dm: simplify request based suspend

The semantics of bio-based dm were changed recently in the case of
suspend with "--nolockfs" but without "--noflush".
Before 2.6.30, I/Os submitted before the suspend invocation were always
flushed.  From 2.6.30 onwards, I/Os submitted before the suspend
invocation might not be flushed.  (For details, see
http://marc.info/?t=123994433400003&r=1&w=2)

This patch brings the behaviour of request-based dm into line with
bio-based dm, simplifying the code and preparing for a subsequent patch
that will wait for all in_flight I/Os to complete without stopping
request_queue and use dm_wait_for_completion() for it.

This change in semantics simplifies the suspend code as follows:
  o Suspend is implemented as stopping request_queue
    in request-based dm, and all I/Os are queued in the request_queue
    even after suspend is invoked.
  o In the old semantics, we had to track whether I/Os were
    queued before or after the suspend invocation, so a special
    barrier-like request called 'suspend marker' was introduced.
  o With the new semantics, we don't need to flush any I/O
    so we can remove the marker and the code related to the marker
    handling and I/O flushing.

After removing this codes, the suspend sequence is now:
  1. Flush all I/Os by lock_fs() if needed.
  2. Stop dispatching any I/O by stopping the request_queue.
  3. Wait for all in-flight I/Os to be completed or requeued.

Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 14 insertions, 144 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 30f5dc8e52bc..634b1daab2d4 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -178,9 +178,6 @@ struct mapped_device {
         /* forced geometry settings */
         struct hd_geometry geometry;
 
-        /* marker of flush suspend for request-based dm */
-        struct request suspend_rq;
-
         /* For saving the address of __make_request for request based dm */
         make_request_fn *saved_make_request_fn;
 
@@ -1471,11 +1468,6 @@ static struct request *clone_rq(struct request *rq, struct mapped_device *md,
         return clone;
 }
 
-static int dm_rq_flush_suspending(struct mapped_device *md)
-{
-        return !md->suspend_rq.special;
-}
-
 /*
  * Called with the queue lock held.
  */
@@ -1484,14 +1476,6 @@ static int dm_prep_fn(struct request_queue *q, struct request *rq)
         struct mapped_device *md = q->queuedata;
         struct request *clone;
 
-        if (unlikely(rq == &md->suspend_rq)) {
-                if (dm_rq_flush_suspending(md))
-                        return BLKPREP_OK;
-                else
-                        /* The flush suspend was interrupted */
-                        return BLKPREP_KILL;
-        }
-
         if (unlikely(rq->special)) {
                 DMWARN("Already has something in rq->special.");
                 return BLKPREP_KILL;
@@ -1560,27 +1544,15 @@ static void dm_request_fn(struct request_queue *q)
         struct request *rq;
 
         /*
-         * For noflush suspend, check blk_queue_stopped() to immediately
-         * quit I/O dispatching.
+         * For suspend, check blk_queue_stopped() and don't increment
+         * the number of in-flight I/Os after the queue is stopped
+         * in dm_suspend().
          */
         while (!blk_queue_plugged(q) && !blk_queue_stopped(q)) {
                 rq = blk_peek_request(q);
                 if (!rq)
                         goto plug_and_out;
 
-                if (unlikely(rq == &md->suspend_rq)) { /* Flush suspend maker */
-                        if (queue_in_flight(q))
-                                /* Not quiet yet.  Wait more */
-                                goto plug_and_out;
-
-                        /* This device should be quiet now */
-                        __stop_queue(q);
-                        blk_start_request(rq);
-                        __blk_end_request_all(rq, 0);
-                        wake_up(&md->wait);
-                        goto out;
-                }
-
                 ti = dm_table_find_target(map, blk_rq_pos(rq));
                 if (ti->type->busy && ti->type->busy(ti))
                         goto plug_and_out;
@@ -2112,7 +2084,7 @@ static int dm_wait_for_completion(struct mapped_device *md, int interruptible)
                 smp_mb();
                 if (dm_request_based(md)) {
                         spin_lock_irqsave(q->queue_lock, flags);
-                        if (!queue_in_flight(q) && blk_queue_stopped(q)) {
+                        if (!queue_in_flight(q)) {
                                 spin_unlock_irqrestore(q->queue_lock, flags);
                                 break;
                         }
@@ -2245,67 +2217,6 @@ out:
         return r;
 }
 
-static void dm_rq_invalidate_suspend_marker(struct mapped_device *md)
-{
-        md->suspend_rq.special = (void *)0x1;
-}
-
-static void dm_rq_abort_suspend(struct mapped_device *md, int noflush)
-{
-        struct request_queue *q = md->queue;
-        unsigned long flags;
-
-        spin_lock_irqsave(q->queue_lock, flags);
-        if (!noflush)
-                dm_rq_invalidate_suspend_marker(md);
-        __start_queue(q);
-        spin_unlock_irqrestore(q->queue_lock, flags);
-}
-
-static void dm_rq_start_suspend(struct mapped_device *md, int noflush)
-{
-        struct request *rq = &md->suspend_rq;
-        struct request_queue *q = md->queue;
-
-        if (noflush)
-                stop_queue(q);
-        else {
-                blk_rq_init(q, rq);
-                blk_insert_request(q, rq, 0, NULL);
-        }
-}
-
-static int dm_rq_suspend_available(struct mapped_device *md, int noflush)
-{
-        int r = 1;
-        struct request *rq = &md->suspend_rq;
-        struct request_queue *q = md->queue;
-        unsigned long flags;
-
-        if (noflush)
-                return r;
-
-        /* The marker must be protected by queue lock if it is in use */
-        spin_lock_irqsave(q->queue_lock, flags);
-        if (unlikely(rq->ref_count)) {
-                /*
-                 * This can happen, when the previous flush suspend was
-                 * interrupted, the marker is still in the queue and
-                 * this flush suspend has been invoked, because we don't
-                 * remove the marker at the time of suspend interruption.
-                 * We have only one marker per mapped_device, so we can't
-                 * start another flush suspend while it is in use.
-                 */
-                BUG_ON(!rq->special); /* The marker should be invalidated */
-                DMWARN("Invalidating the previous flush suspend is still in"
-                       " progress.  Please retry later.");
-                r = 0;
-        }
-        spin_unlock_irqrestore(q->queue_lock, flags);
-
-        return r;
-}
-
 /*
  * Functions to lock and unlock any filesystem running on the
  * device.
@@ -2348,49 +2259,11 @@ static void unlock_fs(struct mapped_device *md)
 /*
  * Suspend mechanism in request-based dm.
  *
- * After the suspend starts, further incoming requests are kept in
- * the request_queue and deferred.
- * Remaining requests in the request_queue at the start of suspend are flushed
- * if it is flush suspend.
- * The suspend completes when the following conditions have been satisfied,
- * so wait for it:
- *    1. q->in_flight is 0 (which means no in_flight request)
- *    2. queue has been stopped (which means no request dispatching)
- *
- *
- * Noflush suspend
- * ---------------
- * Noflush suspend doesn't need to dispatch remaining requests.
- * So stop the queue immediately.  Then, wait for all in_flight requests
- * to be completed or requeued.
- *
- * To abort noflush suspend, start the queue.
+ * 1. Flush all I/Os by lock_fs() if needed.
+ * 2. Stop dispatching any I/O by stopping the request_queue.
+ * 3. Wait for all in-flight I/Os to be completed or requeued.
  *
- *
- * Flush suspend
- * -------------
- * Flush suspend needs to dispatch remaining requests.  So stop the queue
- * after the remaining requests are completed. (Requeued request must be also
- * re-dispatched and completed.  Until then, we can't stop the queue.)
- *
- * During flushing the remaining requests, further incoming requests are also
- * inserted to the same queue.  To distinguish which requests are to be
- * flushed, we insert a marker request to the queue at the time of starting
- * flush suspend, like a barrier.
- * The dispatching is blocked when the marker is found on the top of the queue.
- * And the queue is stopped when all in_flight requests are completed, since
- * that means the remaining requests are completely flushed.
- * Then, the marker is removed from the queue.
- *
- * To abort flush suspend, we also need to take care of the marker, not only
- * starting the queue.
- * We don't remove the marker forcibly from the queue since it's against
- * the block-layer manner.  Instead, we put a invalidated mark on the marker.
- * When the invalidated marker is found on the top of the queue, it is
- * immediately removed from the queue, so it doesn't block dispatching.
- * Because we have only one marker per mapped_device, we can't start another
- * flush suspend until the invalidated marker is removed from the queue.
- * So fail and return with -EBUSY in such a case.
+ * To abort suspend, start the request_queue.
  */
 int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
 {
@@ -2406,11 +2279,6 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
                 goto out_unlock;
         }
 
-        if (dm_request_based(md) && !dm_rq_suspend_available(md, noflush)) {
-                r = -EBUSY;
-                goto out_unlock;
-        }
-
         map = dm_get_table(md);
 
         /*
@@ -2424,8 +2292,10 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
         dm_table_presuspend_targets(map);
 
         /*
-         * Flush I/O to the device. noflush supersedes do_lockfs,
-         * because lock_fs() needs to flush I/Os.
+         * Flush I/O to the device.
+         * Any I/O submitted after lock_fs() may not be flushed.
+         * noflush takes precedence over do_lockfs.
+         * (lock_fs() flushes I/Os and waits for them to complete.)
          */
         if (!noflush && do_lockfs) {
                 r = lock_fs(md);
@@ -2457,7 +2327,7 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
         flush_workqueue(md->wq);
 
         if (dm_request_based(md))
-                dm_rq_start_suspend(md, noflush);
+                stop_queue(md->queue);
 
         /*
          * At this point no more requests are entering target request routines.
@@ -2476,7 +2346,7 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
                 dm_queue_flush(md);
 
                 if (dm_request_based(md))
-                        dm_rq_abort_suspend(md, noflush);
+                        start_queue(md->queue);
 
                 unlock_fs(md);
                 goto out; /* pushback list is already flushed, so skip flush */