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    SubjectRe: Strange block/scsi/workqueue issue
    On 2011-04-11 19:18, Tejun Heo wrote:
    > Hello,
    >
    > (cc'ing James. The original message is http://lkml.org/lkml/2011/4/11/175 )
    >
    > Please read from the bottom up.
    >
    > On Mon, Apr 11, 2011 at 03:56:03PM +0100, Steven Whitehouse wrote:
    >> [<ffffffff8167b8e5>] schedule_timeout+0x295/0x310
    >> [<ffffffff8167a650>] wait_for_common+0x120/0x170
    >> [<ffffffff8167a748>] wait_for_completion+0x18/0x20
    >> [<ffffffff810aba4c>] wait_on_cpu_work+0xec/0x100
    >> [<ffffffff810abb3b>] wait_on_work+0xdb/0x150
    >> [<ffffffff810abc33>] __cancel_work_timer+0x83/0x130
    >> [<ffffffff810abced>] cancel_delayed_work_sync+0xd/0x10
    >
    > 4. which in turn tries to sync cancel q->delay_work. Oops, deadlock.
    >
    >> [<ffffffff813b24b4>] blk_sync_queue+0x24/0x50
    >
    > 3. and calls into blk_sync_queue()
    >
    >> [<ffffffff813b24ef>] blk_cleanup_queue+0xf/0x60
    >> [<ffffffff81479a89>] scsi_free_queue+0x9/0x10
    >> [<ffffffff8147d30b>] scsi_device_dev_release_usercontext+0xeb/0x140
    >> [<ffffffff810ac826>] execute_in_process_context+0x86/0xa0
    >
    > 2. It triggers SCSI device release
    >
    >> [<ffffffff8147d1f7>] scsi_device_dev_release+0x17/0x20
    >> [<ffffffff814609f2>] device_release+0x22/0x90
    >> [<ffffffff813c8165>] kobject_release+0x45/0x90
    >> [<ffffffff813c9767>] kref_put+0x37/0x70
    >> [<ffffffff813c8027>] kobject_put+0x27/0x60
    >> [<ffffffff81460822>] put_device+0x12/0x20
    >> [<ffffffff81478bd9>] scsi_request_fn+0xb9/0x4a0
    >> [<ffffffff813aff2a>] __blk_run_queue+0x6a/0x110
    >> [<ffffffff813b1f66>] blk_delay_work+0x26/0x40
    >
    > 1. Workqueue starting execution of q->delay_work and scsi_request_fn()
    > is run from there.
    >
    >> [<ffffffff810aa9c7>] process_one_work+0x197/0x520
    >> [<ffffffff810acfec>] worker_thread+0x15c/0x330
    >> [<ffffffff810b1f16>] kthread+0xa6/0xb0
    >> [<ffffffff81687064>] kernel_thread_helper+0x4/0x10
    >
    > So, q->delay_work ends up waiting for itself. I'd like to blame SCSI
    > (as it also fits my agenda to kill execute_in_process_context ;-) for
    > diving all the way into blk_cleanup_queue() directly from request_fn.
    >
    > Does the following patch fix the problem?

    Thanks, that looks a lot saner. This is/was a time bomb waiting to blow
    up.

    --
    Jens Axboe



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