15:37 +0200, Peter Zijlstra wrote:

> That said, I can't remember ever having seen a BUG like this, even
> though !PREEMPT is (or at least was) the most popular distro setting.

It requires gcc reordering the code to where a preempt can happen inside
preempt_disable. And also put in a position where the preempt_disable
code it gets added matters.

Then if gcc does this, we need a page fault to occur with a get_user()
operation, which in practice seldom happens as most get user operations
are done on freshly modified memory.

And then, it would require the page fault to cause a schedule. This is
the most likely of the things needed to occur, but itself is not a
problem.

Then, the schedule would have to cause the data that is being protect by
the preempt_disable() to be corrupted. Either by scheduling in another
process that monkeys with the data. Or if it protects per-cpu data,
scheduling to another CPU (for the SMP case only).

If any of the above does not occur, then you wont see a bug. This is
highly unlikely to happen, but that's no excuse to not fix it. But it
probably explains why we never saw a bug report. Heck, it may have
happened, but it would be hard to reproduce, and just forgotten about.

-- Steve


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