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SubjectRe: mmotm 2010-03-09-19-15: Lot of scheduling while atomic warnings related to RCU
On Wed, Mar 10, 2010 at 07:57:46PM +0000, Mel Gorman wrote:
> On Tue, Mar 09, 2010 at 07:16:03PM -0800, akpm@linux-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
>
Yeah, its probably this chunk:
diff -puN
lib/kobject_uevent.c~sysctl-fix-up-remaining-references-to-uevent_helper-fix
lib/kobject_uevent.c
---
a/lib/kobject_uevent.c~sysctl-fix-up-remaining-references-to-uevent_helper-fix
+++ a/lib/kobject_uevent.c
@@ -273,10 +273,11 @@ int kobject_uevent_env(struct kobject *k
#endif

/* call uevent_helper, usually only enabled during early boot */
+ rcu_read_lock();
helper = rcu_dereference(uevent_helper);
if (helper[0])
retval = uevent_call_helper(subsystem, env);
-
+ rcu_read_unlock();
exit:
kfree(devpath);
kfree(env);
_
My bad, I'm not sure why I didn't catch that, I tested using lots of module
loads/unloads while changing uevent_helper. Anywho, should be easy to fix,
Mel, can you modify the code to copy helper to a private variable, then unlock
the rcu read side prior to calling uevent_call_helper? I imagine that would be
the best fix.


Thanks & Regards
Neil



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