Messages in this thread Patch in this message | | | Date | Fri, 12 Mar 2010 10:29:22 -0500 | From | Neil Horman <> | Subject | Re: mmotm 2010-03-09-19-15: Lot of scheduling while atomic warnings related to RCU |
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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@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. > > > > 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@csn.ul.ie> > --- > 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.
Neil
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