When cfq I/O scheduler is selected, get_request() in __make_request() calls

__cfq_get_queue().  __cfq_get_queue() finds an existing queue (struct
cfq_queue) of the current process for the device and returns it.  If it's not
found, __cfq_get_queue() creates and returns a new one if __cfq_get_queue() is
called with __GFP_WAIT flag, or __cfq_get_queue() returns NULL (this means that
get_request() fails) if no __GFP_WAIT flag.

On the other hand, in __make_request(), get_request() is called without
__GFP_WAIT flag at the first time.  Thus, the get_request() fails when there is
no existing queue, typically when it's called for the first I/O request of the
process to the device.

Though it will be followed by get_request_wait() for general case,
__make_request() will just end the I/O with an error (EWOULDBLOCK) when the
request was for read-ahead.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>

drivers/block/cfq-iosched.c

@@ -1202,13 +1202,16 @@ __cfq_get_queue(struct cfq_data *cfqd, unsigned long key, int gfp_mask)
 		if (new_cfqq) {
 			cfqq = new_cfqq;
 			new_cfqq = NULL;
-		} else if (gfp_mask & __GFP_WAIT) {
+		} else {
 			spin_unlock_irq(cfqd->queue->queue_lock);
 			new_cfqq = kmem_cache_alloc(cfq_pool, gfp_mask);
 			spin_lock_irq(cfqd->queue->queue_lock);
+
+			if (!new_cfqq && !(gfp_mask & __GFP_WAIT))
+				goto out;
+
 			goto retry;
-		} else
-			goto out;
+		}
 
 		memset(cfqq, 0, sizeof(*cfqq));