dm snapshot: Use fine-grained locking scheme
Substitute the global locking scheme with a fine grained one, employing the read-write semaphore and the scalable exception tables with per-bucket locks introduced by the previous two commits. Summarizing, we now 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. Finally, we use an extra spinlock (pe_allocation_lock) to serialize the allocation of new exceptions by the exception store. This allocation is really fast, so the extra spinlock doesn't hurt the performance. This scheme allows dm-snapshot to scale better, resulting in increased IOPS and reduced latency. Following are some benchmark results using the null_blk device: modprobe null_blk gb=1024 bs=512 submit_queues=8 hw_queue_depth=4096 \ queue_mode=2 irqmode=1 completion_nsec=1 nr_devices=1 * Benchmark fio_origin_randwrite_throughput_N, from the device mapper test suite [1] (direct IO, random 4K writes to origin device, IO engine libaio): +--------------+-------------+------------+ | # of workers | IOPS Before | IOPS After | +--------------+-------------+------------+ | 1 | 57708 | 66421 | | 2 | 63415 | 77589 | | 4 | 67276 | 98839 | | 8 | 60564 | 109258 | +--------------+-------------+------------+ * Benchmark fio_origin_randwrite_latency_N, from the device mapper test suite [1] (direct IO, random 4K writes to origin device, IO engine psync): +--------------+-----------------------+----------------------+ | # of workers | Latency (usec) Before | Latency (usec) After | +--------------+-----------------------+----------------------+ | 1 | 16.25 | 13.27 | | 2 | 31.65 | 25.08 | | 4 | 55.28 | 41.08 | | 8 | 121.47 | 74.44 | +--------------+-----------------------+----------------------+ * Benchmark fio_snapshot_randwrite_throughput_N, from the device mapper test suite [1] (direct IO, random 4K writes to snapshot device, IO engine libaio): +--------------+-------------+------------+ | # of workers | IOPS Before | IOPS After | +--------------+-------------+------------+ | 1 | 72593 | 84938 | | 2 | 97379 | 134973 | | 4 | 90610 | 143077 | | 8 | 90537 | 180085 | +--------------+-------------+------------+ * Benchmark fio_snapshot_randwrite_latency_N, from the device mapper test suite [1] (direct IO, random 4K writes to snapshot device, IO engine psync): +--------------+-----------------------+----------------------+ | # of workers | Latency (usec) Before | Latency (usec) After | +--------------+-----------------------+----------------------+ | 1 | 12.53 | 10.6 | | 2 | 19.78 | 14.89 | | 4 | 40.37 | 23.47 | | 8 | 89.32 | 48.48 | +--------------+-----------------------+----------------------+ [1] https://github.com/jthornber/device-mapper-test-suite 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>
This commit is contained in:
Nikos Tsironis
Mike Snitzer
parent
f79ae415b6
commit
3f1637f210
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@ -77,7 +77,9 @@ struct dm_snapshot {
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atomic_t pending_exceptions_count;
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/* Protected by "lock" */
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spinlock_t pe_allocation_lock;
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/* Protected by "pe_allocation_lock" */
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sector_t exception_start_sequence;
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/* Protected by kcopyd single-threaded callback */
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@ -1245,6 +1247,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
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s->snapshot_overflowed = 0;
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s->active = 0;
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atomic_set(&s->pending_exceptions_count, 0);
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spin_lock_init(&s->pe_allocation_lock);
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s->exception_start_sequence = 0;
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s->exception_complete_sequence = 0;
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s->out_of_order_tree = RB_ROOT;
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@ -1522,6 +1525,13 @@ static void __invalidate_snapshot(struct dm_snapshot *s, int err)
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dm_table_event(s->ti->table);
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}
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static void invalidate_snapshot(struct dm_snapshot *s, int err)
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{
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down_write(&s->lock);
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__invalidate_snapshot(s, err);
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up_write(&s->lock);
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}
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static void pending_complete(void *context, int success)
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{
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struct dm_snap_pending_exception *pe = context;
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@ -1537,8 +1547,7 @@ static void pending_complete(void *context, int success)
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if (!success) {
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/* Read/write error - snapshot is unusable */
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down_write(&s->lock);
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__invalidate_snapshot(s, -EIO);
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invalidate_snapshot(s, -EIO);
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error = 1;
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dm_exception_table_lock(&lock);
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@ -1547,8 +1556,7 @@ static void pending_complete(void *context, int success)
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e = alloc_completed_exception(GFP_NOIO);
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if (!e) {
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down_write(&s->lock);
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__invalidate_snapshot(s, -ENOMEM);
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invalidate_snapshot(s, -ENOMEM);
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error = 1;
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dm_exception_table_lock(&lock);
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@ -1556,11 +1564,13 @@ static void pending_complete(void *context, int success)
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}
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*e = pe->e;
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down_write(&s->lock);
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down_read(&s->lock);
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dm_exception_table_lock(&lock);
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if (!s->valid) {
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up_read(&s->lock);
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free_completed_exception(e);
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error = 1;
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goto out;
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}
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@ -1572,13 +1582,12 @@ static void pending_complete(void *context, int success)
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* merging can overwrite the chunk in origin.
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*/
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dm_insert_exception(&s->complete, e);
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up_read(&s->lock);
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/* Wait for conflicting reads to drain */
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if (__chunk_is_tracked(s, pe->e.old_chunk)) {
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dm_exception_table_unlock(&lock);
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up_write(&s->lock);
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__check_for_conflicting_io(s, pe->e.old_chunk);
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down_write(&s->lock);
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dm_exception_table_lock(&lock);
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}
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@ -1595,8 +1604,6 @@ static void pending_complete(void *context, int success)
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full_bio->bi_end_io = pe->full_bio_end_io;
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increment_pending_exceptions_done_count();
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up_write(&s->lock);
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/* Submit any pending write bios */
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if (error) {
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if (full_bio)
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@ -1738,8 +1745,8 @@ __lookup_pending_exception(struct dm_snapshot *s, chunk_t chunk)
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/*
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* Inserts a pending exception into the pending table.
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*
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* NOTE: a write lock must be held on snap->lock before calling
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* this.
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* NOTE: a write lock must be held on the chunk's pending exception table slot
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* before calling this.
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*/
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static struct dm_snap_pending_exception *
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__insert_pending_exception(struct dm_snapshot *s,
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@ -1751,12 +1758,15 @@ __insert_pending_exception(struct dm_snapshot *s,
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pe->started = 0;
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pe->full_bio = NULL;
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spin_lock(&s->pe_allocation_lock);
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if (s->store->type->prepare_exception(s->store, &pe->e)) {
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spin_unlock(&s->pe_allocation_lock);
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free_pending_exception(pe);
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return NULL;
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}
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pe->exception_sequence = s->exception_start_sequence++;
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spin_unlock(&s->pe_allocation_lock);
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dm_insert_exception(&s->pending, &pe->e);
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@ -1768,8 +1778,8 @@ __insert_pending_exception(struct dm_snapshot *s,
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* for this chunk, otherwise it allocates a new one and inserts
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* it into the pending table.
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*
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* NOTE: a write lock must be held on snap->lock before calling
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* this.
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* NOTE: a write lock must be held on the chunk's pending exception table slot
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* before calling this.
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*/
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static struct dm_snap_pending_exception *
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__find_pending_exception(struct dm_snapshot *s,
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@ -1820,7 +1830,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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if (!s->valid)
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return DM_MAPIO_KILL;
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down_write(&s->lock);
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down_read(&s->lock);
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dm_exception_table_lock(&lock);
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if (!s->valid || (unlikely(s->snapshot_overflowed) &&
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@ -1845,17 +1855,9 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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pe = __lookup_pending_exception(s, chunk);
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if (!pe) {
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dm_exception_table_unlock(&lock);
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up_write(&s->lock);
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pe = alloc_pending_exception(s);
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down_write(&s->lock);
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dm_exception_table_lock(&lock);
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if (!s->valid || s->snapshot_overflowed) {
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free_pending_exception(pe);
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r = DM_MAPIO_KILL;
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goto out_unlock;
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}
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e = dm_lookup_exception(&s->complete, chunk);
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if (e) {
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free_pending_exception(pe);
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@ -1866,10 +1868,15 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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pe = __find_pending_exception(s, pe, chunk);
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if (!pe) {
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dm_exception_table_unlock(&lock);
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up_read(&s->lock);
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down_write(&s->lock);
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if (s->store->userspace_supports_overflow) {
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s->snapshot_overflowed = 1;
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DMERR("Snapshot overflowed: Unable to allocate exception.");
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if (s->valid && !s->snapshot_overflowed) {
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s->snapshot_overflowed = 1;
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DMERR("Snapshot overflowed: Unable to allocate exception.");
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}
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} else
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__invalidate_snapshot(s, -ENOMEM);
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up_write(&s->lock);
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@ -1887,8 +1894,10 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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bio->bi_iter.bi_size ==
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(s->store->chunk_size << SECTOR_SHIFT)) {
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pe->started = 1;
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dm_exception_table_unlock(&lock);
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up_write(&s->lock);
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up_read(&s->lock);
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start_full_bio(pe, bio);
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goto out;
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}
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@ -1896,10 +1905,12 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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bio_list_add(&pe->snapshot_bios, bio);
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if (!pe->started) {
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/* this is protected by snap->lock */
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/* this is protected by the exception table lock */
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pe->started = 1;
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dm_exception_table_unlock(&lock);
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up_write(&s->lock);
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up_read(&s->lock);
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start_copy(pe);
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goto out;
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}
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@ -1910,7 +1921,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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out_unlock:
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dm_exception_table_unlock(&lock);
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up_write(&s->lock);
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up_read(&s->lock);
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out:
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return r;
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}
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@ -2234,7 +2245,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
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chunk = sector_to_chunk(snap->store, sector);
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dm_exception_table_lock_init(snap, chunk, &lock);
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down_write(&snap->lock);
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down_read(&snap->lock);
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dm_exception_table_lock(&lock);
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/* Only deal with valid and active snapshots */
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@ -2253,16 +2264,9 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
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goto next_snapshot;
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dm_exception_table_unlock(&lock);
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up_write(&snap->lock);
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pe = alloc_pending_exception(snap);
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down_write(&snap->lock);
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dm_exception_table_lock(&lock);
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if (!snap->valid) {
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free_pending_exception(pe);
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goto next_snapshot;
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}
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pe2 = __lookup_pending_exception(snap, chunk);
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if (!pe2) {
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@ -2275,9 +2279,9 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
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pe = __insert_pending_exception(snap, pe, chunk);
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if (!pe) {
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dm_exception_table_unlock(&lock);
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__invalidate_snapshot(snap, -ENOMEM);
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up_write(&snap->lock);
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up_read(&snap->lock);
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invalidate_snapshot(snap, -ENOMEM);
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continue;
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}
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} else {
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@ -2310,7 +2314,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
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next_snapshot:
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dm_exception_table_unlock(&lock);
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up_write(&snap->lock);
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up_read(&snap->lock);
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if (pe_to_start_now) {
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start_copy(pe_to_start_now);
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