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@xxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: James Bottomley <JBottomley@xxxxxxxxxxxxx>
Cc: Alasdair G Kergon <agk@xxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
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;
- /*
- * Some request_fn implementations, e.g. scsi_request_fn(), unlock
- * the queue lock internally. As a result multiple threads may be
- * running such a request function concurrently. Keep track of the
- * number of active request_fn invocations such that blk_drain_queue()
- * can wait until all these request_fn calls have finished.
- */
q->request_fn_active++;
q->request_fn(q);
q->request_fn_active--;
--
1.7.10.4
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