[PATCH RT 1/2 v2] dm-snapshot: fix crash with the realtime kernel

[Mikulas Patocka]

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).

We can't change hlist_bl to use non-raw spinlocks, this triggers warnings 
in dentry lookup code, because the dentry lookup code uses hlist_bl while 
holding a seqlock.

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 |   23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

Index: linux-2.6/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-snap.c	2019-11-12 16:44:36.000000000 +0100
+++ linux-2.6/drivers/md/dm-snap.c	2019-11-12 17:01:46.000000000 +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 +629,46 @@ 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;
+#else
+	spinlock_t *lock;
+#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)];
+#else
+	lock->lock = &s->exception_table_lock;
+#endif
 }
 
 static void dm_exception_table_lock(struct dm_exception_table_lock *lock)
 {
+#ifndef CONFIG_PREEMPT_RT_BASE
 	hlist_bl_lock(lock->complete_slot);
 	hlist_bl_lock(lock->pending_slot);
+#else
+	spin_lock(lock->lock);
+#endif
 }
 
 static void dm_exception_table_unlock(struct dm_exception_table_lock *lock)
 {
+#ifndef CONFIG_PREEMPT_RT_BASE
 	hlist_bl_unlock(lock->pending_slot);
 	hlist_bl_unlock(lock->complete_slot);
+#else
+	spin_unlock(lock->lock);
+#endif
 }
 
 static int dm_exception_table_init(struct dm_exception_table *et,
@@ -1318,6 +1338,9 @@ 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)) {