sg: fix unkillable I/O wait deadlock with scsi-mq

When using the write()/read() interface for submitting commands, the SCSI generic driver does not call blk_put_request() on a completed SCSI command until userspace calls read() to get the command completion. Since scsi-mq uses a fixed number of preallocated requests, this makes it possible for userspace to exhaust the entire preallocated supply of requests, leading to deadlock with the user process stuck in a permanent unkillable I/O wait in sg_write() -> ... -> blk_get_request() -> ... -> bt_get(). Note that this deadlock can happen only if scsi-mq is enabled. Prevent the deadlock by calling blk_put_request() as soon as the SCSI command completes instead of waiting for userspace to call read().

---

For inclusion in kernel 3.20.

I encountered this problem using mptsas (can_queue == 127) and 8 disks connected via an expander. I have a test program called cydiskbench that spawns multiple threads, opens multiple /dev/sg* file descriptors, and sends multiple disk read/write commands to each /dev/sg* file descriptor. I can vary the # of disks being tested and the command queue depth per disk. Whenever I chose test parameters such that (n_disks * queue_depth_per_disk) > shost->can_queue, the test deadlocked as described when scsi-mq was enabled but worked just fine with scsi-mq disabled.

I will send a separate patch to fix the same problem in the bsg driver.

@@ -1350,6 +1350,17 @@ sg_rq_end_io(struct request *rq, int upt
 	}
 	/* Rely on write phase to clean out srp status values, so no "else" */
 
+	/*
+	 * Free the request as soon as it is complete so that its resources
+	 * can be reused without waiting for userspace to read() the
+	 * result.  But keep the associated bio (if any) around until
+	 * blk_rq_unmap_user() can be called from user context.
+	 */
+	srp->rq = NULL;
+	if (rq->cmd != rq->__cmd)
+		kfree(rq->cmd);
+	__blk_put_request(rq->q, rq);
+
 	write_lock_irqsave(&sfp->rq_list_lock, iflags);
 	if (unlikely(srp->orphan)) {
 		if (sfp->keep_orphan)

@@ -1777,10 +1788,10 @@ sg_finish_rem_req(Sg_request *srp)
 	SCSI_LOG_TIMEOUT(4, sg_printk(KERN_INFO, sfp->parentdp,
 				      "sg_finish_rem_req: res_used=%d\n",
 				      (int) srp->res_used));
+	if (srp->bio)
+		ret = blk_rq_unmap_user(srp->bio);
+
 	if (srp->rq) {
-		if (srp->bio)
-			ret = blk_rq_unmap_user(srp->bio);
-
 		if (srp->rq->cmd != srp->rq->__cmd)
 			kfree(srp->rq->cmd);
 		blk_put_request(srp->rq);