[PATCH] cfq-iosched.c: minor fixes

One critical fix and two minor fixes for 2.6.13-rc7:

- Max depth must currently be 2 to allow barriers to function on SCSI
- Prefer sync request over async in choosing the next request
- Never allow async request to preempt or disturb the "anticipation" for
  a single cfq process context. This is as-designed, the code right now
  is buggy in that area.

Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 21 insertions, 10 deletions

diff --git a/drivers/block/cfq-iosched.c b/drivers/block/cfq-iosched.c
index 2435a7c99b2b..cd056e7e64ec 100644
--- a/drivers/block/cfq-iosched.c
+++ b/drivers/block/cfq-iosched.c
@@ -47,7 +47,7 @@ static int cfq_slice_idle = HZ / 100;
 /*
  * disable queueing at the driver/hardware level
  */
-static int cfq_max_depth = 1;
+static int cfq_max_depth = 2;
 
 /*
  * for the hash of cfqq inside the cfqd
@@ -385,9 +385,15 @@ cfq_choose_req(struct cfq_data *cfqd, struct cfq_rq *crq1, struct cfq_rq *crq2)
                 return crq2;
         if (crq2 == NULL)
                 return crq1;
-        if (cfq_crq_requeued(crq1))
+
+        if (cfq_crq_requeued(crq1) && !cfq_crq_requeued(crq2))
                 return crq1;
-        if (cfq_crq_requeued(crq2))
+        else if (cfq_crq_requeued(crq2) && !cfq_crq_requeued(crq1))
+                return crq2;
+
+        if (cfq_crq_is_sync(crq1) && !cfq_crq_is_sync(crq2))
+                return crq1;
+        else if (cfq_crq_is_sync(crq2) && !cfq_crq_is_sync(crq1))
                 return crq2;
 
         s1 = crq1->request->sector;
@@ -1769,18 +1775,23 @@ static void
 cfq_crq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                  struct cfq_rq *crq)
 {
-        const int sync = cfq_crq_is_sync(crq);
+        struct cfq_io_context *cic;
 
         cfqq->next_crq = cfq_choose_req(cfqd, cfqq->next_crq, crq);
 
-        if (sync) {
-                struct cfq_io_context *cic = crq->io_context;
+        /*
+         * we never wait for an async request and we don't allow preemption
+         * of an async request. so just return early
+         */
+        if (!cfq_crq_is_sync(crq))
+                return;
 
-                cfq_update_io_thinktime(cfqd, cic);
-                cfq_update_idle_window(cfqd, cfqq, cic);
+        cic = crq->io_context;
 
-                cic->last_queue = jiffies;
-        }
+        cfq_update_io_thinktime(cfqd, cic);
+        cfq_update_idle_window(cfqd, cfqq, cic);
+
+        cic->last_queue = jiffies;
 
         if (cfqq == cfqd->active_queue) {
                 /*