This is a note to let you know that I've just added the patch titled
block: __elv_next_request() shouldn't call into the elevator if bypassing
to the 3.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
block-__elv_next_request-shouldn-t-call-into-the-elevator-if-bypassing.patch
and it can be found in the queue-3.12 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 556ee818c06f37b2e583af0363e6b16d0e0270de Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@xxxxxxxxxx>
Date: Wed, 29 Jan 2014 14:56:16 -0700
Subject: block: __elv_next_request() shouldn't call into the elevator if bypassing
From: Tejun Heo <tj@xxxxxxxxxx>
commit 556ee818c06f37b2e583af0363e6b16d0e0270de upstream.
request_queue bypassing is used to suppress higher-level function of a
request_queue so that they can be switched, reconfigured and shut
down. A request_queue does the followings while bypassing.
* bypasses elevator and io_cq association and queues requests directly
to the FIFO dispatch queue.
* bypasses block cgroup request_list lookup and always uses the root
request_list.
Once confirmed to be bypassing, specific elevator and block cgroup
policy implementations can assume that nothing is in flight for them
and perform various operations which would be dangerous otherwise.
Such confirmation is acheived by short-circuiting all new requests
directly to the dispatch queue and waiting for all the requests which
were issued before to finish. Unfortunately, while the request
allocating and draining sides were properly handled, we forgot to
actually plug the request dispatch path. Even after bypassing mode is
confirmed, if the attached driver tries to fetch a request and the
dispatch queue is empty, __elv_next_request() would invoke the current
elevator's elevator_dispatch_fn() callback. As all in-flight requests
were drained, the elevator wouldn't contain any request but once
bypass is confirmed we don't even know whether the elevator is even
there. It might be in the process of being switched and half torn
down.
Frank Mayhar reports that this actually happened while switching
elevators, leading to an oops.
Let's fix it by making __elv_next_request() avoid invoking the
elevator_dispatch_fn() callback if the queue is bypassing. It already
avoids invoking the callback if the queue is dying. As a dying queue
is guaranteed to be bypassing, we can simply replace blk_queue_dying()
check with blk_queue_bypass().
Reported-by: Frank Mayhar <fmayhar@xxxxxxxxxx>
References: http://lkml.kernel.org/g/1390319905.20232.38.camel@xxxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: Frank Mayhar <fmayhar@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
block/blk.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/block/blk.h
+++ b/block/blk.h
@@ -96,7 +96,7 @@ static inline struct request *__elv_next
q->flush_queue_delayed = 1;
return NULL;
}
- if (unlikely(blk_queue_dying(q)) ||
+ if (unlikely(blk_queue_bypass(q)) ||
!q->elevator->type->ops.elevator_dispatch_fn(q, 0))
return NULL;
}
Patches currently in stable-queue which might be from tj@xxxxxxxxxx are
queue-3.12/block-__elv_next_request-shouldn-t-call-into-the-elevator-if-bypassing.patch
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