linux-kernel - Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for !PREEMPT

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Date:	Mon, 08 Apr 2013 10:31:10 -0400
From:	Steven Rostedt <srostedt@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christian Ruppert <christian.ruppert@...lis.com>,
	Pierrick Hascoet <pierrick.hascoet@...lis.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for
 !PREEMPT_COUNT

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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