linux-kernel - Re: [tip:timers/urgent] clocksource: Resume clocksource without taking the clocksource mutex

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Message-ID: <20091001082640.GB12200@elte.hu>
Date:	Thu, 1 Oct 2009 10:26:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	john stultz <johnstul@...ibm.com>
Cc:	mschmidt@...hat.com, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, schwidefsky@...ibm.com,
	tglx@...utronix.de, xtfeng@...il.com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/urgent] clocksource: Resume clocksource without
	taking the clocksource mutex


* john stultz <johnstul@...ibm.com> wrote:

> On Fri, 2009-09-25 at 06:20 +0000, tip-bot for Martin Schwidefsky wrote:
> > Commit-ID:  89133f93508137231251543d1732da638e6022e1
> > Gitweb:     http://git.kernel.org/tip/89133f93508137231251543d1732da638e6022e1
> > Author:     Martin Schwidefsky <schwidefsky@...ibm.com>
> > AuthorDate: Thu, 24 Sep 2009 17:29:52 +0200
> > Committer:  Ingo Molnar <mingo@...e.hu>
> > CommitDate: Thu, 24 Sep 2009 22:37:53 +0200
> > 
> > clocksource: Resume clocksource without taking the clocksource mutex
> > 
> > git commit 75c5158f70c065b9 converted the clocksource spinlock to a
> > mutex. This causes the following BUG:
> > 
> > BUG: sleeping function called from invalid context at
> > kernel/mutex.c:280 in_atomic(): 0, irqs_disabled(): 1, pid: 2473,
> > name: pm-suspend 2 locks held by pm-suspend/2473:
> >  #0:  (&buffer->mutex){......}, at: [<ffffffff8115ab13>]
> > sysfs_write_file+0x3c/0x137
> >  #1:  (pm_mutex){......}, at: [<ffffffff810865b5>]
> > enter_state+0x39/0x130 Pid: 2473, comm: pm-suspend Not tainted 2.6.31
> > #1 Call Trace:
> >  [<ffffffff810792f0>] ? __debug_show_held_locks+0x22/0x24
> >  [<ffffffff8104a2ef>] __might_sleep+0x107/0x10b
> >  [<ffffffff8141fca9>] mutex_lock_nested+0x25/0x43
> >  [<ffffffff81073537>] clocksource_resume+0x1c/0x60
> >  [<ffffffff81072902>] timekeeping_resume+0x1e/0x1c8
> >  [<ffffffff812aee62>] __sysdev_resume+0x25/0xcf
> >  [<ffffffff812aef79>] sysdev_resume+0x6d/0xae
> >  [<ffffffff810864f8>] suspend_devices_and_enter+0x12b/0x1af
> >  [<ffffffff8108665b>] enter_state+0xdf/0x130
> >  [<ffffffff81085dc3>] state_store+0xb6/0xd3
> >  [<ffffffff81204c73>] kobj_attr_store+0x17/0x19
> >  [<ffffffff8115abd2>] sysfs_write_file+0xfb/0x137
> >  [<ffffffff811057d2>] vfs_write+0xae/0x10b
> >  [<ffffffff81208392>] ? __up_read+0x1a/0x7f
> >  [<ffffffff811058ef>] sys_write+0x4a/0x6e
> >  [<ffffffff81011b82>] system_call_fastpath+0x16/0x1b
> > 
> > clocksource_resume is called early in the resume process, there is 
> > only one cpu, no processes are running and the interrupts are 
> > disabled. It is therefore possible to resume the clocksources 
> > without taking the clocksource mutex.
> 
> Should a comment to this effect be included in the code?

Yeah. (I suspect if such a question ever arises the answer is always yes 
;-)

	Ingo
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