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    Subject[patch]block: fix ioc locking warning
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    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.

    Signed-off-by: Shaohua Li <shaohua.li@intel.com>
    Reported-by: Meelis Roos <mroos@linux.ee>
    Reported-by: Knut Petersen <Knut_Petersen@t-online.de>
    Tested-by: Knut Petersen <Knut_Petersen@t-online.de>

    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)
    spin_unlock(last_q->queue_lock);
    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))
    break;
    last_q = this_q;
    continue;



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