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    SubjectRe: blk_throtl_exit taking q->queue_lock is problematic
    On Thu, 17 Feb 2011 11:59:06 -0500 Vivek Goyal <vgoyal@redhat.com> wrote:

    > On Thu, Feb 17, 2011 at 04:55:01PM +1100, NeilBrown wrote:
    > > On Wed, 16 Feb 2011 20:10:29 -0500 Vivek Goyal <vgoyal@redhat.com> wrote:
    > >
    > > > So is it possible to keep the spinlock intact when md is calling up
    > > > blk_cleanup_queue()?
    > > >
    > >
    > > It would be possible, yes - but messy. I would probably end up just making
    > > ->queue_lock always point to __queue_lock, and then only take it at the
    > > places where I call 'block' code which wants to test to see if it is
    > > currently held (like the plugging routines).
    > >
    > > The queue lock (and most of the request queue) is simply irrelevant for md.
    > > I would prefer to get away from having to touch it at all...
    >
    > If queue lock is mostly irrelevant for md, then why md should provide an
    > external lock and not use queue's internal spin lock?

    See other email - historical reasons mostly.

    >
    > >
    > > I'll see how messy it would be to stop using it completely and it can just be
    > > __queue_lock.
    > >
    > > Though for me - it would be much easier if you just used __queue_lock .....
    >
    > Ok, here is the simple patch which splits the queue lock and uses
    > throtl_lock for throttling logic. I booted and it seems to be working.
    >
    > Having said that, this now introduces the possibility of races for any
    > services I take from request queue. I need to see if I need to take
    > queue lock and that makes me little uncomfortable.
    >
    > I am using kblockd_workqueue to queue throtl work. Looks like I don't
    > need queue lock for that. I am also using block tracing infrastructure
    > and my understanding is that I don't need queue lock for that as well.
    >
    > So if we do this change for performance reasons, it still makes sense
    > but doing this change because md provided a q->queue_lock and took away that
    > lock without notifying block layer hence we do this change, is still not
    > the right reason, IMHO.

    Well...I like that patch, as it makes my life easier....

    But I agree that md is doing something wrong. Now that ->queue_lock is
    always initialised, it is wrong to leave it in a state where it not defined.

    So maybe I'll apply this (after testing it a bit. The only reason for taking
    the lock queue_lock in a couple of places is to silence some warnings.

    Thanks,
    NeilBrown


    diff --git a/drivers/md/linear.c b/drivers/md/linear.c
    index 8a2f767..0ed7f6b 100644
    --- a/drivers/md/linear.c
    +++ b/drivers/md/linear.c
    @@ -216,7 +216,6 @@ static int linear_run (mddev_t *mddev)

    if (md_check_no_bitmap(mddev))
    return -EINVAL;
    - mddev->queue->queue_lock = &mddev->queue->__queue_lock;
    conf = linear_conf(mddev, mddev->raid_disks);

    if (!conf)
    diff --git a/drivers/md/multipath.c b/drivers/md/multipath.c
    index 6d7ddf3..3a62d44 100644
    --- a/drivers/md/multipath.c
    +++ b/drivers/md/multipath.c
    @@ -435,7 +435,6 @@ static int multipath_run (mddev_t *mddev)
    * bookkeeping area. [whatever we allocate in multipath_run(),
    * should be freed in multipath_stop()]
    */
    - mddev->queue->queue_lock = &mddev->queue->__queue_lock;

    conf = kzalloc(sizeof(multipath_conf_t), GFP_KERNEL);
    mddev->private = conf;
    diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
    index 75671df..c0ac457 100644
    --- a/drivers/md/raid0.c
    +++ b/drivers/md/raid0.c
    @@ -361,7 +361,6 @@ static int raid0_run(mddev_t *mddev)
    if (md_check_no_bitmap(mddev))
    return -EINVAL;
    blk_queue_max_hw_sectors(mddev->queue, mddev->chunk_sectors);
    - mddev->queue->queue_lock = &mddev->queue->__queue_lock;

    /* if private is not null, we are here after takeover */
    if (mddev->private == NULL) {
    diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
    index a23ffa3..909282d 100644
    --- a/drivers/md/raid1.c
    +++ b/drivers/md/raid1.c
    @@ -593,7 +593,9 @@ static int flush_pending_writes(conf_t *conf)
    if (conf->pending_bio_list.head) {
    struct bio *bio;
    bio = bio_list_get(&conf->pending_bio_list);
    + spin_lock(conf->mddev->queue->queue_lock);
    blk_remove_plug(conf->mddev->queue);
    + spin_unlock(conf->mddev->queue->queue_lock);
    spin_unlock_irq(&conf->device_lock);
    /* flush any pending bitmap writes to
    * disk before proceeding w/ I/O */
    @@ -959,7 +961,9 @@ static int make_request(mddev_t *mddev, struct bio * bio)
    atomic_inc(&r1_bio->remaining);
    spin_lock_irqsave(&conf->device_lock, flags);
    bio_list_add(&conf->pending_bio_list, mbio);
    + spin_lock(mddev->queue->queue_lock);
    blk_plug_device(mddev->queue);
    + spin_unlock(mddev->queue->queue_lock);
    spin_unlock_irqrestore(&conf->device_lock, flags);
    }
    r1_bio_write_done(r1_bio, bio->bi_vcnt, behind_pages, behind_pages != NULL);
    @@ -2021,7 +2025,6 @@ static int run(mddev_t *mddev)
    if (IS_ERR(conf))
    return PTR_ERR(conf);

    - mddev->queue->queue_lock = &conf->device_lock;
    list_for_each_entry(rdev, &mddev->disks, same_set) {
    disk_stack_limits(mddev->gendisk, rdev->bdev,
    rdev->data_offset << 9);
    diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
    index 3b607b2..60e6cb1 100644
    --- a/drivers/md/raid10.c
    +++ b/drivers/md/raid10.c
    @@ -662,7 +662,9 @@ static int flush_pending_writes(conf_t *conf)
    if (conf->pending_bio_list.head) {
    struct bio *bio;
    bio = bio_list_get(&conf->pending_bio_list);
    + spin_lock(conf->mddev->queue->queue_lock);
    blk_remove_plug(conf->mddev->queue);
    + spin_unlock(conf->mddev->queue->queue_lock);
    spin_unlock_irq(&conf->device_lock);
    /* flush any pending bitmap writes to disk
    * before proceeding w/ I/O */
    @@ -970,8 +972,10 @@ static int make_request(mddev_t *mddev, struct bio * bio)

    atomic_inc(&r10_bio->remaining);
    spin_lock_irqsave(&conf->device_lock, flags);
    + spin_lock(mddev->queue->queue_lock);
    bio_list_add(&conf->pending_bio_list, mbio);
    blk_plug_device(mddev->queue);
    + spin_unlock(mddev->queue->queue_lock);
    spin_unlock_irqrestore(&conf->device_lock, flags);
    }

    @@ -2304,8 +2308,6 @@ static int run(mddev_t *mddev)
    if (!conf)
    goto out;

    - mddev->queue->queue_lock = &conf->device_lock;
    -
    mddev->thread = conf->thread;
    conf->thread = NULL;

    diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
    index 7028128..78536fd 100644
    --- a/drivers/md/raid5.c
    +++ b/drivers/md/raid5.c
    @@ -5204,7 +5204,6 @@ static int run(mddev_t *mddev)

    mddev->queue->backing_dev_info.congested_data = mddev;
    mddev->queue->backing_dev_info.congested_fn = raid5_congested;
    - mddev->queue->queue_lock = &conf->device_lock;
    mddev->queue->unplug_fn = raid5_unplug_queue;

    chunk_size = mddev->chunk_sectors << 9;


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