Re: mmotm 2010-03-09-19-15: Lot of scheduling while atomicwarnings related to RCU
From: Mel Gorman
Date: Fri Mar 12 2010 - 10:41:44 EST
On Fri, Mar 12, 2010 at 10:29:22AM -0500, Neil Horman wrote:
> On Fri, Mar 12, 2010 at 11:46:00AM +0000, Mel Gorman wrote:
> > On Wed, Mar 10, 2010 at 03:32:42PM -0500, Neil Horman wrote:
> > > On Wed, Mar 10, 2010 at 07:57:46PM +0000, Mel Gorman wrote:
> > > > On Tue, Mar 09, 2010 at 07:16:03PM -0800, akpm@xxxxxxxxxxxxxxxxxxxx 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.
> > >
> >
> > I am missing why it is even necessary to dereference this string. helper
> > is not passed to uevent_call_helper() and it makes a proper copy of the
> > string. Why not just make the empty-string check on the copy like the
> > following?
> >
> > ==== CUT HERE ====
> > kobject: Do not call uevent_helper with RCU lock held
> >
> > With sleep debugging enabled, the call to uevent_helper spews warnings
> > all over the console because the RCU lock is held when the process goes to
> > sleep. The RCU lock is only held so that the system is sure that
> > uevent_helper is not an empty string.
> >
> > As uevent_call_helper() already makes a proper copy of the uevent_helper
> > string, this patch changes when and where the empty-string check is
> > made so that the check is made against the copy. Assuming the helper
> > exists, it is then called without the RCU lock held.
> >
> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > ---
> > lib/kobject_uevent.c | 13 +++++++------
> > 1 files changed, 7 insertions(+), 6 deletions(-)
> >
> > diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> > index a7520d0..f411457 100644
> > --- a/lib/kobject_uevent.c
> > +++ b/lib/kobject_uevent.c
> > @@ -88,6 +88,11 @@ static int uevent_call_helper(const char *subsystem, struct kobj_uevent_env *env
> > if (!helper)
> > return -ENOMEM;
> >
> > + if (!helper[0]) {
> > + retval = -EINVAL;
> > + goto error;
> > + }
> > +
> > retval = -E2BIG;
> > argv[0] = helper;
> > argv[1] = (char *)subsystem;
> > @@ -126,7 +131,6 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
> > struct kset *kset;
> > const struct kset_uevent_ops *uevent_ops;
> > u64 seq;
> > - const char *helper;
> > int i = 0;
> > int retval = 0;
> >
> > @@ -273,11 +277,8 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
> > #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();
> > + retval = uevent_call_helper(subsystem, env);
> > +
> > exit:
> > kfree(devpath);
> > kfree(env);
> >
>
> We could, but all the rcu string copy stuff got trashed, so we've got to re-diff
> this all anyway. I'll probalby do exactly what you're suggesting as I fix this
> all up though.
>
Grand. I'm going to ignore this for the moment so and will keep an eye
out for it in the future.
Thanks
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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