dm: rework queueing and suspension

Rework shutting down on suspend and document the associated rules.

Drop write lock in __split_and_process_bio to allow more processing
concurrency.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 32 insertions, 8 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index bb97ec8d6644..9746c1eb9ef7 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1434,18 +1434,21 @@ static void dm_wq_work(struct work_struct *work)
 
         down_write(&md->io_lock);
 
-        while (1) {
+        while (!test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags)) {
                 spin_lock_irq(&md->deferred_lock);
                 c = bio_list_pop(&md->deferred);
                 spin_unlock_irq(&md->deferred_lock);
 
                 if (!c) {
-                        clear_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags);
                         clear_bit(DMF_QUEUE_IO_TO_THREAD, &md->flags);
                         break;
                 }
 
+                up_write(&md->io_lock);
+
                 __split_and_process_bio(md, c);
+
+                down_write(&md->io_lock);
         }
 
         up_write(&md->io_lock);
@@ -1453,8 +1456,9 @@ static void dm_wq_work(struct work_struct *work)
 
 static void dm_queue_flush(struct mapped_device *md)
 {
+        clear_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags);
+        smp_mb__after_clear_bit();
         queue_work(md->wq, &md->work);
-        flush_workqueue(md->wq);
 }
 
 /*
@@ -1572,22 +1576,36 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
         }
 
         /*
-         * First we set the DMF_QUEUE_IO_TO_THREAD flag so no more ios
-         * will be mapped.
+         * Here we must make sure that no processes are submitting requests
+         * to target drivers i.e. no one may be executing
+         * __split_and_process_bio. This is called from dm_request and
+         * dm_wq_work.
+         *
+         * To get all processes out of __split_and_process_bio in dm_request,
+         * we take the write lock. To prevent any process from reentering
+         * __split_and_process_bio from dm_request, we set
+         * DMF_QUEUE_IO_TO_THREAD.
+         *
+         * To quiesce the thread (dm_wq_work), we set DMF_BLOCK_IO_FOR_SUSPEND
+         * and call flush_workqueue(md->wq). flush_workqueue will wait until
+         * dm_wq_work exits and DMF_BLOCK_IO_FOR_SUSPEND will prevent any
+         * further calls to __split_and_process_bio from dm_wq_work.
          */
         down_write(&md->io_lock);
         set_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags);
         set_bit(DMF_QUEUE_IO_TO_THREAD, &md->flags);
-
         up_write(&md->io_lock);
 
+        flush_workqueue(md->wq);
+
         /*
-         * Wait for the already-mapped ios to complete.
+         * At this point no more requests are entering target request routines.
+         * We call dm_wait_for_completion to wait for all existing requests
+         * to finish.
          */
         r = dm_wait_for_completion(md, TASK_INTERRUPTIBLE);
 
         down_write(&md->io_lock);
-
         if (noflush)
                 clear_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
         up_write(&md->io_lock);
@@ -1600,6 +1618,12 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
                 goto out; /* pushback list is already flushed, so skip flush */
         }
 
+        /*
+         * If dm_wait_for_completion returned 0, the device is completely
+         * quiescent now. There is no request-processing activity. All new
+         * requests are being added to md->deferred list.
+         */
+
         dm_table_postsuspend_targets(map);
 
         set_bit(DMF_SUSPENDED, &md->flags);