Message-ID: <20100310203401.GA14715@csn.ul.ie>
Date:	Wed, 10 Mar 2010 20:34:01 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <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: Re: mmotm 2010-03-09-19-15: Lot of scheduling while atomic
	warnings related to RCU

On Wed, Mar 10, 2010 at 12:21:38PM -0800, Andrew Morton wrote:
> On Wed, 10 Mar 2010 19:57:46 +0000
> Mel Gorman <mel@....ul.ie> wrote:
> 
> > 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.
> > 
> 
> Thanks.  It's Neil's
> sysctl-fix-up-remaining-references-to-uevent_helper-fix.patch which does
> 
> --- 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);
> 
> 

Indeed, I spotted that but the large number of dependencies around it made
me wonder was it really that patch or a problem with the whole thing. I
confess I didn't actually look to see what they were doing. Shortly after
I sent the first mail, I removed all these patches from the series just so
it would boot without crapping all over the console.

> Ho hum.  I guess all of
> 
> #rcu-add-rcustring-adt-for-rcu-protected-strings.patch: TBU
> rcu-add-rcustring-adt-for-rcu-protected-strings.patch
> #add-a-kernel_address-that-works-for-data-too.patch: TBU
> add-a-kernel_address-that-works-for-data-too.patch
> sysctl-add-proc_rcu_string-to-manage-sysctls-using-rcu-strings.patch
> sysctl-use-rcu-strings-for-core_pattern-sysctl.patch
> sysctl-add-call_usermodehelper_cleanup.patch
> sysctl-convert-modprobe_path-to-proc_rcu_string.patch
> sysctl-convert-poweroff_command-to-proc_rcu_string.patch
> #sysctl-convert-hotplug-helper-string-to-proc_rcu_string.patch: jirislaby crash
> sysctl-convert-hotplug-helper-string-to-proc_rcu_string.patch
> #sysctl-use-rcu-protected-sysctl-for-ocfs-group-add-helper.patch: Joel nack
> sysctl-use-rcu-protected-sysctl-for-ocfs-group-add-helper.patch
> #
> sysctl-fix-up-remaining-references-to-uevent_helper.patch
> sysctl-fix-up-remaining-references-to-uevent_helper-fix.patch
> #
> sysctl-clean-up-vm-related-variable-declarations.patch
> sysctl-clean-up-vm-related-variable-declarations-fix.patch
> #
> sysctl-remove-references-to-ctl_unnumbered-which-has-been-removed.patch
> #
> #
> sysctl-extern-cleanup-c_a_d.patch
> sysctl-extern-cleanup-signal.patch
> sysctl-extern-cleanup-binfmts.patch
> sysctl-extern-cleanup-pid.patch
> sysctl-extern-cleanup-mm.patch
> sysctl-extern-cleanup-compat.patch
> sysctl-extern-cleanup-latencytop.patch
> sysctl-extern-cleanup-file-nr.patch
> sysctl-extern-cleanup-rcu.patch
> sysctl-extern-cleanup-block-iopoll.patch
> sysctl-extern-cleanup-module.patch
> sysctl-extern-cleanup-sg.patch
> sysctl-extern-cleanup-acct.patch
> sysctl-extern-cleanup-rtmutex.patch
> sysctl-extern-cleanup-lockdep.patch
> sysctl-extern-cleanup-poll.patch
> #
> kmod-add-init-function-to-usermodehelper.patch
> kmod-add-init-function-to-usermodehelper-fix.patch
> kmod-replace-call_usermodehelper_pipe-with-use-of-umh-init-function-and-resolve-limit.patch
> kmod-replace-call_usermodehelper_pipe-with-use-of-umh-init-function-and-resolve-limitcleanup.patch
> 
> are not a prospect for 2.6.34. There are issues in the rcustring
> patches and the others have dependencies.  I suppose I can rebase
> sysctl-extern-cleanup-* though.
> 

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