[PATCH] blk: reduce locking

Change around locking a bit for a result of 1-2 less spin lock unlock pairs in
request submission paths.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/drivers/block/ll_rw_blk.c b/drivers/block/ll_rw_blk.c
index 67431f2..5caebe2 100644 (file)
--- a/drivers/block/ll_rw_blk.c
+++ b/drivers/block/ll_rw_blk.c
@@ -1867,19 +1867,20 @@ static void freed_request(request_queue_t *q, int rw)
 
 #define blkdev_free_rq(list) list_entry((list)->next, struct request, queuelist)
 /*
- * Get a free request, queue_lock must not be held
+ * Get a free request, queue_lock must be held.
+ * Returns NULL on failure, with queue_lock held.
+ * Returns !NULL on success, with queue_lock *not held*.
  */
 static struct request *get_request(request_queue_t *q, int rw, struct bio *bio,
                                   int gfp_mask)
 {
        struct request *rq = NULL;
        struct request_list *rl = &q->rq;
-       struct io_context *ioc = get_io_context(gfp_mask);
+       struct io_context *ioc = get_io_context(GFP_ATOMIC);
 
        if (unlikely(test_bit(QUEUE_FLAG_DRAIN, &q->queue_flags)))
                goto out;
 
-       spin_lock_irq(q->queue_lock);
        if (rl->count[rw]+1 >= q->nr_requests) {
                /*
                 * The queue will fill after this allocation, so set it as
@@ -1907,7 +1908,6 @@ static struct request *get_request(request_queue_t *q, int rw, struct bio *bio,
                 * The queue is full and the allocating process is not a
                 * "batcher", and not exempted by the IO scheduler
                 */
-               spin_unlock_irq(q->queue_lock);
                goto out;
        }
 
@@ -1950,7 +1950,6 @@ rq_starved:
                if (unlikely(rl->count[rw] == 0))
                        rl->starved[rw] = 1;
 
-               spin_unlock_irq(q->queue_lock);
                goto out;
        }
 
@@ -1967,6 +1966,8 @@ out:
 /*
  * No available requests for this queue, unplug the device and wait for some
  * requests to become available.
+ *
+ * Called with q->queue_lock held, and returns with it unlocked.
  */
 static struct request *get_request_wait(request_queue_t *q, int rw,
                                        struct bio *bio)
@@ -1986,7 +1987,8 @@ static struct request *get_request_wait(request_queue_t *q, int rw,
                if (!rq) {
                        struct io_context *ioc;
 
-                       generic_unplug_device(q);
+                       __generic_unplug_device(q);
+                       spin_unlock_irq(q->queue_lock);
                        io_schedule();
 
                        /*
@@ -1998,6 +2000,8 @@ static struct request *get_request_wait(request_queue_t *q, int rw,
                        ioc = get_io_context(GFP_NOIO);
                        ioc_set_batching(q, ioc);
                        put_io_context(ioc);
+
+                       spin_lock_irq(q->queue_lock);
                }
                finish_wait(&rl->wait[rw], &wait);
        }
@@ -2011,14 +2015,18 @@ struct request *blk_get_request(request_queue_t *q, int rw, int gfp_mask)
 
        BUG_ON(rw != READ && rw != WRITE);
 
-       if (gfp_mask & __GFP_WAIT)
+       spin_lock_irq(q->queue_lock);
+       if (gfp_mask & __GFP_WAIT) {
                rq = get_request_wait(q, rw, NULL);
-       else
+       } else {
                rq = get_request(q, rw, NULL, gfp_mask);
+               if (!rq)
+                       spin_unlock_irq(q->queue_lock);
+       }
+       /* q->queue_lock is unlocked at this point */
 
        return rq;
 }
-
 EXPORT_SYMBOL(blk_get_request);
 
 /**
@@ -2605,9 +2613,10 @@ static int __make_request(request_queue_t *q, struct bio *bio)
 get_rq:
        /*
         * Grab a free request. This is might sleep but can not fail.
+        * Returns with the queue unlocked.
         */
-       spin_unlock_irq(q->queue_lock);
        req = get_request_wait(q, rw, bio);
+
        /*
         * After dropping the lock and possibly sleeping here, our request
         * may now be mergeable after it had proven unmergeable (above).