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[dm-devel] [PATCH 1/3] block: flush queued bios when process blocks to avoid deadlock



The block layer uses per-process bio list to avoid recursion in
generic_make_request.  When generic_make_request is called recursively,
the bio is added to current->bio_list and generic_make_request returns
immediately.  The top-level instance of generic_make_request takes bios
from current->bio_list and processes them.

Commit df2cb6daa4 ("block: Avoid deadlocks with bio allocation by
stacking drivers") created a workqueue for every bio set and code
in bio_alloc_bioset() that tries to resolve some low-memory deadlocks by
redirecting bios queued on current->bio_list to the workqueue if the
system is low on memory.  However another deadlock (see below **) may
happen, without any low memory condition, because generic_make_request
is queuing bios to current->bio_list (rather than submitting them).

Fix this deadlock by redirecting any bios on current->bio_list to the
bio_set's rescue workqueue on every schedule call.  Consequently, when
the process blocks on a mutex, the bios queued on current->bio_list are
dispatched to independent workqueus and they can complete without
waiting for the mutex to be available.

Also, now we can remove punt_bios_to_rescuer() and bio_alloc_bioset()'s
calls to it because bio_alloc_bioset() will implicitly punt all bios on
current->bio_list if it performs a blocking allocation.

** Here is the dm-snapshot deadlock that was observed:

1) Process A sends one-page read bio to the dm-snapshot target. The bio
spans snapshot chunk boundary and so it is split to two bios by device
mapper.

2) Device mapper creates the first sub-bio and sends it to the snapshot
driver.

3) The function snapshot_map calls track_chunk (that allocates a structure
dm_snap_tracked_chunk and adds it to tracked_chunk_hash) and then remaps
the bio to the underlying device and exits with DM_MAPIO_REMAPPED.

4) The remapped bio is submitted with generic_make_request, but it isn't
issued - it is added to current->bio_list instead.

5) Meanwhile, process B (dm's kcopyd) executes pending_complete for the
chunk affected be the first remapped bio, it takes down_write(&s->lock)
and then loops in __check_for_conflicting_io, waiting for
dm_snap_tracked_chunk created in step 3) to be released.

6) Process A continues, it creates a second sub-bio for the rest of the
original bio.

7) snapshot_map is called for this new bio, it waits on
down_write(&s->lock) that is held by Process B (in step 5).

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1267650
Signed-off-by: Mikulas Patocka <mpatocka redhat com>
Signed-off-by: Mike Snitzer <snitzer redhat com>
Depends-on: df2cb6daa4 ("block: Avoid deadlocks with bio allocation by stacking drivers")
Cc: stable vger kernel org

---
 block/bio.c            |   77 +++++++++++++++++++------------------------------
 include/linux/blkdev.h |   24 ++++++++++-----
 kernel/sched/core.c    |    7 +---
 3 files changed, 50 insertions(+), 58 deletions(-)

Index: linux-4.7-rc6/block/bio.c
===================================================================
--- linux-4.7-rc6.orig/block/bio.c	2016-07-04 23:00:17.000000000 +0200
+++ linux-4.7-rc6/block/bio.c	2016-07-05 00:02:01.000000000 +0200
@@ -349,35 +349,37 @@ static void bio_alloc_rescue(struct work
 	}
 }
 
-static void punt_bios_to_rescuer(struct bio_set *bs)
+/**
+ * blk_flush_bio_list
+ * @tsk: task_struct whose bio_list must be flushed
+ *
+ * Pop bios queued on @tsk->bio_list and submit each of them to
+ * their rescue workqueue.
+ *
+ * If the bio doesn't have a bio_set, we leave it on @tsk->bio_list.
+ * If the bio is allocated from fs_bio_set, we must leave it to avoid
+ * deadlock on loopback block device.
+ * Stacking bio drivers should use bio_set, so this shouldn't be
+ * an issue.
+ */
+void blk_flush_bio_list(struct task_struct *tsk)
 {
-	struct bio_list punt, nopunt;
 	struct bio *bio;
+	struct bio_list list = *tsk->bio_list;
+	bio_list_init(tsk->bio_list);
 
-	/*
-	 * In order to guarantee forward progress we must punt only bios that
-	 * were allocated from this bio_set; otherwise, if there was a bio on
-	 * there for a stacking driver higher up in the stack, processing it
-	 * could require allocating bios from this bio_set, and doing that from
-	 * our own rescuer would be bad.
-	 *
-	 * Since bio lists are singly linked, pop them all instead of trying to
-	 * remove from the middle of the list:
-	 */
-
-	bio_list_init(&punt);
-	bio_list_init(&nopunt);
-
-	while ((bio = bio_list_pop(current->bio_list)))
-		bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio);
-
-	*current->bio_list = nopunt;
-
-	spin_lock(&bs->rescue_lock);
-	bio_list_merge(&bs->rescue_list, &punt);
-	spin_unlock(&bs->rescue_lock);
+	while ((bio = bio_list_pop(&list))) {
+		struct bio_set *bs = bio->bi_pool;
+		if (unlikely(!bs) || bs == fs_bio_set) {
+			bio_list_add(tsk->bio_list, bio);
+			continue;
+		}
 
-	queue_work(bs->rescue_workqueue, &bs->rescue_work);
+		spin_lock(&bs->rescue_lock);
+		bio_list_add(&bs->rescue_list, bio);
+		queue_work(bs->rescue_workqueue, &bs->rescue_work);
+		spin_unlock(&bs->rescue_lock);
+	}
 }
 
 /**
@@ -417,7 +419,6 @@ static void punt_bios_to_rescuer(struct 
  */
 struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
 {
-	gfp_t saved_gfp = gfp_mask;
 	unsigned front_pad;
 	unsigned inline_vecs;
 	unsigned long idx = BIO_POOL_NONE;
@@ -452,23 +453,11 @@ struct bio *bio_alloc_bioset(gfp_t gfp_m
 		 * reserve.
 		 *
 		 * We solve this, and guarantee forward progress, with a rescuer
-		 * workqueue per bio_set. If we go to allocate and there are
-		 * bios on current->bio_list, we first try the allocation
-		 * without __GFP_DIRECT_RECLAIM; if that fails, we punt those
-		 * bios we would be blocking to the rescuer workqueue before
-		 * we retry with the original gfp_flags.
+		 * workqueue per bio_set. If an allocation would block (due to
+		 * __GFP_DIRECT_RECLAIM) the scheduler will first punt all bios
+		 * on current->bio_list to the rescuer workqueue.
 		 */
-
-		if (current->bio_list && !bio_list_empty(current->bio_list))
-			gfp_mask &= ~__GFP_DIRECT_RECLAIM;
-
 		p = mempool_alloc(bs->bio_pool, gfp_mask);
-		if (!p && gfp_mask != saved_gfp) {
-			punt_bios_to_rescuer(bs);
-			gfp_mask = saved_gfp;
-			p = mempool_alloc(bs->bio_pool, gfp_mask);
-		}
-
 		front_pad = bs->front_pad;
 		inline_vecs = BIO_INLINE_VECS;
 	}
@@ -481,12 +470,6 @@ struct bio *bio_alloc_bioset(gfp_t gfp_m
 
 	if (nr_iovecs > inline_vecs) {
 		bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx, bs->bvec_pool);
-		if (!bvl && gfp_mask != saved_gfp) {
-			punt_bios_to_rescuer(bs);
-			gfp_mask = saved_gfp;
-			bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx, bs->bvec_pool);
-		}
-
 		if (unlikely(!bvl))
 			goto err_free;
 
Index: linux-4.7-rc6/include/linux/blkdev.h
===================================================================
--- linux-4.7-rc6.orig/include/linux/blkdev.h	2016-07-04 23:00:17.000000000 +0200
+++ linux-4.7-rc6/include/linux/blkdev.h	2016-07-04 23:58:02.000000000 +0200
@@ -1114,6 +1114,22 @@ static inline bool blk_needs_flush_plug(
 		 !list_empty(&plug->cb_list));
 }
 
+extern void blk_flush_bio_list(struct task_struct *tsk);
+
+static inline void blk_flush_queued_io(struct task_struct *tsk)
+{
+	/*
+	 * Flush any queued bios to corresponding rescue threads.
+	 */
+	if (tsk->bio_list && !bio_list_empty(tsk->bio_list))
+		blk_flush_bio_list(tsk);
+	/*
+	 * Flush any plugged IO that is queued.
+	 */
+	if (blk_needs_flush_plug(tsk))
+		blk_schedule_flush_plug(tsk);
+}
+
 /*
  * tag stuff
  */
@@ -1722,16 +1738,10 @@ static inline void blk_flush_plug(struct
 {
 }
 
-static inline void blk_schedule_flush_plug(struct task_struct *task)
+static inline void blk_flush_queued_io(struct task_struct *tsk)
 {
 }
 
-
-static inline bool blk_needs_flush_plug(struct task_struct *tsk)
-{
-	return false;
-}
-
 static inline int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
 				     sector_t *error_sector)
 {
Index: linux-4.7-rc6/kernel/sched/core.c
===================================================================
--- linux-4.7-rc6.orig/kernel/sched/core.c	2016-07-04 23:00:17.000000000 +0200
+++ linux-4.7-rc6/kernel/sched/core.c	2016-07-04 23:01:29.000000000 +0200
@@ -3359,11 +3359,10 @@ static inline void sched_submit_work(str
 	if (!tsk->state || tsk_is_pi_blocked(tsk))
 		return;
 	/*
-	 * If we are going to sleep and we have plugged IO queued,
+	 * If we are going to sleep and we have queued IO,
 	 * make sure to submit it to avoid deadlocks.
 	 */
-	if (blk_needs_flush_plug(tsk))
-		blk_schedule_flush_plug(tsk);
+	blk_flush_queued_io(tsk);
 }
 
 asmlinkage __visible void __sched schedule(void)
@@ -4977,7 +4976,7 @@ long __sched io_schedule_timeout(long ti
 	long ret;
 
 	current->in_iowait = 1;
-	blk_schedule_flush_plug(current);
+	blk_flush_queued_io(current);
 
 	delayacct_blkio_start();
 	rq = raw_rq();


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