linux-kernel - Re: [patch]block: fix ioc locking warning

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Date:	Mon, 6 Feb 2012 11:37:21 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Shaohua Li <vivek.goyal2008@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>,
	Knut Petersen <Knut_Petersen@...nline.de>, mroos@...ux.ee
Subject: Re: [patch]block: fix ioc locking warning

On Mon, Feb 06, 2012 at 05:09:05PM +0100, Jens Axboe wrote:

[..]
> > Changlog was not obivious atleast to me. I wished it was little more
> > descriptive. Anyway, patch is already committed..
> 
> We can always amend the changelog, so don't worry about it already being
> committed. If you want to add/change something, just send it in.

How about something like as follows.


block: Do not lock try to lock already locked queue again

put_io_context() can be called with one of the request queue lock 
already held. But if the locked queue's cic is not first in the
ioc->ioc_list, then we have two possible issues.

- For above condition, current code bails out and schedules the worker
  thread for freeing up ioc. This can be optimized.

- It might happen that we received the queue locked but we still do the
  trylock on the queue. For SMP case that's not a problem as we will
  fail to lock already locked queue, but in case of UP, we seem to
  succeed and in the process increment the preempt count. Once we are
  done with ioc_exit_icq(), we do not call spin_unlock() on locked
  queue as we are not supposed to. This leads to imbalance in preemtion
  count and following warning was reported.

This patch fixes both the above issues by making sure we do not try to
lock already locked queue again.

WARNING: at kernel/timer.c:1122 run_timer_softirq+0x199/0x1ec()
Hardware name: 939Dual-SATA2
timer: cfq_idle_slice_timer+0x0/0xaa preempt leak: 00000102 -> 00000103
Modules linked in: sr_mod cdrom videodev media drm_kms_helper ohci_hcd ehci_hcd
v4l2_compat_ioctl32 usbcore i2c_ali15x3 snd_seq drm snd_timer snd_seq
Pid: 0, comm: swapper Not tainted 3.3.0-rc2-00110-gd125666 #176
Call Trace:
<IRQ>  [<ffffffff81022aaa>] warn_slowpath_common+0x7e/0x96
[<ffffffff8114c485>] ? cfq_slice_expired+0x1d/0x1d
[<ffffffff81022b56>] warn_slowpath_fmt+0x41/0x43
[<ffffffff8114c526>] ? cfq_idle_slice_timer+0xa1/0xaa
[<ffffffff8114c485>] ? cfq_slice_expired+0x1d/0x1d
[<ffffffff8102c124>] run_timer_softirq+0x199/0x1ec
[<ffffffff81047a53>] ? timekeeping_get_ns+0x12/0x31
[<ffffffff810145fd>] ? apic_write+0x11/0x13
[<ffffffff81027475>] __do_softirq+0x74/0xfa
[<ffffffff812f337a>] call_softirq+0x1a/0x30
[<ffffffff81002ff9>] do_softirq+0x31/0x68
[<ffffffff810276cf>] irq_exit+0x3d/0xa3
[<ffffffff81014aca>] smp_apic_timer_interrupt+0x6b/0x77
[<ffffffff812f2de9>] apic_timer_interrupt+0x69/0x70
<EOI>  [<ffffffff81040136>] ? sched_clock_cpu+0x73/0x7d
[<ffffffff81040136>] ? sched_clock_cpu+0x73/0x7d
[<ffffffff8100801f>] ? default_idle+0x1e/0x32
[<ffffffff81008019>] ? default_idle+0x18/0x32
[<ffffffff810008b1>] cpu_idle+0x87/0xd1
[<ffffffff812de861>] rest_init+0x85/0x89
[<ffffffff81659a4d>] start_kernel+0x2eb/0x2f8
[<ffffffff8165926e>] x86_64_start_reservations+0x7e/0x82
[<ffffffff81659362>] x86_64_start_kernel+0xf0/0xf7

Signed-off-by: Shaohua Li <shaohua.li@...el.com>
Reported-by: Meelis Roos <mroos@...ux.ee>
Reported-by: Knut Petersen <Knut_Petersen@...nline.de>
Tested-by: Knut Petersen <Knut_Petersen@...nline.de>

Thanks
Vivek
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