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dm: simplify request based suspend
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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>
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Kiyoshi Ueda authored and Alasdair G Kergon committed Dec 10, 2009
1 parent 6facdaf commit 9f518b2
Showing 1 changed file with 14 additions and 144 deletions.
158 changes: 14 additions & 144 deletions drivers/md/dm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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;

Expand Down Expand Up @@ -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.
*/
Expand All @@ -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;
Expand Down Expand Up @@ -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;
Expand Down Expand Up @@ -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;
}
Expand Down Expand Up @@ -2245,67 +2217,6 @@ int dm_swap_table(struct mapped_device *md, struct dm_table *table)
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.
Expand Down Expand Up @@ -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)
{
Expand All @@ -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);

/*
Expand All @@ -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);
Expand Down Expand Up @@ -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.
Expand All @@ -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 */
Expand Down

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