[patch]block: fix ioc locking warning

From: Shaohua Li
Date: Mon Feb 06 2012 - 02:53:27 EST

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
2. In SMP case, the loop breaks too earlier.

Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx>
Reported-by: Meelis Roos <mroos@xxxxxxxx>
Reported-by: Knut Petersen <Knut_Petersen@xxxxxxxxxxx>
Tested-by: Knut Petersen <Knut_Petersen@xxxxxxxxxxx>

diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index 27a06e0..7490b6d 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -204,7 +204,9 @@ void put_io_context(struct io_context *ioc, struct request_queue *locked_q)
last_q = NULL;

- if (!spin_trylock(this_q->queue_lock))
+ /* spin_trylock() always successes in UP case */
+ if (this_q != locked_q &&
+ !spin_trylock(this_q->queue_lock))
last_q = this_q;

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