[PATCH 1/2] dm-snapshot: fix crash with the realtime kernel

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Snapshot doesn't work with realtime kernels since the commit f79ae415b64c.
hlist_bl is implemented as a raw spinlock and the code takes two non-raw
spinlocks while holding hlist_bl (non-raw spinlocks are blocking mutexes
in the realtime kernel, so they couldn't be taken inside a raw spinlock).

This patch fixes the problem by using non-raw spinlock
exception_table_lock instead of the hlist_bl lock.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Fixes: f79ae415b64c ("dm snapshot: Make exception tables scalable")

---
 drivers/md/dm-snap.c |   65 ++++++++++++++++++++++++++++++++-------------------
 1 file changed, 42 insertions(+), 23 deletions(-)

Index: linux-2.6/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-snap.c	2019-11-08 15:51:42.000000000 +0100
+++ linux-2.6/drivers/md/dm-snap.c	2019-11-08 15:54:58.000000000 +0100
@@ -141,6 +141,10 @@ struct dm_snapshot {
 	 * for them to be committed.
 	 */
 	struct bio_list bios_queued_during_merge;
+
+#ifdef CONFIG_PREEMPT_RT_BASE
+	spinlock_t exception_table_lock;
+#endif
 };
 
 /*
@@ -625,30 +629,42 @@ static uint32_t exception_hash(struct dm
 
 /* Lock to protect access to the completed and pending exception hash tables. */
 struct dm_exception_table_lock {
+#ifndef CONFIG_PREEMPT_RT_BASE
 	struct hlist_bl_head *complete_slot;
 	struct hlist_bl_head *pending_slot;
+#endif
 };
 
 static void dm_exception_table_lock_init(struct dm_snapshot *s, chunk_t chunk,
 					 struct dm_exception_table_lock *lock)
 {
+#ifndef CONFIG_PREEMPT_RT_BASE
 	struct dm_exception_table *complete = &s->complete;
 	struct dm_exception_table *pending = &s->pending;
 
 	lock->complete_slot = &complete->table[exception_hash(complete, chunk)];
 	lock->pending_slot = &pending->table[exception_hash(pending, chunk)];
+#endif
 }
 
-static void dm_exception_table_lock(struct dm_exception_table_lock *lock)
+static void dm_exception_table_lock(struct dm_snapshot *s, struct dm_exception_table_lock *lock)
 {
+#ifdef CONFIG_PREEMPT_RT_BASE
+	spin_lock(&s->exception_table_lock);
+#else
 	hlist_bl_lock(lock->complete_slot);
 	hlist_bl_lock(lock->pending_slot);
+#endif
 }
 
-static void dm_exception_table_unlock(struct dm_exception_table_lock *lock)
+static void dm_exception_table_unlock(struct dm_snapshot *s, struct dm_exception_table_lock *lock)
 {
+#ifdef CONFIG_PREEMPT_RT_BASE
+	spin_unlock(&s->exception_table_lock);
+#else
 	hlist_bl_unlock(lock->pending_slot);
 	hlist_bl_unlock(lock->complete_slot);
+#endif
 }
 
 static int dm_exception_table_init(struct dm_exception_table *et,
@@ -835,9 +851,9 @@ static int dm_add_exception(void *contex
 	 */
 	dm_exception_table_lock_init(s, old, &lock);
 
-	dm_exception_table_lock(&lock);
+	dm_exception_table_lock(s, &lock);
 	dm_insert_exception(&s->complete, e);
-	dm_exception_table_unlock(&lock);
+	dm_exception_table_unlock(s, &lock);
 
 	return 0;
 }
@@ -1318,6 +1334,9 @@ static int snapshot_ctr(struct dm_target
 	s->first_merging_chunk = 0;
 	s->num_merging_chunks = 0;
 	bio_list_init(&s->bios_queued_during_merge);
+#ifdef CONFIG_PREEMPT_RT_BASE
+	spin_lock_init(&s->exception_table_lock);
+#endif
 
 	/* Allocate hash table for COW data */
 	if (init_hash_tables(s)) {
@@ -1651,7 +1670,7 @@ static void pending_complete(void *conte
 		invalidate_snapshot(s, -EIO);
 		error = 1;
 
-		dm_exception_table_lock(&lock);
+		dm_exception_table_lock(s, &lock);
 		goto out;
 	}
 
@@ -1660,13 +1679,13 @@ static void pending_complete(void *conte
 		invalidate_snapshot(s, -ENOMEM);
 		error = 1;
 
-		dm_exception_table_lock(&lock);
+		dm_exception_table_lock(s, &lock);
 		goto out;
 	}
 	*e = pe->e;
 
 	down_read(&s->lock);
-	dm_exception_table_lock(&lock);
+	dm_exception_table_lock(s, &lock);
 	if (!s->valid) {
 		up_read(&s->lock);
 		free_completed_exception(e);
@@ -1687,16 +1706,16 @@ static void pending_complete(void *conte
 
 	/* Wait for conflicting reads to drain */
 	if (__chunk_is_tracked(s, pe->e.old_chunk)) {
-		dm_exception_table_unlock(&lock);
+		dm_exception_table_unlock(s, &lock);
 		__check_for_conflicting_io(s, pe->e.old_chunk);
-		dm_exception_table_lock(&lock);
+		dm_exception_table_lock(s, &lock);
 	}
 
 out:
 	/* Remove the in-flight exception from the list */
 	dm_remove_exception(&pe->e);
 
-	dm_exception_table_unlock(&lock);
+	dm_exception_table_unlock(s, &lock);
 
 	snapshot_bios = bio_list_get(&pe->snapshot_bios);
 	origin_bios = bio_list_get(&pe->origin_bios);
@@ -1968,7 +1987,7 @@ static int snapshot_map(struct dm_target
 	}
 
 	down_read(&s->lock);
-	dm_exception_table_lock(&lock);
+	dm_exception_table_lock(s, &lock);
 
 	if (!s->valid || (unlikely(s->snapshot_overflowed) &&
 	    bio_data_dir(bio) == WRITE)) {
@@ -1997,7 +2016,7 @@ static int snapshot_map(struct dm_target
 		remap_exception(s, e, bio, chunk);
 		if (unlikely(bio_op(bio) == REQ_OP_DISCARD) &&
 		    io_overlaps_chunk(s, bio)) {
-			dm_exception_table_unlock(&lock);
+			dm_exception_table_unlock(s, &lock);
 			up_read(&s->lock);
 			zero_exception(s, e, bio, chunk);
 			r = DM_MAPIO_SUBMITTED; /* discard is not issued */
@@ -2024,9 +2043,9 @@ static int snapshot_map(struct dm_target
 	if (bio_data_dir(bio) == WRITE) {
 		pe = __lookup_pending_exception(s, chunk);
 		if (!pe) {
-			dm_exception_table_unlock(&lock);
+			dm_exception_table_unlock(s, &lock);
 			pe = alloc_pending_exception(s);
-			dm_exception_table_lock(&lock);
+			dm_exception_table_lock(s, &lock);
 
 			e = dm_lookup_exception(&s->complete, chunk);
 			if (e) {
@@ -2037,7 +2056,7 @@ static int snapshot_map(struct dm_target
 
 			pe = __find_pending_exception(s, pe, chunk);
 			if (!pe) {
-				dm_exception_table_unlock(&lock);
+				dm_exception_table_unlock(s, &lock);
 				up_read(&s->lock);
 
 				down_write(&s->lock);
@@ -2063,7 +2082,7 @@ static int snapshot_map(struct dm_target
 		if (!pe->started && io_overlaps_chunk(s, bio)) {
 			pe->started = 1;
 
-			dm_exception_table_unlock(&lock);
+			dm_exception_table_unlock(s, &lock);
 			up_read(&s->lock);
 
 			start_full_bio(pe, bio);
@@ -2076,7 +2095,7 @@ static int snapshot_map(struct dm_target
 			/* this is protected by the exception table lock */
 			pe->started = 1;
 
-			dm_exception_table_unlock(&lock);
+			dm_exception_table_unlock(s, &lock);
 			up_read(&s->lock);
 
 			start_copy(pe);
@@ -2088,7 +2107,7 @@ static int snapshot_map(struct dm_target
 	}
 
 out_unlock:
-	dm_exception_table_unlock(&lock);
+	dm_exception_table_unlock(s, &lock);
 	up_read(&s->lock);
 out:
 	return r;
@@ -2449,7 +2468,7 @@ static int __origin_write(struct list_he
 		dm_exception_table_lock_init(snap, chunk, &lock);
 
 		down_read(&snap->lock);
-		dm_exception_table_lock(&lock);
+		dm_exception_table_lock(snap, &lock);
 
 		/* Only deal with valid and active snapshots */
 		if (!snap->valid || !snap->active)
@@ -2466,9 +2485,9 @@ static int __origin_write(struct list_he
 			if (e)
 				goto next_snapshot;
 
-			dm_exception_table_unlock(&lock);
+			dm_exception_table_unlock(snap, &lock);
 			pe = alloc_pending_exception(snap);
-			dm_exception_table_lock(&lock);
+			dm_exception_table_lock(snap, &lock);
 
 			pe2 = __lookup_pending_exception(snap, chunk);
 
@@ -2481,7 +2500,7 @@ static int __origin_write(struct list_he
 
 				pe = __insert_pending_exception(snap, pe, chunk);
 				if (!pe) {
-					dm_exception_table_unlock(&lock);
+					dm_exception_table_unlock(snap, &lock);
 					up_read(&snap->lock);
 
 					invalidate_snapshot(snap, -ENOMEM);
@@ -2516,7 +2535,7 @@ static int __origin_write(struct list_he
 		}
 
 next_snapshot:
-		dm_exception_table_unlock(&lock);
+		dm_exception_table_unlock(snap, &lock);
 		up_read(&snap->lock);
 
 		if (pe_to_start_now) {

--
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