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Re: [dm-devel] block_abort_queue (blk_abort_request) racing with scsi_request_fn



On 05/12/2010 12:23 AM, Mike Anderson wrote:
I was looking at a dump from a weekend run and I believe I am seeing a
case where blk_abort_request through blk_abort_queue picked up a request
for timeout that scsi_request_fn decided not to start. This test was under
error injection.

I assume the case in scsi_request_fn this is hitting is that a request has
been put on the timeout_list with blk_start_request and then one of the
not_ready checks is hit and the request is decided not to be started. I
believe the drop

It appears that my usage of walking the timeout_list in block_abort_queue
and using blk_mark_rq_complete in block_abort_request will not work in
this case.

While it would be good to have way to ensure a command is started, it is
unclear if even at a low timeout of 1 second that a user other than
blk_abort_queue would hit this race.

The dropping / acquiring of host_lock and queue_lock in scsi_request_fn
and scsi_dispatch_cmd make it unclear to me if usage of
blk_mark_rq_complete will cover all cases.

I looked at checking serial_number in scsi_times_out along with a couple
blk_mark_rq_complete additions, but unclear if this would be good and / or
work in all cases.

I looked at just accelerating deadline by some default value but unclear
if that would be acceptable.

I also looked at just using just the mark interface I previously posted
and not calling blk_abort_request at all, but that would change current
behavior that has been in use for a while.


Did you ever solve this? I am hitting this with the dm-multipath blk_abort_queue case (the email I sent you a couple weeks ago).

It seems we could fix this by just having blk_requeue_request do a check for if the request timedout similar to what scsi used to do. A hacky way might be to have 2 requeue functions.

blk_requeue_completed_request - This is the blk_requeue_request we have today. It unconditionally requeues the request. It should only be used if the command has been completed either from blk_complete_request or from block layer timeout handling (blk_rq_timed_out_timer->blk_rq_timed_out->rq_timed_out_fn).

blk_requeue_running_request - This checks if the timer is running before requeueing the request. If blk_rq_timed_out_timer/blk_rq_timed_out has taken over the request and is going to handle it then this function just returns and does not requeue. So for this we could just have it check if the queue has a rq_timed_out_fn and if rq->timeout_list is empty or not.

I think this might be confusing to use, so I tried something slightly different below.


I also tried a patch where we just add another req bit. We set it in blk_rq_timed_out_timer and clear it in a new function that clears it then calls blk_requeue_reqeust. The new function:

blk_requeue_timedout_request - used when request is to be requeued if a LLD q->rq_timed_out_fn returned BLK_EH_NOT_HANDLED and has resolved the problem and wants the request to be requeued. This function clears REQ_ATOM_TIMEDOUT and then calls blk_requeue_request.

blk_reqeuue_request would then check if REQ_ATOM_TIMEDOUT is set and if it was just drop the request assuming the rq_timed_out_fn was handling it. This still requires the caller to know how the command is supposed to be reqeueud. But I think it might be easier since the driver here has returned BLK_EH_NOT_HANDLED in the q timeout fn so they know that they are going to be handling the request in a special way.

I attached the last idea here. Made over Linus's tree. Only compile tested.
diff --git a/block/blk-core.c b/block/blk-core.c
index f0834e2..92279d4 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -981,6 +981,9 @@ EXPORT_SYMBOL(blk_make_request);
  */
 void blk_requeue_request(struct request_queue *q, struct request *rq)
 {
+	if (test_bit(REQ_ATOM_TIMEDOUT, &rq->atomic_flags))
+		return;
+
 	blk_delete_timer(rq);
 	blk_clear_rq_complete(rq);
 	trace_block_rq_requeue(q, rq);
diff --git a/block/blk-softirq.c b/block/blk-softirq.c
index ee9c216..e0a8d11 100644
--- a/block/blk-softirq.c
+++ b/block/blk-softirq.c
@@ -156,8 +156,10 @@ void blk_complete_request(struct request *req)
 {
 	if (unlikely(blk_should_fake_timeout(req->q)))
 		return;
-	if (!blk_mark_rq_complete(req))
+	if (!blk_mark_rq_complete(req)) {
+		blk_clear_rq_timedout(req);
 		__blk_complete_request(req);
+	}
 }
 EXPORT_SYMBOL(blk_complete_request);
 
diff --git a/block/blk-timeout.c b/block/blk-timeout.c
index 4f0c06c..afc6e5f 100644
--- a/block/blk-timeout.c
+++ b/block/blk-timeout.c
@@ -97,6 +97,7 @@ static void blk_rq_timed_out(struct request *req)
 		 * and we can move more of the generic scsi eh code to
 		 * the blk layer.
 		 */
+		blk_mark_rq_timedout(req);
 		break;
 	default:
 		printk(KERN_ERR "block: bad eh return: %d\n", ret);
@@ -104,6 +105,25 @@ static void blk_rq_timed_out(struct request *req)
 	}
 }
 
+/**
+ * blk_requeue_timedout_request - put a request that timedout back on queue
+ * @q:		request queue where request should be inserted
+ * @rq:		request to be inserted
+ *
+ * Description:
+ *	If a module has returned BLK_EH_NOT_HANDLED from its
+ *	rq_timed_out_fn and needs to requeue the request this
+ *	function should be used instead of blk_requeue_request.
+ *
+ *	queue_lock must be held.
+ */
+void blk_requeue_timedout_request(struct request_queue *q, struct request *req)
+{
+	blk_clear_rq_timedout(req);
+	blk_requeue_request(q, req);
+}
+EXPORT_SYMBOL_GPL(blk_requeue_timedout_request);
+
 void blk_rq_timed_out_timer(unsigned long data)
 {
 	struct request_queue *q = (struct request_queue *) data;
diff --git a/block/blk.h b/block/blk.h
index 2db8f32..ad93258 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -30,6 +30,7 @@ void __generic_unplug_device(struct request_queue *);
  */
 enum rq_atomic_flags {
 	REQ_ATOM_COMPLETE = 0,
+	REQ_ATOM_TIMEDOUT = 1,
 };
 
 /*
@@ -46,7 +47,15 @@ static inline void blk_clear_rq_complete(struct request *rq)
 	clear_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
 }
 
-/*
+static inline int blk_mark_rq_timedout(struct request *rq)
+{
+	return test_and_set_bit(REQ_ATOM_TIMEDOUT, &rq->atomic_flags);
+}
+
+static inline void blk_clear_rq_timedout(struct request *rq)
+{
+	clear_bit(REQ_ATOM_TIMEDOUT, &rq->atomic_flags);
+}/*
  * Internal elevator interface
  */
 #define ELV_ON_HASH(rq)		(!hlist_unhashed(&(rq)->hash))
diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index 1de30eb..06df25a 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -1680,7 +1680,11 @@ void scsi_eh_flush_done_q(struct list_head *done_q)
 							  " retry cmd: %p\n",
 							  current->comm,
 							  scmd));
-				scsi_queue_insert(scmd, SCSI_MLQUEUE_EH_RETRY);
+				printk(KERN_ERR "scmd %p %p %p\n", scmd,
+					scmd->eh_entry.next, scmd->eh_entry.prev);
+
+				scsi_queue_insert(scmd,
+						SCSI_MLQUEUE_EH_TIMEDOUT_RETRY);
 		} else {
 			/*
 			 * If just we got sense for the device (called
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index eafeeda..cef49b2 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -158,7 +158,10 @@ static int __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, int unbusy)
 	 * and plugs the queue appropriately.
          */
 	spin_lock_irqsave(q->queue_lock, flags);
-	blk_requeue_request(q, cmd->request);
+	if (reason == SCSI_MLQUEUE_EH_TIMEDOUT_RETRY)
+		blk_requeue_timedout_request(q, cmd->request);
+	else
+		blk_requeue_request(q, cmd->request);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 
 	scsi_run_queue(q);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 5027a59..e56f28e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -205,7 +205,10 @@ typedef int (dma_drain_needed_fn)(struct request *);
 typedef int (lld_busy_fn) (struct request_queue *q);
 
 enum blk_eh_timer_return {
-	BLK_EH_NOT_HANDLED,
+	BLK_EH_NOT_HANDLED,	/* If this is returned the module must
+				 * call blk_requeue_timedout_request to
+				 * requeue it
+				 */
 	BLK_EH_HANDLED,
 	BLK_EH_RESET_TIMER,
 };
@@ -653,6 +656,8 @@ extern struct request *blk_get_request(struct request_queue *, int, gfp_t);
 extern struct request *blk_make_request(struct request_queue *, struct bio *,
 					gfp_t);
 extern void blk_insert_request(struct request_queue *, struct request *, int, void *);
+extern void blk_requeue_timedout_request(struct request_queue *,
+					 struct request *);
 extern void blk_requeue_request(struct request_queue *, struct request *);
 extern void blk_add_request_payload(struct request *rq, struct page *page,
 		unsigned int len);
diff --git a/include/scsi/scsi.h b/include/scsi/scsi.h
index 216af85..5bde952 100644
--- a/include/scsi/scsi.h
+++ b/include/scsi/scsi.h
@@ -442,6 +442,7 @@ static inline int scsi_is_wlun(unsigned int lun)
 #define SCSI_MLQUEUE_DEVICE_BUSY 0x1056
 #define SCSI_MLQUEUE_EH_RETRY    0x1057
 #define SCSI_MLQUEUE_TARGET_BUSY 0x1058
+#define SCSI_MLQUEUE_EH_TIMEDOUT_RETRY	0x1059
 
 /*
  *  Use these to separate status msg and our bytes

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