dm snapshot: Replace mutex with rw semaphore

dm-snapshot uses a single mutex to serialize every access to the
snapshot state. This includes all accesses to the complete and pending
exception tables, which occur at every origin write, every snapshot
read/write and every exception completion.

The lock statistics indicate that this mutex is a bottleneck (average
wait time ~480 usecs for 8 processes doing random 4K writes to the
origin device) preventing dm-snapshot to scale as the number of threads
doing IO increases.

The major contention points are __origin_write()/snapshot_map() and
pending_complete(), i.e., the submission and completion of pending
exceptions.

Replace this mutex with a rw semaphore.

We essentially revert commit ae1093be5a0ef9 ("dm snapshot: use mutex
instead of rw_semaphore") and together with the next two patches we
substitute the single mutex with a fine-grained locking scheme, where we
use a read-write semaphore to protect the mostly read fields of the
snapshot structure, e.g., valid, active, etc., and per-bucket bit
spinlocks to protect accesses to the complete and pending exception
tables.

Co-developed-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 051e4d076323d2aaef1600ff6a8e76eb5baaec91..5a67f408876ece6d12129e482bf89938e3bbad2f 100644 (file)
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -48,7 +48,7 @@ struct dm_exception_table {
 };
 
 struct dm_snapshot {
-       struct mutex lock;
+       struct rw_semaphore lock;
 
        struct dm_dev *origin;
        struct dm_dev *cow;
@@ -457,9 +457,9 @@ static int __find_snapshots_sharing_cow(struct dm_snapshot *snap,
                if (!bdev_equal(s->cow->bdev, snap->cow->bdev))
                        continue;
 
-               mutex_lock(&s->lock);
+               down_read(&s->lock);
                active = s->active;
-               mutex_unlock(&s->lock);
+               up_read(&s->lock);
 
                if (active) {
                        if (snap_src)
@@ -927,7 +927,7 @@ static int remove_single_exception_chunk(struct dm_snapshot *s)
        int r;
        chunk_t old_chunk = s->first_merging_chunk + s->num_merging_chunks - 1;
 
-       mutex_lock(&s->lock);
+       down_write(&s->lock);
 
        /*
         * Process chunks (and associated exceptions) in reverse order
@@ -942,7 +942,7 @@ static int remove_single_exception_chunk(struct dm_snapshot *s)
        b = __release_queued_bios_after_merge(s);
 
 out:
-       mutex_unlock(&s->lock);
+       up_write(&s->lock);
        if (b)
                flush_bios(b);
 
@@ -1001,9 +1001,9 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
                if (linear_chunks < 0) {
                        DMERR("Read error in exception store: "
                              "shutting down merge");
-                       mutex_lock(&s->lock);
+                       down_write(&s->lock);
                        s->merge_failed = 1;
-                       mutex_unlock(&s->lock);
+                       up_write(&s->lock);
                }
                goto shut;
        }
@@ -1044,10 +1044,10 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
                previous_count = read_pending_exceptions_done_count();
        }
 
-       mutex_lock(&s->lock);
+       down_write(&s->lock);
        s->first_merging_chunk = old_chunk;
        s->num_merging_chunks = linear_chunks;
-       mutex_unlock(&s->lock);
+       up_write(&s->lock);
 
        /* Wait until writes to all 'linear_chunks' drain */
        for (i = 0; i < linear_chunks; i++)
@@ -1089,10 +1089,10 @@ static void merge_callback(int read_err, unsigned long write_err, void *context)
        return;
 
 shut:
-       mutex_lock(&s->lock);
+       down_write(&s->lock);
        s->merge_failed = 1;
        b = __release_queued_bios_after_merge(s);
-       mutex_unlock(&s->lock);
+       up_write(&s->lock);
        error_bios(b);
 
        merge_shutdown(s);
@@ -1191,7 +1191,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
        s->exception_start_sequence = 0;
        s->exception_complete_sequence = 0;
        s->out_of_order_tree = RB_ROOT;
-       mutex_init(&s->lock);
+       init_rwsem(&s->lock);
        INIT_LIST_HEAD(&s->list);
        spin_lock_init(&s->pe_lock);
        s->state_bits = 0;
@@ -1357,9 +1357,9 @@ static void snapshot_dtr(struct dm_target *ti)
        /* Check whether exception handover must be cancelled */
        (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
        if (snap_src && snap_dest && (s == snap_src)) {
-               mutex_lock(&snap_dest->lock);
+               down_write(&snap_dest->lock);
                snap_dest->valid = 0;
-               mutex_unlock(&snap_dest->lock);
+               up_write(&snap_dest->lock);
                DMERR("Cancelling snapshot handover.");
        }
        up_read(&_origins_lock);
@@ -1390,8 +1390,6 @@ static void snapshot_dtr(struct dm_target *ti)
 
        dm_exception_store_destroy(s->store);
 
-       mutex_destroy(&s->lock);
-
        dm_put_device(ti, s->cow);
 
        dm_put_device(ti, s->origin);
@@ -1479,7 +1477,7 @@ static void pending_complete(void *context, int success)
 
        if (!success) {
                /* Read/write error - snapshot is unusable */
-               mutex_lock(&s->lock);
+               down_write(&s->lock);
                __invalidate_snapshot(s, -EIO);
                error = 1;
                goto out;
@@ -1487,14 +1485,14 @@ static void pending_complete(void *context, int success)
 
        e = alloc_completed_exception(GFP_NOIO);
        if (!e) {
-               mutex_lock(&s->lock);
+               down_write(&s->lock);
                __invalidate_snapshot(s, -ENOMEM);
                error = 1;
                goto out;
        }
        *e = pe->e;
 
-       mutex_lock(&s->lock);
+       down_write(&s->lock);
        if (!s->valid) {
                free_completed_exception(e);
                error = 1;
@@ -1512,9 +1510,9 @@ static void pending_complete(void *context, int success)
 
        /* Wait for conflicting reads to drain */
        if (__chunk_is_tracked(s, pe->e.old_chunk)) {
-               mutex_unlock(&s->lock);
+               up_write(&s->lock);
                __check_for_conflicting_io(s, pe->e.old_chunk);
-               mutex_lock(&s->lock);
+               down_write(&s->lock);
        }
 
 out:
@@ -1527,7 +1525,7 @@ out:
                full_bio->bi_end_io = pe->full_bio_end_io;
        increment_pending_exceptions_done_count();
 
-       mutex_unlock(&s->lock);
+       up_write(&s->lock);
 
        /* Submit any pending write bios */
        if (error) {
@@ -1750,7 +1748,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
        if (!s->valid)
                return DM_MAPIO_KILL;
 
-       mutex_lock(&s->lock);
+       down_write(&s->lock);
 
        if (!s->valid || (unlikely(s->snapshot_overflowed) &&
            bio_data_dir(bio) == WRITE)) {
@@ -1773,9 +1771,9 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
        if (bio_data_dir(bio) == WRITE) {
                pe = __lookup_pending_exception(s, chunk);
                if (!pe) {
-                       mutex_unlock(&s->lock);
+                       up_write(&s->lock);
                        pe = alloc_pending_exception(s);
-                       mutex_lock(&s->lock);
+                       down_write(&s->lock);
 
                        if (!s->valid || s->snapshot_overflowed) {
                                free_pending_exception(pe);
@@ -1810,7 +1808,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
                    bio->bi_iter.bi_size ==
                    (s->store->chunk_size << SECTOR_SHIFT)) {
                        pe->started = 1;
-                       mutex_unlock(&s->lock);
+                       up_write(&s->lock);
                        start_full_bio(pe, bio);
                        goto out;
                }
@@ -1820,7 +1818,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
                if (!pe->started) {
                        /* this is protected by snap->lock */
                        pe->started = 1;
-                       mutex_unlock(&s->lock);
+                       up_write(&s->lock);
                        start_copy(pe);
                        goto out;
                }
@@ -1830,7 +1828,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
        }
 
 out_unlock:
-       mutex_unlock(&s->lock);
+       up_write(&s->lock);
 out:
        return r;
 }
@@ -1866,7 +1864,7 @@ static int snapshot_merge_map(struct dm_target *ti, struct bio *bio)
 
        chunk = sector_to_chunk(s->store, bio->bi_iter.bi_sector);
 
-       mutex_lock(&s->lock);
+       down_write(&s->lock);
 
        /* Full merging snapshots are redirected to the origin */
        if (!s->valid)
@@ -1897,12 +1895,12 @@ redirect_to_origin:
        bio_set_dev(bio, s->origin->bdev);
 
        if (bio_data_dir(bio) == WRITE) {
-               mutex_unlock(&s->lock);
+               up_write(&s->lock);
                return do_origin(s->origin, bio);
        }
 
 out_unlock:
-       mutex_unlock(&s->lock);
+       up_write(&s->lock);
 
        return r;
 }
@@ -1934,7 +1932,7 @@ static int snapshot_preresume(struct dm_target *ti)
        down_read(&_origins_lock);
        (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
        if (snap_src && snap_dest) {
-               mutex_lock(&snap_src->lock);
+               down_read(&snap_src->lock);
                if (s == snap_src) {
                        DMERR("Unable to resume snapshot source until "
                              "handover completes.");
@@ -1944,7 +1942,7 @@ static int snapshot_preresume(struct dm_target *ti)
                              "source is suspended.");
                        r = -EINVAL;
                }
-               mutex_unlock(&snap_src->lock);
+               up_read(&snap_src->lock);
        }
        up_read(&_origins_lock);
 
@@ -1990,11 +1988,11 @@ static void snapshot_resume(struct dm_target *ti)
 
        (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
        if (snap_src && snap_dest) {
-               mutex_lock(&snap_src->lock);
-               mutex_lock_nested(&snap_dest->lock, SINGLE_DEPTH_NESTING);
+               down_write(&snap_src->lock);
+               down_write_nested(&snap_dest->lock, SINGLE_DEPTH_NESTING);
                __handover_exceptions(snap_src, snap_dest);
-               mutex_unlock(&snap_dest->lock);
-               mutex_unlock(&snap_src->lock);
+               up_write(&snap_dest->lock);
+               up_write(&snap_src->lock);
        }
 
        up_read(&_origins_lock);
@@ -2009,9 +2007,9 @@ static void snapshot_resume(struct dm_target *ti)
        /* Now we have correct chunk size, reregister */
        reregister_snapshot(s);
 
-       mutex_lock(&s->lock);
+       down_write(&s->lock);
        s->active = 1;
-       mutex_unlock(&s->lock);
+       up_write(&s->lock);
 }
 
 static uint32_t get_origin_minimum_chunksize(struct block_device *bdev)
@@ -2051,7 +2049,7 @@ static void snapshot_status(struct dm_target *ti, status_type_t type,
        switch (type) {
        case STATUSTYPE_INFO:
 
-               mutex_lock(&snap->lock);
+               down_write(&snap->lock);
 
                if (!snap->valid)
                        DMEMIT("Invalid");
@@ -2076,7 +2074,7 @@ static void snapshot_status(struct dm_target *ti, status_type_t type,
                                DMEMIT("Unknown");
                }
 
-               mutex_unlock(&snap->lock);
+               up_write(&snap->lock);
 
                break;
 
@@ -2142,7 +2140,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
                if (dm_target_is_snapshot_merge(snap->ti))
                        continue;
 
-               mutex_lock(&snap->lock);
+               down_write(&snap->lock);
 
                /* Only deal with valid and active snapshots */
                if (!snap->valid || !snap->active)
@@ -2169,9 +2167,9 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
                        if (e)
                                goto next_snapshot;
 
-                       mutex_unlock(&snap->lock);
+                       up_write(&snap->lock);
                        pe = alloc_pending_exception(snap);
-                       mutex_lock(&snap->lock);
+                       down_write(&snap->lock);
 
                        if (!snap->valid) {
                                free_pending_exception(pe);
@@ -2221,7 +2219,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
                }
 
 next_snapshot:
-               mutex_unlock(&snap->lock);
+               up_write(&snap->lock);
 
                if (pe_to_start_now) {
                        start_copy(pe_to_start_now);