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    SubjectRe: [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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