linux-kernel - [ 38/70] virtio-blk: Use block layer provided spinlock

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Date:	Tue, 07 Aug 2012 04:28:33 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Asias He <asias@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: [ 38/70] virtio-blk: Use block layer provided spinlock

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Asias He <asias@...hat.com>

commit 2c95a3290919541b846bee3e0fbaa75860929f53 upstream.

Block layer will allocate a spinlock for the queue if the driver does
not provide one in blk_init_queue().

The reason to use the internal spinlock is that blk_cleanup_queue() will
switch to use the internal spinlock in the cleanup code path.

        if (q->queue_lock != &q->__queue_lock)
                q->queue_lock = &q->__queue_lock;

However, processes which are in D state might have taken the driver
provided spinlock, when the processes wake up, they would release the
block provided spinlock.

=====================================
[ BUG: bad unlock balance detected! ]
3.4.0-rc7+ #238 Not tainted
-------------------------------------
fio/3587 is trying to release lock (&(&q->__queue_lock)->rlock) at:
[<ffffffff813274d2>] blk_queue_bio+0x2a2/0x380
but there are no more locks to release!

other info that might help us debug this:
1 lock held by fio/3587:
 #0:  (&(&vblk->lock)->rlock){......}, at:
[<ffffffff8132661a>] get_request_wait+0x19a/0x250

Other drivers use block layer provided spinlock as well, e.g. SCSI.

Switching to the block layer provided spinlock saves a bit of memory and
does not increase lock contention. Performance test shows no real
difference is observed before and after this patch.

Changes in v2: Improve commit log as Michael suggested.

Cc: virtualization@...ts.linux-foundation.org
Cc: kvm@...r.kernel.org
Signed-off-by: Asias He <asias@...hat.com>
Acked-by: Michael S. Tsirkin <mst@...hat.com>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/block/virtio_blk.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -20,8 +20,6 @@ struct workqueue_struct *virtblk_wq;
 
 struct virtio_blk
 {
-	spinlock_t lock;
-
 	struct virtio_device *vdev;
 	struct virtqueue *vq;
 
@@ -62,7 +60,7 @@ static void blk_done(struct virtqueue *v
 	unsigned int len;
 	unsigned long flags;
 
-	spin_lock_irqsave(&vblk->lock, flags);
+	spin_lock_irqsave(vblk->disk->queue->queue_lock, flags);
 	while ((vbr = virtqueue_get_buf(vblk->vq, &len)) != NULL) {
 		int error;
 
@@ -97,7 +95,7 @@ static void blk_done(struct virtqueue *v
 	}
 	/* In case queue is stopped waiting for more buffers. */
 	blk_start_queue(vblk->disk->queue);
-	spin_unlock_irqrestore(&vblk->lock, flags);
+	spin_unlock_irqrestore(vblk->disk->queue->queue_lock, flags);
 }
 
 static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
@@ -384,7 +382,6 @@ static int __devinit virtblk_probe(struc
 	}
 
 	INIT_LIST_HEAD(&vblk->reqs);
-	spin_lock_init(&vblk->lock);
 	vblk->vdev = vdev;
 	vblk->sg_elems = sg_elems;
 	sg_init_table(vblk->sg, vblk->sg_elems);
@@ -410,7 +407,7 @@ static int __devinit virtblk_probe(struc
 		goto out_mempool;
 	}
 
-	q = vblk->disk->queue = blk_init_queue(do_virtblk_request, &vblk->lock);
+	q = vblk->disk->queue = blk_init_queue(do_virtblk_request, NULL);
 	if (!q) {
 		err = -ENOMEM;
 		goto out_put_disk;


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