linux-kernel - Re: [RFC][PATCH] preempt: Debug for possible missed preemption checks

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Message-Id: <20140119115253.d6974d9c188c6d9e88d04b80@canb.auug.org.au>
Date:	Sun, 19 Jan 2014 11:52:53 +1100
From:	Stephen Rothwell <sfr@...b.auug.org.au>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>
Subject: Re: [RFC][PATCH] preempt: Debug for possible missed preemption
 checks

Hi Steve,

On Sat, 18 Jan 2014 18:44:01 -0500 Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Thu, 16 Jan 2014 21:12:14 -0800
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Thu, 16 Jan 2014 23:57:51 -0500 Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > When PROVE_LOCKING and PREEMPT is configured, the preempt state
> > > tracking is active. Testing this out, I added a module that did the
> > > following:
> > 
> > So I assume your kernel at least has no instances of this bug, so we
> > don't need the patch ;) It *is* a fairly daft thing to do.
> > 
> > Maybe stick it in -next for a few months, see if anyone hits it?
> 
> Do you have any objections if I add this change to my for-next branch?
> I'll do it as a merge as I do not plan on having it go into the next
> release. But this is an extension to lockdep that when both
> PROVE_LOCKING and PREEMPT are enabled, it can catch a certain bug. But
> as Andrew has stated, it did not find any in the kernel that I'm
> running.
> 
> What I propose is to have this go into linux-next, as I assume that
> people test it with PROVE_LOCKING and PREEMPT enabled, and if someone
> adds this bug this patch will catch it (if the bug path is taken).
> Hopefully it would be reported and we know two things. One, someone
> added a bug, and two, this patch is useful to add to mainline.
> 
> Here's the catch 22, it may not be worth adding to mainline if it never
> catches any bugs. But we wont know that unless we add it to mainline.
> Maybe adding it to linux-next might be good enough for now.

Given that the merge window will probably open today or tomorrow, I would
prefer any new code not intended for 3.14 not be added to linux-next
until after v3.14-rc1 to avoid unneeded conflicts.  If, however, Andrew
thinks it is still worth the (maybe minimal) pain, then fine.
-- 
Cheers,
Stephen Rothwell                    sfr@...b.auug.org.au

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