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From: Ilya Dryomov <idryomov@gmail.com>
To: Hannes Reinecke <hare@suse.de>
Cc: Sage Weil <sage@redhat.com>, Daniel Disseldorp <ddiss@suse.com>,
	Jens Axboe <axboe@kernel.dk>,
	Ceph Development <ceph-devel@vger.kernel.org>,
	linux-block <linux-block@vger.kernel.org>
Subject: Re: [PATCH 11/15] rbd: drop state_mutex in __rbd_img_handle_request()
Date: Mon, 3 Feb 2020 19:01:49 +0100	[thread overview]
Message-ID: <CAOi1vP-x5whO0U8oECsgGB0K2FyEnMFUKW=uQJrjQ_1cgeRYww@mail.gmail.com> (raw)
In-Reply-To: <20200131103739.136098-12-hare@suse.de>

On Fri, Jan 31, 2020 at 11:38 AM Hannes Reinecke <hare@suse.de> wrote:
>
> The use of READ_ONCE/WRITE_ONCE for the image request state allows
> us to drop the state_mutex in __rbd_img_handle_request().
>
> Signed-off-by: Hannes Reinecke <hare@suse.de>
> ---
>  drivers/block/rbd.c | 26 +++++++++-----------------
>  1 file changed, 9 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
> index 671e941d6edf..db04401c4d8b 100644
> --- a/drivers/block/rbd.c
> +++ b/drivers/block/rbd.c
> @@ -349,7 +349,6 @@ struct rbd_img_request {
>         struct list_head        object_extents; /* obj_req.ex structs */
>         struct mutex            object_mutex;
>
> -       struct mutex            state_mutex;
>         int                     pending_result;
>         struct work_struct      work;
>         struct kref             kref;
> @@ -1674,7 +1673,6 @@ static struct rbd_img_request *rbd_img_request_create(
>
>         INIT_LIST_HEAD(&img_request->lock_item);
>         INIT_LIST_HEAD(&img_request->object_extents);
> -       mutex_init(&img_request->state_mutex);
>         mutex_init(&img_request->object_mutex);
>         kref_init(&img_request->kref);
>
> @@ -2529,7 +2527,7 @@ static int __rbd_img_fill_request(struct rbd_img_request *img_req)
>                 }
>         }
>         mutex_unlock(&img_req->object_mutex);
> -       img_req->state = RBD_IMG_START;
> +       WRITE_ONCE(img_req->state, RBD_IMG_START);
>         return 0;
>  }
>
> @@ -3652,15 +3650,15 @@ static bool rbd_img_advance(struct rbd_img_request *img_req, int *result)
>         int ret;
>
>         dout("%s: img %p state %d\n", __func__, img_req, img_req->state);
> -       switch (img_req->state) {
> +       switch (READ_ONCE(img_req->state)) {
>         case RBD_IMG_START:
>                 rbd_assert(!*result);
>
> -               img_req->state = RBD_IMG_EXCLUSIVE_LOCK;
> +               WRITE_ONCE(img_req->state, RBD_IMG_EXCLUSIVE_LOCK);
>                 ret = rbd_img_exclusive_lock(img_req);
>                 if (ret < 0) {
>                         *result = ret;
> -                       img_req->state = RBD_IMG_DONE;
> +                       WRITE_ONCE(img_req->state, RBD_IMG_DONE);
>                         return true;
>                 }
>                 if (ret == 0)
> @@ -3668,17 +3666,17 @@ static bool rbd_img_advance(struct rbd_img_request *img_req, int *result)
>                 /* fall through */
>         case RBD_IMG_EXCLUSIVE_LOCK:
>                 if (*result) {
> -                       img_req->state = RBD_IMG_DONE;
> +                       WRITE_ONCE(img_req->state, RBD_IMG_DONE);
>                         return true;
>                 }
>
>                 rbd_assert(!need_exclusive_lock(img_req) ||
>                            __rbd_is_lock_owner(rbd_dev));
>
> -               img_req->state = RBD_IMG_OBJECT_REQUESTS;
> +               WRITE_ONCE(img_req->state, RBD_IMG_OBJECT_REQUESTS);
>                 if (!rbd_img_object_requests(img_req)) {
>                         *result = img_req->pending_result;
> -                       img_req->state = RBD_IMG_DONE;
> +                       WRITE_ONCE(img_req->state, RBD_IMG_DONE);
>                         return true;
>                 }
>                 return false;
> @@ -3686,7 +3684,7 @@ static bool rbd_img_advance(struct rbd_img_request *img_req, int *result)
>                 if (rbd_img_object_requests_pending(img_req))
>                         return false;
>                 *result = img_req->pending_result;
> -               img_req->state = RBD_IMG_DONE;
> +               WRITE_ONCE(img_req->state, RBD_IMG_DONE);
>                 /* fall through */
>         case RBD_IMG_DONE:
>                 return true;
> @@ -3706,16 +3704,12 @@ static bool __rbd_img_handle_request(struct rbd_img_request *img_req,
>
>         if (need_exclusive_lock(img_req)) {
>                 down_read(&rbd_dev->lock_rwsem);
> -               mutex_lock(&img_req->state_mutex);
>                 done = rbd_img_advance(img_req, result);
>                 if (done)
>                         rbd_lock_del_request(img_req);
> -               mutex_unlock(&img_req->state_mutex);
>                 up_read(&rbd_dev->lock_rwsem);
>         } else {
> -               mutex_lock(&img_req->state_mutex);
>                 done = rbd_img_advance(img_req, result);
> -               mutex_unlock(&img_req->state_mutex);
>         }
>
>         if (done && *result) {
> @@ -3985,10 +3979,8 @@ static void wake_lock_waiters(struct rbd_device *rbd_dev, int result)
>         }
>
>         list_for_each_entry(img_req, &rbd_dev->acquiring_list, lock_item) {
> -               mutex_lock(&img_req->state_mutex);
> -               rbd_assert(img_req->state == RBD_IMG_EXCLUSIVE_LOCK);
> +               rbd_assert(READ_ONCE(img_req->state) == RBD_IMG_EXCLUSIVE_LOCK);
>                 rbd_img_schedule(img_req, result);
> -               mutex_unlock(&img_req->state_mutex);
>         }
>
>         list_splice_tail_init(&rbd_dev->acquiring_list, &rbd_dev->running_list);

->state_mutex doesn't just protect ->state or ->pending_result,
it is meant to be a code lock.  In the future, we will be adding
support for timeouts and forceful unmapping of rbd devices, which
means cancelling requests at arbitrary points.  These state machines
need to be reentrant, not just from the inside (i.e. object requests)
but also from the outside.  Getting that right when ->state is managed
through READ/WRITE_ONCE and must be carefully set before dispatching
anything that might change it is going to be very challenging.

In the cover letter, this patch is listed as one of the required
steps for up to 25% speedup.  Is that really the case?  It doesn't
make top-30 contended locks in my tests...

Do you have the numbers without this and any of the preceding patches
or possibly just with patch 15?

Thanks,

                Ilya

  reply	other threads:[~2020-02-03 18:01 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-31 10:37 [PATCH 00/15] rbd: switch to blk-mq Hannes Reinecke
2020-01-31 10:37 ` [PATCH 01/15] rbd: lock object request list Hannes Reinecke
2020-02-03 16:38   ` Ilya Dryomov
2020-01-31 10:37 ` [PATCH 02/15] rbd: use READ_ONCE() when checking the mapping size Hannes Reinecke
2020-02-03 16:50   ` Ilya Dryomov
2020-02-04  7:05     ` Hannes Reinecke
2020-01-31 10:37 ` [PATCH 03/15] rbd: reorder rbd_img_advance() Hannes Reinecke
2020-01-31 10:37 ` [PATCH 04/15] rbd: reorder switch statement in rbd_advance_read() Hannes Reinecke
2020-01-31 10:37 ` [PATCH 05/15] rbd: reorder switch statement in rbd_advance_write() Hannes Reinecke
2020-01-31 10:37 ` [PATCH 06/15] rbd: add 'done' state for rbd_obj_advance_copyup() Hannes Reinecke
2020-01-31 10:37 ` [PATCH 07/15] rbd: use callback for image request completion Hannes Reinecke
2020-02-03 17:13   ` Ilya Dryomov
2020-01-31 10:37 ` [PATCH 08/15] rbd: add debugging statements for the state machine Hannes Reinecke
2020-01-31 10:37 ` [PATCH 09/15] rbd: count pending object requests in-line Hannes Reinecke
2020-02-03 17:47   ` Ilya Dryomov
2020-02-04  6:59     ` Hannes Reinecke
2020-01-31 10:37 ` [PATCH 10/15] rbd: kill 'work_result' Hannes Reinecke
2020-01-31 10:37 ` [PATCH 11/15] rbd: drop state_mutex in __rbd_img_handle_request() Hannes Reinecke
2020-02-03 18:01   ` Ilya Dryomov [this message]
2020-01-31 10:37 ` [PATCH 12/15] rbd: kill img_request kref Hannes Reinecke
2020-01-31 10:37 ` [PATCH 13/15] rbd: schedule image_request after preparation Hannes Reinecke
2020-02-03 18:40   ` Ilya Dryomov
2020-01-31 10:37 ` [PATCH 14/15] rbd: embed image request as blk_mq request payload Hannes Reinecke
2020-01-31 10:37 ` [PATCH 15/15] rbd: switch to blk-mq Hannes Reinecke
2020-02-03  8:36   ` Christoph Hellwig

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