Re: [PATCH] block: Make blk_drain_queue() work for stopped queues

[Bart Van Assche <bvanassche@acm.org>]

On 03/18/12 15:57, Tejun Heo wrote:
> On Sun, Mar 18, 2012 at 01:18:21PM +0000, Bart Van Assche wrote:
>> All queued requests must be processed eventually. Hence make sure
>> that blk_drain_queue() drains the queue even if the queue is in the
>> stopped state. This patch makes it safe to invoke blk_cleanup_queue()
>> on a stopped queue.
> ...
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 3a78b00..bdcec86 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -300,10 +300,8 @@ EXPORT_SYMBOL(blk_sync_queue);
>>   */
>>  void __blk_run_queue(struct request_queue *q)
>>  {
>> -	if (unlikely(blk_queue_stopped(q)))
>> -		return;
>> -
>> -	q->request_fn(q);
>> +	if (!blk_queue_stopped(q) || blk_queue_dead(q))
>> +		q->request_fn(q);
> So, this allows calling request_fn for dead && stopped queue.  Have
> you seen something which requires this?

Not servicing queued SCSI requests can e.g. cause user space processes
to hang. See also http://lkml.org/lkml/2011/8/27/6 for an example. Hence
commit 3308511c93e6ad0d3c58984ecd6e5e57f96b12c8 which causes pending
SCSI commands to be killed just before blk_cleanup_queue() is invoked.
However, there is still a tiny race window left by that patch - new
requests can get queued after the SCSI request function has been invoked
by scsi_free_queue() and before blk_cleanup_queue() gets invoked. Hence
the proposal to change the block layer to make sure that all queued
requests get processed eventually.

Bart.