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>

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index bb97ec8..9746c1e 100644 (file)
--- 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);