Re: [PATCH 2/9] ublk: properly serialize all FETCH_REQs

[Uday Shankar <ushankar@purestorage.com>]

On Mon, Apr 14, 2025 at 02:39:33PM -0600, Jens Axboe wrote:
> On 4/14/25 1:58 PM, Uday Shankar wrote:
> > +static int ublk_fetch(struct io_uring_cmd *cmd, struct ublk_device *ub,
> > +		      struct ublk_queue *ubq, struct ublk_io *io,
> > +		      const struct ublksrv_io_cmd *ub_cmd,
> > +		      unsigned int issue_flags)
> > +{
> > +	int ret = 0;
> > +
> > +	if (issue_flags & IO_URING_F_NONBLOCK)
> > +		return -EAGAIN;
> > +
> > +	mutex_lock(&ub->mutex);
> 
> This looks like overkill, if we can trylock the mutex that should surely
> be fine? And I would imagine succeed most of the time, hence making the
> inline/fastpath fine with F_NONBLOCK?

Yeah, makes sense. How about this?

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index cdb1543fa4a9..bf4a88cb1413 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -1832,8 +1832,8 @@ static void ublk_nosrv_work(struct work_struct *work)
 
 /* device can only be started after all IOs are ready */
 static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
+	__must_hold(&ub->mutex)
 {
-	mutex_lock(&ub->mutex);
 	ubq->nr_io_ready++;
 	if (ublk_queue_ready(ubq)) {
 		ubq->ubq_daemon = current;
@@ -1845,7 +1845,6 @@ static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
 	}
 	if (ub->nr_queues_ready == ub->dev_info.nr_hw_queues)
 		complete_all(&ub->completion);
-	mutex_unlock(&ub->mutex);
 }
 
 static void ublk_handle_need_get_data(struct ublk_device *ub, int q_id,
@@ -1929,6 +1928,55 @@ static int ublk_unregister_io_buf(struct io_uring_cmd *cmd,
 	return io_buffer_unregister_bvec(cmd, index, issue_flags);
 }
 
+static int ublk_fetch(struct io_uring_cmd *cmd, struct ublk_device *ub,
+		      struct ublk_queue *ubq, struct ublk_io *io,
+		      const struct ublksrv_io_cmd *ub_cmd,
+		      unsigned int issue_flags)
+{
+	int ret = 0;
+
+	if (!mutex_trylock(&ub->mutex)) {
+		if (issue_flags & IO_URING_F_NONBLOCK)
+			return -EAGAIN;
+		else
+			mutex_lock(&ub->mutex);
+	}
+
+	/* UBLK_IO_FETCH_REQ is only allowed before queue is setup */
+	if (ublk_queue_ready(ubq)) {
+		ret = -EBUSY;
+		goto out;
+	}
+
+	/* allow each command to be FETCHed at most once */
+	if (io->flags & UBLK_IO_FLAG_ACTIVE) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	WARN_ON_ONCE(io->flags & UBLK_IO_FLAG_OWNED_BY_SRV);
+
+	if (ublk_need_map_io(ubq)) {
+		/*
+		 * FETCH_RQ has to provide IO buffer if NEED GET
+		 * DATA is not enabled
+		 */
+		if (!ub_cmd->addr && !ublk_need_get_data(ubq))
+			goto out;
+	} else if (ub_cmd->addr) {
+		/* User copy requires addr to be unset */
+		ret = -EINVAL;
+		goto out;
+	}
+
+	ublk_fill_io_cmd(io, cmd, ub_cmd->addr);
+	ublk_mark_io_ready(ub, ubq);
+
+out:
+	mutex_unlock(&ub->mutex);
+	return ret;
+}
+
 static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
 			       unsigned int issue_flags,
 			       const struct ublksrv_io_cmd *ub_cmd)
@@ -1985,34 +2033,7 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
 	case UBLK_IO_UNREGISTER_IO_BUF:
 		return ublk_unregister_io_buf(cmd, ub_cmd->addr, issue_flags);
 	case UBLK_IO_FETCH_REQ:
-		/* UBLK_IO_FETCH_REQ is only allowed before queue is setup */
-		if (ublk_queue_ready(ubq)) {
-			ret = -EBUSY;
-			goto out;
-		}
-		/*
-		 * The io is being handled by server, so COMMIT_RQ is expected
-		 * instead of FETCH_REQ
-		 */
-		if (io->flags & UBLK_IO_FLAG_OWNED_BY_SRV)
-			goto out;
-
-		if (ublk_need_map_io(ubq)) {
-			/*
-			 * FETCH_RQ has to provide IO buffer if NEED GET
-			 * DATA is not enabled
-			 */
-			if (!ub_cmd->addr && !ublk_need_get_data(ubq))
-				goto out;
-		} else if (ub_cmd->addr) {
-			/* User copy requires addr to be unset */
-			ret = -EINVAL;
-			goto out;
-		}
-
-		ublk_fill_io_cmd(io, cmd, ub_cmd->addr);
-		ublk_mark_io_ready(ub, ubq);
-		break;
+		return ublk_fetch(cmd, ub, ubq, io, ub_cmd, issue_flags);
 	case UBLK_IO_COMMIT_AND_FETCH_REQ:
 		req = blk_mq_tag_to_rq(ub->tag_set.tags[ub_cmd->q_id], tag);