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Date: Tue, 1 Jun 2010 08:55:00 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Jens Axboe <axboe@kernel.dk>
Cc: Peter Zijlstra <peterz@infradead.org>, piotr@hosowicz.com,
       linux-kernel@vger.kernel.org, Divyesh Shah <dpshah@google.com>
Subject: Re: BUG: using smp_processor_id() in preemptible [00000000] code:
 icedove-bin/5449
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* Jens Axboe <axboe@kernel.dk> wrote:

> On Tue, Jun 01 2010, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > > On Mon, 2010-05-24 at 05:03 +0200, Piotr Hosowicz wrote:
> > > > [  720.313607] BUG: using smp_processor_id() in preemptible [00000000] code: icedove-bin/5449
> > > > [  720.313612] caller is native_sched_clock+0x3c/0x68
> > > > [  720.313616] Pid: 5449, comm: icedove-bin Tainted: P            2.6.34-20100524-0407 #1
> > > > [  720.313618] Call Trace:
> > > > [  720.313624]  [<ffffffff811a533b>] debug_smp_processor_id+0xc7/0xe0
> > > > [  720.313629]  [<ffffffff81009b87>] native_sched_clock+0x3c/0x68
> > > > [  720.313634]  [<ffffffff81009a4d>] sched_clock+0x9/0xd
> > > > [  720.313637]  [<ffffffff811823ec>] blk_rq_init+0x92/0x9d
> > > > [  720.313641]  [<ffffffff81184227>] get_request+0x1bf/0x2c7
> > > > [  720.313646]  [<ffffffff8118435c>] get_request_wait+0x2d/0x19d 
> > > 
> > > This comes from wreckage in the blk tree..
> > > 
> > > ---
> > > commit 9195291e5f05e01d67f9a09c756b8aca8f009089
> > > Author: Divyesh Shah <dpshah@google.com>
> > > Date:   Thu Apr 1 15:01:41 2010 -0700
> > > 
> > >     blkio: Increment the blkio cgroup stats for real now
> > 
> > Jens, this regression is still in .35-rc1 and triggers in about 25% of all 
> > -tip boot tests.
> > 
> > The above commit is using sched_clock() in an unsafe way - please fix it or 
> > revert it.
> > 
> > The local_clock() changes PeterZ is working on are still WIP, it's not sure 
> > we'll have it before .36.
> 
> OK, I guess we'll have to solve this differently for .35 - I'll cook up 
> something simple, if need be revert the change.

I suspect you can put get_cpu/put_cpu around it and use cpu_clock(). The 
cross-CPU effects will still be there and there might be weird stats.

	Ingo
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