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Date:	Wed, 10 Mar 2010 19:57:46 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	akpm@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org, Neil Horman <nhorman@...driver.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: mmotm 2010-03-09-19-15: Lot of scheduling while atomic warnings
	related to RCU

On Tue, Mar 09, 2010 at 07:16:03PM -0800, akpm@...ux-foundation.org wrote:
> The mm-of-the-moment snapshot 2010-03-09-19-15 has been uploaded to
> 
>    http://userweb.kernel.org/~akpm/mmotm/
> 
> and will soon be available at
> 
>    git://zen-kernel.org/kernel/mmotm.git
> 
> It contains the following patches against 2.6.34-rc1:
> 

With preempt and sleeping spinlock debugging on, I'm seeing a lot of warnings
about scheduling while atomic. Starts off with messages like

[    0.572278] BUG: scheduling while atomic: swapper/1/0x00000002
[    0.576004] 1 lock held by swapper/1:
[    0.580002]  #0:  (rcu_read_lock){.+.+.+}, at: [<ffffffff811b6950>] kobject_uevent_env+0x363/0x4d0
[    0.600001] Modules linked in:
[    0.608003] Pid: 1, comm: swapper Not tainted 2.6.34-rc1-mm1-vanilla #1
[    0.612001] Call Trace:
[    0.616005]  [<ffffffff810727fe>] ?  __debug_show_held_locks+0x22/0x24
[    0.620005]  [<ffffffff810395ed>] __schedule_bug+0x77/0x7c
[    0.624004]  [<ffffffff812f4cc5>] schedule+0xcc/0x723
[    0.628003]  [<ffffffff812f55b6>] schedule_timeout+0x2d/0x28d
[    0.632003]  [<ffffffff8107315d>] ? mark_held_locks+0x4d/0x6b
[    0.636004]  [<ffffffff812f81e9>] ? _raw_spin_unlock_irq+0x30/0x5d
[    0.640003]  [<ffffffff810733de>] ?  trace_hardirqs_on_caller+0x110/0x134
[    0.644003]  [<ffffffff812f4a0f>] wait_for_common+0xdc/0x152
[    0.648003]  [<ffffffff8103a43b>] ? default_wake_function+0x0/0x14
[    0.652004]  [<ffffffff812f4b28>] wait_for_completion+0x1d/0x1f
[    0.656004]  [<ffffffff8105cdf0>] call_usermodehelper_exec+0xad/0xfa
[    0.660004]  [<ffffffff81068429>] ?  prepare_usermodehelper_creds+0x136/0x168
[    0.664003]  [<ffffffff811b6a37>] kobject_uevent_env+0x44a/0x4d0
[    0.668003]  [<ffffffff811b6950>] ? kobject_uevent_env+0x363/0x4d0
[    0.672003]  [<ffffffff811b6ac8>] kobject_uevent+0xb/0xd
[    0.676002]  [<ffffffff811b5e65>] kset_register+0x37/0x3f
[    0.680004]  [<ffffffff8123239a>] bus_register+0x11f/0x2b1
[    0.684005]  [<ffffffff8175973e>] platform_bus_init+0x2c/0x44
[    0.688003]  [<ffffffff817597b9>] driver_init+0x1d/0x29
[    0.692004]  [<ffffffff8172c69b>] kernel_init+0x12e/0x1e3
[    0.696003]  [<ffffffff81003be4>] kernel_thread_helper+0x4/0x10
[    0.700004]  [<ffffffff812f8794>] ? restore_args+0x0/0x30
[    0.704003]  [<ffffffff8172c56d>] ? kernel_init+0x0/0x1e3
[    0.708002]  [<ffffffff81003be0>] ? kernel_thread_helper+0x0/0x10

There are a lot of warnings along these lines during boot. This system did
boot *eventually* but it was in obvious trouble. The warnings appear to
be all around kobject_uevent_env acquiring the RCU lock and then going to
sleep. Most likely candidate problem patches in mmotm are

sysctl-fix-up-remaining-references-to-uevent_helper
sysctl-fix-up-remaining-references-to-uevent_helper-fix

But there are also a whole host of other sysctl-related changes around RCU in
there. The changelog implies that these patches are part of some long-lived
review process so I didn't delve too deeply. The people most likely to be
involved based on comments in the changelog are cc'd.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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