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Date: Wed, 18 Jan 2012 17:31:06 +0100
From: Jens Axboe <axboe@...nel.dk>
To: Tejun Heo <tj@...nel.org>
CC: Vivek Goyal <vgoyal@...hat.com>, Shaohua Li <shaohua.li@...el.com>,
linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: Kernel crash in icq_free_icq_rcu
On 2012-01-18 17:24, Jens Axboe wrote:
> On 2012-01-18 17:09, Tejun Heo wrote:
>> On Wed, Jan 18, 2012 at 09:20:05AM -0500, Vivek Goyal wrote:
>>>> Not allocating icq if request is never going to go to elevator as elevator
>>>> switch was happening makes sense to me.
>>>>
>>>> I tried this patch. It went little further and crashed at a different
>>>> place. I think this seems to be separate merging issue Tejun is trying
>>>> to track down.
>>>
>>> Applied Tejun's debug patch to return early and not call into elevator
>>> for checking whether merge is allowed or not. Things seems to be stable
>>> now for me.
>>
>> Yeah, plug merge is calling into elevator code without any
>> synchronization, so it's bound to be broken. Given plugging is
>> per-task, I don't think we really need to query elevator about merging
>> bio's. The request is not on elevator and plugging is part of issuing
>> mechanism, not scheduling, after all. Jens, what do you think?
>
> Hmmm. We can bypass asking the elevator, as long as we query the
> restrictions. Does the below, by itself, resolve the crash? If yes, let
> me cook up a patch splitting the elv and blk rq merging logic.
Something like the below, completely untested.
But thinking about this a bit while doing it, why is the IO scheduler
going away while we have plugged requests that are elvpriv?
diff --git a/block/blk-core.c b/block/blk-core.c
index e6c05a9..75eba5c 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1283,7 +1283,7 @@ static bool attempt_plug_merge(struct request_queue *q, struct bio *bio,
if (rq->q != q)
continue;
- el_ret = elv_try_merge(rq, bio);
+ el_ret = blk_try_merge(rq, bio);
if (el_ret == ELEVATOR_BACK_MERGE) {
ret = bio_attempt_back_merge(q, rq, bio);
if (ret)
diff --git a/block/blk-merge.c b/block/blk-merge.c
index cfcc37c..ee9ec90 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -471,3 +471,59 @@ int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
{
return attempt_merge(q, rq, next);
}
+
+int blk_rq_merge_ok(struct request *rq, struct bio *bio)
+{
+ if (!rq_mergeable(rq))
+ return 0;
+
+ /*
+ * Don't merge file system requests and discard requests
+ */
+ if ((bio->bi_rw & REQ_DISCARD) != (rq->bio->bi_rw & REQ_DISCARD))
+ return 0;
+
+ /*
+ * Don't merge discard requests and secure discard requests
+ */
+ if ((bio->bi_rw & REQ_SECURE) != (rq->bio->bi_rw & REQ_SECURE))
+ return 0;
+
+ /*
+ * different data direction or already started, don't merge
+ */
+ if (bio_data_dir(bio) != rq_data_dir(rq))
+ return 0;
+
+ /*
+ * must be same device and not a special request
+ */
+ if (rq->rq_disk != bio->bi_bdev->bd_disk || rq->special)
+ return 0;
+
+ /*
+ * only merge integrity protected bio into ditto rq
+ */
+ if (bio_integrity(bio) != blk_integrity_rq(rq))
+ return 0;
+
+ return 1;
+}
+EXPORT_SYMBOL(blk_rq_merge_ok);
+
+int blk_try_merge(struct request *__rq, struct bio *bio)
+{
+ int ret = ELEVATOR_NO_MERGE;
+
+ /*
+ * we can merge and sequence is ok, check if it's possible
+ */
+ if (blk_rq_merge_ok(__rq, bio)) {
+ if (blk_rq_pos(__rq) + blk_rq_sectors(__rq) == bio->bi_sector)
+ ret = ELEVATOR_BACK_MERGE;
+ else if (blk_rq_pos(__rq) - bio_sectors(bio) == bio->bi_sector)
+ ret = ELEVATOR_FRONT_MERGE;
+ }
+
+ return ret;
+}
diff --git a/block/blk.h b/block/blk.h
index 7efd772..a117fa9 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -137,6 +137,8 @@ int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
struct request *next);
void blk_recalc_rq_segments(struct request *rq);
void blk_rq_set_mixed_merge(struct request *rq);
+int blk_rq_merge_ok(struct request *rq, struct bio *bio);
+int blk_try_merge(struct request *rq, struct bio *bio);
void blk_queue_congestion_threshold(struct request_queue *q);
diff --git a/block/elevator.c b/block/elevator.c
index 91e18f8..a1a75f7 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -72,39 +72,8 @@ static int elv_iosched_allow_merge(struct request *rq, struct bio *bio)
*/
int elv_rq_merge_ok(struct request *rq, struct bio *bio)
{
- if (!rq_mergeable(rq))
+ if (!blk_rq_merge_ok(rq, bio))
return 0;
-
- /*
- * Don't merge file system requests and discard requests
- */
- if ((bio->bi_rw & REQ_DISCARD) != (rq->bio->bi_rw & REQ_DISCARD))
- return 0;
-
- /*
- * Don't merge discard requests and secure discard requests
- */
- if ((bio->bi_rw & REQ_SECURE) != (rq->bio->bi_rw & REQ_SECURE))
- return 0;
-
- /*
- * different data direction or already started, don't merge
- */
- if (bio_data_dir(bio) != rq_data_dir(rq))
- return 0;
-
- /*
- * must be same device and not a special request
- */
- if (rq->rq_disk != bio->bi_bdev->bd_disk || rq->special)
- return 0;
-
- /*
- * only merge integrity protected bio into ditto rq
- */
- if (bio_integrity(bio) != blk_integrity_rq(rq))
- return 0;
-
if (!elv_iosched_allow_merge(rq, bio))
return 0;
@@ -114,19 +83,13 @@ EXPORT_SYMBOL(elv_rq_merge_ok);
int elv_try_merge(struct request *__rq, struct bio *bio)
{
- int ret = ELEVATOR_NO_MERGE;
-
/*
* we can merge and sequence is ok, check if it's possible
*/
- if (elv_rq_merge_ok(__rq, bio)) {
- if (blk_rq_pos(__rq) + blk_rq_sectors(__rq) == bio->bi_sector)
- ret = ELEVATOR_BACK_MERGE;
- else if (blk_rq_pos(__rq) - bio_sectors(bio) == bio->bi_sector)
- ret = ELEVATOR_FRONT_MERGE;
- }
+ if (elv_rq_merge_ok(__rq, bio))
+ return blk_try_merge(__rq, bio);
- return ret;
+ return ELEVATOR_NO_MERGE;
}
static struct elevator_type *elevator_find(const char *name)
--
Jens Axboe
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