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Re: [dm-devel] [PATCH 1/2] block: Avoid invoking blk_run_queue() recursively



Hello, Bart.

On Fri, Feb 22, 2013 at 11:46:14AM +0100, Bart Van Assche wrote:
> Some block drivers, e.g. dm and SCSI, need to trigger a queue
> run from inside functions that may be invoked by their request_fn()
> implementation. Make sure that invoking blk_run_queue() instead
> of blk_run_queue_async() from such functions does not trigger
> recursion. Making blk_run_queue() skip queue processing when
> invoked recursively is safe because the only two affected
> request_fn() implementations (dm and SCSI) guarantee that the
> request queue will be reexamined sooner or later before returning
> from their request_fn() implementation.
> 
> Signed-off-by: Bart Van Assche <bvanassche acm org>
> Cc: Jens Axboe <axboe kernel dk>
> Cc: Tejun Heo <tj kernel org>
> Cc: James Bottomley <JBottomley parallels com>
> Cc: Alasdair G Kergon <agk redhat com>
> Cc: Mike Snitzer <snitzer redhat com>
> Cc: <stable vger kernel org>
> ---
>  block/blk-core.c |   21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index c973249..cf26e3a 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -304,19 +304,24 @@ EXPORT_SYMBOL(blk_sync_queue);
>   *    This variant runs the queue whether or not the queue has been
>   *    stopped. Must be called with the queue lock held and interrupts
>   *    disabled. See also @blk_run_queue.
> + *
> + * Note:
> + *    Request handling functions are allowed to invoke __blk_run_queue() or
> + *    blk_run_queue() directly or indirectly. This will not result in a
> + *    recursive call of the request handler. However, such request handling
> + *    functions must, before they return, either reexamine the request queue
> + *    or invoke blk_delay_queue() to avoid that queue processing stops.
> + *
> + *    Some request handler implementations, e.g. scsi_request_fn() and
> + *    dm_request_fn(), unlock the queue lock internally. Returning immediately
> + *    if q->request_fn_active > 0 avoids that for the same queue multiple
> + *    threads execute the request handling function concurrently.
>   */
>  inline void __blk_run_queue_uncond(struct request_queue *q)
>  {
> -	if (unlikely(blk_queue_dead(q)))
> +	if (unlikely(blk_queue_dead(q) || q->request_fn_active))
>  		return;

Hmmm... I can't say I like this.  Silently ignoring explicit
blk_run_queue() call like this is likely to introduce nasty queue hang
bugs later on even if it's okay for the current users and there isn't
even debugging aid to detect what's going on.  If somebody invokes
blk_run_queue(), block layer better guarantee that the queue will be
run at least once afterwards no matter what, so please don't it this
way.

Thanks.

-- 
tejun


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