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Subject: Re: 2.6.19-rc3-git7: scsi_device_unbusy: inconsistent lock state
From: Arjan van de Ven <arjan@infradead.org>
To: Jens Axboe <jens.axboe@oracle.com>
Cc: Mark Lord <liml@rtr.ca>,
       IDE/ATA development list <linux-ide@vger.kernel.org>,
       Linux Kernel <linux-kernel@vger.kernel.org>, mingo@elte.hu
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Date: Mon, 30 Oct 2006 17:16:42 +0100
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> 
> > so to me it looks like lockdep at least has the appearance of moaning
> > about a reasonably fishy situation...
> 
> To me it looks more about lockdep complaining because it doesn't grok
> the full picture. The question is how to shut it up.

ok that is quite possible. But I do think you read the original output
incorrectly so let me at least phrase it in english:


__queue_lock is used in softirq context like this:
  [<c0361c59>] _spin_lock+0x29/0x40
  [<c029fa24>] scsi_device_unbusy+0x64/0x90
  [<c029a5bc>] scsi_finish_command+0x1c/0xa0
  [<c02115c2>] blk_done_softirq+0x62/0x70
  [<c0122a27>] __do_softirq+0x87/0x100
  [<c0122af5>] do_softirq+0x55/0x60
  [<c0122f3c>] ksoftirqd+0x7c/0xd0
  [<c0130f76>] kthread+0xf6/0x100

which means that it always has to be taken _irq / _irqsave and one never
can enable interrupts while holding this lock. This backtrace is from
the first time the lock was taken in irq context.

Now a new situation has arisen that violates this constraint, and it
looks like this:


 [<c0219091>] cfq_set_request+0x351/0x3b0
 [<c020c7fc>] elv_set_request+0x1c/0x40
 [<c020fcff>] get_request+0x23f/0x270
 [<c0210537>] get_request_wait+0x27/0x120
 [<c02107ca>] __make_request+0x5a/0x350
 [<c020f40f>] generic_make_request+0x16f/0x220
 [<c02117e4>] submit_bio+0x64/0x110

now cfq_set_request() uses several inlines which muddies the situation,
but lockdep claims one of them is not done correctly. (eg either it
takes the lock incorrectly or something does spin_unlock_irq while the
lock is held)


I get the impression you assumed lockdep was complaining about
scsi_device_unbusy; but it's not; that function is only referenced since
it's the first place since boot where the lock was taken in softirq
context... not because the violation is occuring there.



-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org

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