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

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Message-ID: <4F2FFB21.9000202@kernel.dk>
Date:	Mon, 06 Feb 2012 17:09:05 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Vivek Goyal <vgoyal@...hat.com>
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 02/06/2012 04:12 PM, Vivek Goyal wrote:
> On Mon, Feb 06, 2012 at 03:50:11PM +0800, Shaohua Li wrote:
>> Meelis reported a warning:
>>
>> 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
>>
>> this_q == locked_q is possible. There are two problems here:
>> 1. In UP case, there is preemption counter issue as spin_trylock always
>> successes.
>> 2. In SMP case, the loop breaks too earlier.
> 
> Thanks Shaohua. So is it the case where there are more than one cic's on
> ioc->ioc_list and first cic's queue is not same as locked_queue. But some
> other cic other than first has queue same as locked_queue.
> 
> In that case current code will still defer freeing of ioc and cic to a
> worker thread. So this patch will introduce one optimization to handle
> those cases and avoid calling worker thread.
> 
> Secondly it fixes the discrepancy of preemption count on UP machines, 
> where we have one extra preemption count after finish of function
> put_io_context(). So for UP case spin_trylock() increases the preemption
> count and always returns success. As this_q == locked_q we never try to do
> unlock on this queue and hence never decrement the preemption count
> hence resulting in preemption count warning.
> 
> 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.

-- 
Jens Axboe

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