Re: [PATCH] backing dev unplugging

Andrew Morton
2004-03-10T23:07:53Z

(Please use reply-to-all)

"Miquel van Smoorenburg" <miquels@cistron.nl> wrote:
>
> In article <20040310210207.GL15087@suse.de>,
> Jens Axboe <axboe@suse.de> wrote:
> >On Wed, Mar 10 2004, Andrew Morton wrote:
> >> Jens Axboe <axboe@suse.de> wrote:
> >> >
> >> > Here's a first cut at killing global plugging of block devices to reduce
> >> > the nasty contention blk_plug_lock caused.
> >>
> >> Shouldn't we take read_lock(&md->map_lock) in dm_table_unplug_all()?
> >
> >Ugh yes, we certainly should.
>
> With the latest patches from Joe it would be more like
>
> map = dm_get_table(md);
> if (map) {
> dm_table_unplug_all(map);
> dm_table_put(map);
> }
>
> No lock ranking issues, you just get a refcounted map (table, really).

Ah, OK. Jens, you'll be needing this (on rc2-mm1):

dm.c: protect md->map with a rw spin lock rather than the md->lock
semaphore. Also ensure that everyone accesses md->map through
dm_get_table(), rather than directly.

---

25-akpm/drivers/md/dm-table.c | 3 +
25-akpm/drivers/md/dm.c | 88 +++++++++++++++++++++++++-----------------
2 files changed, 57 insertions(+), 34 deletions(-)

diff -puN drivers/md/dm.c~dm-map-rwlock-ng drivers/md/dm.c
--- 25/drivers/md/dm.c~dm-map-rwlock-ng Mon Mar 8 13:58:42 2004
+++ 25-akpm/drivers/md/dm.c Mon Mar 8 13:58:42 2004
@@ -49,7 +49,7 @@ struct target_io {

struct mapped_device {
struct rw_semaphore lock;
- rwlock_t maplock;
+ rwlock_t map_lock;
atomic_t holders;

unsigned long flags;
@@ -238,6 +238,24 @@ static int queue_io(struct mapped_device
return 0; /* deferred successfully */
}

+/*
+ * Everyone (including functions in this file), should use this
+ * function to access the md->map field, and make sure they call
+ * dm_table_put() when finished.
+ */
+struct dm_table *dm_get_table(struct mapped_device *md)
+{
+ struct dm_table *t;
+
+ read_lock(&md->map_lock);
+ t = md->map;
+ if (t)
+ dm_table_get(t);
+ read_unlock(&md->map_lock);
+
+ return t;
+}
+
/*-----------------------------------------------------------------
* CRUD START:
* A more elegant soln is in the works that uses the queue
@@ -346,6 +364,7 @@ static void __map_bio(struct dm_target *

struct clone_info {
struct mapped_device *md;
+ struct dm_table *map;
struct bio *bio;
struct dm_io *io;
sector_t sector;
@@ -399,7 +418,7 @@ static struct bio *clone_bio(struct bio
static void __clone_and_map(struct clone_info *ci)
{
struct bio *clone, *bio = ci->bio;
- struct dm_target *ti = dm_table_find_target(ci->md->map, ci->sector);
+ struct dm_target *ti = dm_table_find_target(ci->map, ci->sector);
sector_t len = 0, max = max_io_len(ci->md, ci->sector, ti);
struct target_io *tio;

@@ -460,7 +479,7 @@ static void __clone_and_map(struct clone

ci->sector += max;
ci->sector_count -= max;
- ti = dm_table_find_target(ci->md->map, ci->sector);
+ ti = dm_table_find_target(ci->map, ci->sector);

len = to_sector(bv->bv_len) - max;
clone = split_bvec(bio, ci->sector, ci->idx,
@@ -485,6 +504,7 @@ static void __split_bio(struct mapped_de
struct clone_info ci;

ci.md = md;
+ ci.map = dm_get_table(md);
ci.bio = bio;
ci.io = alloc_io(md);
ci.io->error = 0;
@@ -501,6 +521,7 @@ static void __split_bio(struct mapped_de

/* drop the extra reference count */
dec_pending(ci.io, 0);
+ dm_table_put(ci.map);
}
/*-----------------------------------------------------------------
* CRUD END
@@ -563,15 +584,16 @@ static int dm_request(request_queue_t *q
static int dm_any_congested(void *congested_data, int bdi_bits)
{
int r;
- struct mapped_device *md = congested_data;
+ struct mapped_device *md = (struct mapped_device *) congested_data;
+ struct dm_table *map = dm_get_table(md);

- read_lock(&md->maplock);
- if (md->map == NULL || test_bit(DMF_BLOCK_IO, &md->flags))
+ if (!map || test_bit(DMF_BLOCK_IO, &md->flags))
+ /* FIXME: shouldn't suspended count a congested ? */
r = bdi_bits;
else
- r = dm_table_any_congested(md->map, bdi_bits);
- read_unlock(&md->maplock);
+ r = dm_table_any_congested(map, bdi_bits);

+ dm_table_put(map);
return r;
}

@@ -646,7 +668,7 @@ static struct mapped_device *alloc_dev(u

memset(md, 0, sizeof(*md));
init_rwsem(&md->lock);
- rwlock_init(&md->maplock);
+ rwlock_init(&md->map_lock);
atomic_set(&md->holders, 1);

md->queue = blk_alloc_queue(GFP_KERNEL);
@@ -746,12 +768,12 @@ static int __bind(struct mapped_device *
if (size == 0)
return 0;

- write_lock(&md->maplock);
+ write_lock(&md->map_lock);
md->map = t;
- write_unlock(&md->maplock);
- dm_table_event_callback(md->map, event_callback, md);
+ write_unlock(&md->map_lock);

dm_table_get(t);
+ dm_table_event_callback(md->map, event_callback, md);
dm_table_set_restrictions(t, q);
return 0;
}
@@ -764,9 +786,9 @@ static void __unbind(struct mapped_devic
return;

dm_table_event_callback(map, NULL, NULL);
- write_lock(&md->maplock);
+ write_lock(&md->map_lock);
md->map = NULL;
- write_unlock(&md->maplock);
+ write_unlock(&md->map_lock);
dm_table_put(map);
}

@@ -803,12 +825,16 @@ void dm_get(struct mapped_device *md)

void dm_put(struct mapped_device *md)
{
+ struct dm_table *map = dm_get_table(md);
+
if (atomic_dec_and_test(&md->holders)) {
- if (!test_bit(DMF_SUSPENDED, &md->flags) && md->map)
- dm_table_suspend_targets(md->map);
+ if (!test_bit(DMF_SUSPENDED, &md->flags) && map)
+ dm_table_suspend_targets(map);
__unbind(md);
free_dev(md);
}
+
+ dm_table_put(map);
}

/*
@@ -859,6 +885,7 @@ int dm_swap_table(struct mapped_device *
*/
int dm_suspend(struct mapped_device *md)
{
+ struct dm_table *map;
DECLARE_WAITQUEUE(wait, current);

down_write(&md->lock);
@@ -894,8 +921,11 @@ int dm_suspend(struct mapped_device *md)
down_write(&md->lock);
remove_wait_queue(&md->wait, &wait);
set_bit(DMF_SUSPENDED, &md->flags);
- if (md->map)
- dm_table_suspend_targets(md->map);
+
+ map = dm_get_table(md);
+ if (map)
+ dm_table_suspend_targets(map);
+ dm_table_put(map);
up_write(&md->lock);

return 0;
@@ -904,22 +934,25 @@ int dm_suspend(struct mapped_device *md)
int dm_resume(struct mapped_device *md)
{
struct bio *def;
+ struct dm_table *map = dm_get_table(md);

down_write(&md->lock);
- if (!md->map ||
+ if (!map ||
!test_bit(DMF_SUSPENDED, &md->flags) ||
- !dm_table_get_size(md->map)) {
+ !dm_table_get_size(map)) {
up_write(&md->lock);
+ dm_table_put(map);
return -EINVAL;
}

- dm_table_resume_targets(md->map);
+ dm_table_resume_targets(map);
clear_bit(DMF_SUSPENDED, &md->flags);
clear_bit(DMF_BLOCK_IO, &md->flags);

def = bio_list_get(&md->deferred);
__flush_deferred_io(md, def);
up_write(&md->lock);
+ dm_table_put(map);

blk_run_queues();

@@ -971,19 +1004,6 @@ struct gendisk *dm_disk(struct mapped_de
return md->disk;
}

-struct dm_table *dm_get_table(struct mapped_device *md)
-{
- struct dm_table *t;
-
- down_read(&md->lock);
- t = md->map;
- if (t)
- dm_table_get(t);
- up_read(&md->lock);
-
- return t;
-}
-
int dm_suspended(struct mapped_device *md)
{
return test_bit(DMF_SUSPENDED, &md->flags);
diff -puN drivers/md/dm-table.c~dm-map-rwlock-ng drivers/md/dm-table.c
--- 25/drivers/md/dm-table.c~dm-map-rwlock-ng Mon Mar 8 13:58:42 2004
+++ 25-akpm/drivers/md/dm-table.c Mon Mar 8 13:58:42 2004
@@ -279,6 +279,9 @@ void dm_table_get(struct dm_table *t)

void dm_table_put(struct dm_table *t)
{
+ if (!t)
+ return;
+
if (atomic_dec_and_test(&t->holders))
table_destroy(t);
}

_