Re: spinlock recursion in scsi_end_request() (kernel 2.6.24)

[Tejun Heo <tj@kernel.org>]

Hello,

On 05/20/2010 01:33 PM, Prashant wrote:
> I have a question related to code which is almost same in the
> current kernel.  I don't know whether this is the right mailing list
> for the following question.

linux-scsi would probably fit better (cc'd).

> When a sata drive is unplugged, its corresponding sdev's state is set
> to SDEV_OFFLINE. Now if IO requests are still comming on the same device,
> They will be killed by calling scsi_kill_request().
> 
> 1) scsi_kill_request does following things:
>     i) Unlock request queue
>     ii) Increment host_busy count
>     iii) Lock request queue
>     iv) Calls __scsi_done()
> 
> 2) __scsi_done() does following things:
>      i) set request completion data
>      ii) Calls blk_completion_request()
> 
> 3) blk_completion_request() does following things:
>      i) Adds request->donelist to blk_cpu_done softirq queue
>         and raise the softirq (which is scsi_softirq_done)
> 
> 4) next sequence is:
>     scsi_softirq_done >> scsi_finish_command >> scsi_device_unbusy()
> 
> 5) scsi_device_unbusy() again locks the request_queue. This is the place where
>     we can get into the spinlock recursion.
> 
>     Is this correct? Please correct me if something is wrong.

Raising softirq defers the work to another context and grabbing the
same lock from softirq handler doesn't constitute a recursive locking.
Please try to reproduce the problem on recent kernel w/ lockdep
enabled.

Thanks.

-- 
tejun