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rbd: only get snap context for write requests

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Right now we get the snapshot context for an rbd image (under
protection of the header semaphore) for every request processed.

There's no need to get the snap context if we're doing a read,
so avoid doing so in that case.

Note that we no longer need to hold the header semaphore to
check the rbd_dev's existence flag.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
This commit is contained in:
Alex Elder 2012-11-09 08:43:15 -06:00
parent d78b650a59
commit a7b4c65f4f
1 changed files with 18 additions and 14 deletions
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32
drivers/block/rbd.c
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@ -1331,7 +1331,7 @@ static int rbd_do_op(struct request *rq,
} else {
opcode = CEPH_OSD_OP_READ;
flags = CEPH_OSD_FLAG_READ;
snapc = NULL;
rbd_assert(!snapc);
snapid = rbd_dev->spec->snap_id;
payload_len = 0;
}
@ -1662,22 +1662,25 @@ static void rbd_rq_fn(struct request_queue *q)
}
spin_unlock_irq(q->queue_lock);
/* Stop writes to a read-only device */
/* Write requests need a reference to the snapshot context */
result = -EROFS;
if (read_only && rq_data_dir(rq) == WRITE)
goto out_end_request;
if (rq_data_dir(rq) == WRITE) {
result = -EROFS;
if (read_only) /* Can't write to a read-only device */
goto out_end_request;
/* Grab a reference to the snapshot context */
down_read(&rbd_dev->header_rwsem);
if (atomic_read(&rbd_dev->exists)) {
/*
* Note that each osd request will take its
* own reference to the snapshot context
* supplied. The reference we take here
* just guarantees the one we provide stays
* valid.
*/
down_read(&rbd_dev->header_rwsem);
snapc = ceph_get_snap_context(rbd_dev->header.snapc);
up_read(&rbd_dev->header_rwsem);
rbd_assert(snapc != NULL);
}
up_read(&rbd_dev->header_rwsem);
if (!snapc) {
} else if (!atomic_read(&rbd_dev->exists)) {
rbd_assert(rbd_dev->spec->snap_id != CEPH_NOSNAP);
dout("request for non-existent snapshot");
result = -ENXIO;
@ -1689,7 +1692,8 @@ static void rbd_rq_fn(struct request_queue *q)
blk_rq_pos(rq) * SECTOR_SIZE,
size, rq->bio);
out_end_request:
ceph_put_snap_context(snapc);
if (snapc)
ceph_put_snap_context(snapc);
spin_lock_irq(q->queue_lock);
if (!size || result < 0)
__blk_end_request_all(rq, result);