Re: [RFC PATCH V2] rt/aio: fix rcu garbage collection might_sleep() splat

[Benjamin LaHaise]

On Thu, Jun 26, 2014 at 09:37:14AM +0200, Mike Galbraith wrote:
> Hi Ben,
> 
> On Wed, 2014-06-25 at 11:24 -0400, Benjamin LaHaise wrote:
> 
> > I finally have some time to look at this patch in detail.  I'd rather do the 
> > below variant that does what Kent suggested.  Mike, can you confirm that 
> > this fixes the issue you reported?  It's on top of my current aio-next tree 
> > at git://git.kvack.org/~bcrl/aio-next.git .  If that's okay, I'll queue it 
> > up.  Does this bug fix need to end up in -stable kernels as well or would it 
> > end up in the -rt tree?
> 
> It's an -rt specific problem, so presumably any fix would only go into
> -rt trees until it manages to get merged.
> 
> I knew intervening change wasn't likely to fix the might_sleep() splat
> up, but did the test anyway with fixed up CONFIG_PREEMPT_RT_BASE typo.
> schedule_work() leads to an rtmutex, so -rt still has to ship that out
> from under rcu_read_lock_sched().

So that doesn't fix it.  I think you should fix schedule_work(), because 
that should be callable from any context.  Abusing RCU instead of using 
schedule_work() is not the right way to fix this.

		-ben

> marge:/usr/local/src/kernel/linux-3.14-rt # quilt applied|tail
> patches/mm-memcg-make-refill_stock-use-get_cpu_light.patch
> patches/printk-fix-lockdep-instrumentation-of-console_sem.patch
> patches/aio-block-io_destroy-until-all-context-requests-are-completed.patch
> patches/fs-aio-Remove-ctx-parameter-in-kiocb_cancel.patch
> patches/aio-report-error-from-io_destroy-when-threads-race-in-io_destroy.patch
> patches/aio-cleanup-flatten-kill_ioctx.patch
> patches/aio-fix-aio-request-leak-when-events-are-reaped-by-userspace.patch
> patches/aio-fix-kernel-memory-disclosure-in-io_getevents-introduced-in-v3.10.patch
> patches/aio-change-exit_aio-to-load-mm-ioctx_table-once-and-avoid-rcu_read_lock.patch
> patches/rt-aio-fix-rcu-garbage-collection-might_sleep-splat-ben.patch
> 
> [  191.057656] BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:792
> [  191.057672] in_atomic(): 1, irqs_disabled(): 0, pid: 22, name: rcuc/0
> [  191.057674] 2 locks held by rcuc/0/22:
> [  191.057684]  #0:  (rcu_callback){.+.+..}, at: [<ffffffff810ceb87>] rcu_cpu_kthread+0x2d7/0x840
> [  191.057691]  #1:  (rcu_read_lock_sched){.+.+..}, at: [<ffffffff812e52f6>] percpu_ref_kill_rcu+0xa6/0x1c0
> [  191.057694] Preemption disabled at:[<ffffffff810cebca>] rcu_cpu_kthread+0x31a/0x840
> [  191.057695] 
> [  191.057698] CPU: 0 PID: 22 Comm: rcuc/0 Tainted: GF       W    3.14.8-rt5 #47
> [  191.057699] Hardware name: MEDIONPC MS-7502/MS-7502, BIOS 6.00 PG 12/26/2007
> [  191.057704]  ffff88007c5d8000 ffff88007c5d7c98 ffffffff815696ed 0000000000000000
> [  191.057708]  ffff88007c5d7cb8 ffffffff8108c3e5 ffff88007dc0e120 000000000000e120
> [  191.057711]  ffff88007c5d7cd8 ffffffff8156f404 ffff88007dc0e120 ffff88007dc0e120
> [  191.057712] Call Trace:
> [  191.057716]  [<ffffffff815696ed>] dump_stack+0x4e/0x9c
> [  191.057720]  [<ffffffff8108c3e5>] __might_sleep+0x105/0x180
> [  191.057723]  [<ffffffff8156f404>] rt_spin_lock+0x24/0x70
> [  191.057727]  [<ffffffff81078897>] queue_work_on+0x67/0x1a0
> [  191.057731]  [<ffffffff81216fc2>] free_ioctx_users+0x72/0x80
> [  191.057734]  [<ffffffff812e5404>] percpu_ref_kill_rcu+0x1b4/0x1c0
> [  191.057737]  [<ffffffff812e52f6>] ? percpu_ref_kill_rcu+0xa6/0x1c0
> [  191.057740]  [<ffffffff812e5250>] ? percpu_ref_kill_and_confirm+0x70/0x70
> [  191.057742]  [<ffffffff810cebca>] rcu_cpu_kthread+0x31a/0x840
> [  191.057745]  [<ffffffff810ceb87>] ? rcu_cpu_kthread+0x2d7/0x840
> [  191.057749]  [<ffffffff8108a76d>] smpboot_thread_fn+0x1dd/0x340
> [  191.057752]  [<ffffffff8156c45a>] ? schedule+0x2a/0xa0
> [  191.057755]  [<ffffffff8108a590>] ? smpboot_register_percpu_thread+0x100/0x100
> [  191.057758]  [<ffffffff81081ca6>] kthread+0xd6/0xf0
> [  191.057761]  [<ffffffff81081bd0>] ? __kthread_parkme+0x70/0x70
> [  191.057764]  [<ffffffff815780bc>] ret_from_fork+0x7c/0xb0
> [  191.057767]  [<ffffffff81081bd0>] ? __kthread_parkme+0x70/0x70
> 

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