[Devel] [PATCH rh7 2/2] ploop: fix busyloop on secondary discard bio

[Andrey Smetanin]

After diff-ploop-add-a-separate-queue-for-discard-bio-s, ploop_thread()
skips processing previously queued discard bio-s if any discard bio is
already under processing (fbd->fbd_dbl is not empty).

ploop_wait() must take care about such a case, otherwise a busyloop may
happen: ploop_thread() believes that it has to go to sleep because all
incoming queues are empty excepting plo->bio_discard_list which cannot be
processed by now and calls ploop_wait(); the latter returns immediately
because plo->bio_discard_list is not empty and hence needs for processing.

The patch also fixes a trivial bug in discard bio accounting:

ploop_bio_queue() is called for all bio-s including discard bio-s and
it decrements bio_qlen unconditionally. This is incorrect: it has to
decrement either bio_qlen or discard_bio_qlen dependently on the type of bio.

https://jira.sw.ru/browse/PSBM-30451
https://bugzilla.openvz.org/show_bug.cgi?id=3124

Signed-off-by: Maxim Patlasov <MPatlasov at parallels.com>
Acked-by: Andrew Vagin <avagin at parallels.com>
---
 drivers/block/ploop/dev.c      |  8 ++++++--
 drivers/block/ploop/freeblks.c | 12 ++++++++----
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/block/ploop/dev.c b/drivers/block/ploop/dev.c
index 5baffbb..95b335c 100644
--- a/drivers/block/ploop/dev.c
+++ b/drivers/block/ploop/dev.c
@@ -551,7 +551,10 @@ ploop_bio_queue(struct ploop_device * plo, struct bio * bio,
 
 	__TRACE("A %p %u\n", preq, preq->req_cluster);
 
-	plo->bio_qlen--;
+	if (unlikely(bio->bi_rw & REQ_DISCARD))
+		plo->bio_discard_qlen--;
+	else
+		plo->bio_qlen--;
 	ploop_entry_add(plo, preq);
 
 	if (bio->bi_size && !(bio->bi_rw & REQ_DISCARD))
@@ -2575,7 +2578,8 @@ static void ploop_wait(struct ploop_device * plo, int once, struct blk_plug *plu
 			     !plo->active_reqs))
 				break;
 		} else if (plo->bio_head ||
-			   !bio_list_empty(&plo->bio_discard_list)) {
+			   (!bio_list_empty(&plo->bio_discard_list) &&
+			    !ploop_discard_is_inprogress(plo->fbd))) {
 			/* ready_queue and entry_queue are empty, but
 			 * bio list not. Obviously, we'd like to process
 			 * bio_list instead of sleeping */
diff --git a/drivers/block/ploop/freeblks.c b/drivers/block/ploop/freeblks.c
index cf48d3a..89108c7 100644
--- a/drivers/block/ploop/freeblks.c
+++ b/drivers/block/ploop/freeblks.c
@@ -696,20 +696,24 @@ int ploop_fb_get_free_block(struct ploop_freeblks_desc *fbd,
 
 static void fbd_complete_bio(struct ploop_freeblks_desc *fbd, int err)
 {
+	struct ploop_device *plo = fbd->plo;
 	unsigned int nr_completed = 0;
 
 	while (fbd->fbd_dbl.head) {
 		struct bio * bio = fbd->fbd_dbl.head;
 		fbd->fbd_dbl.head = bio->bi_next;
 		bio->bi_next = NULL;
-		BIO_ENDIO(fbd->plo->queue, bio, err);
+		BIO_ENDIO(plo->queue, bio, err);
 		nr_completed++;
 	}
 	fbd->fbd_dbl.tail = NULL;
 
-	spin_lock_irq(&fbd->plo->lock);
-	fbd->plo->bio_total -= nr_completed;
-	spin_unlock_irq(&fbd->plo->lock);
+	spin_lock_irq(&plo->lock);
+	plo->bio_total -= nr_completed;
+	if (!bio_list_empty(&plo->bio_discard_list) &&
+	    waitqueue_active(&plo->waitq))
+		wake_up_interruptible(&plo->waitq);
+	spin_unlock_irq(&plo->lock);
 }
 
 void ploop_fb_reinit(struct ploop_freeblks_desc *fbd, int err)
-- 
1.8.3.1