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Date: Mon, 22 Feb 2010 19:16:34 +0900
From: Kiyoshi Ueda <k-ueda@...jp.nec.com>
To: Stefan Bader <stefan.bader@...onical.com>,
Greg KH <gregkh@...e.de>, Alasdair G Kergon <agk@...hat.com>
CC: linux-kernel@...r.kernel.org, stable@...nel.org,
Junichi Nomura <j-nomura@...jp.nec.com>,
akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
stable-review@...nel.org, alan@...rguk.ukuu.org.uk,
Kiyoshi Ueda <k-ueda@...jp.nec.com>
Subject: Re: [Stable-review] [93/93] dm mpath: fix stall when requeueing io
Hi Alasdair, Greg,
Please replace this patch with the patch attached below.
This patch seems to have been broken somewhere, since the original
patch (*) for 2.6.33-rc6 cosmetically depends on another patch included
in 2.6.33-rc1.
(*) http://marc.info/?l=dm-devel&m=126518144727377&w=2
Stefan, thank you for spotting this.
BTW, as for your spin lock question, the spin_lock_irq could be spin_lock.
Please see my answer below for details.
On 02/22/2010 01:07 AM +0900, Stefan Bader wrote:
>> @@ -1568,12 +1575,16 @@ static void dm_request_fn(struct request
>>
>> blk_start_request(rq);
>> spin_unlock(q->queue_lock);
>> - map_request(ti, rq, md);
>> + if (map_request(ti, rq, md))
>> + goto requeued;
>> spin_lock_irq(q->queue_lock);
>> }
>
> That is the current state of dm_request_function:
>
> clone = rq->special;
> atomic_inc(&md->pending[rq_data_dir(clone)]);
>
> spin_unlock(q->queue_lock);
> if (map_request(ti, clone, md))
>
> While looking over the code I also noticed that the spinlock is dropped with
> spin_unlock and then reacquired with spin_lock_irq. Isn't the irq version too
> much in that case? Or was the intention to have interrupts enabled when unlocking?
In the current device-mapper code, I would like to go with
spin_unlock/lock here.
However, there was a case to enable irq in map_requst() for request
allocation, and this spin_lock_irq was a work-around for the case.
Now, there is no such case in the device-mapper code, so spin_lock should
be enough here. But I'm still using spin_lock_irq for safeness, since
there might be some more cases to enable irq during request submission
to underlying devices.
I'll remove the _irq in the future after lots of testings.
REVISED PATCH:
This patch fixes the problem that system may stall if target's ->map_rq
returns DM_MAPIO_REQUEUE in map_request().
E.g. stall happens on 1 CPU box when a dm-mpath device with queue_if_no_path
bounces between all-paths-down and paths-up on I/O load.
When target's ->map_rq returns DM_MAPIO_REQUEUE, map_request() requeues
the request and returns to dm_request_fn(). Then, dm_request_fn()
doesn't exit the I/O dispatching loop and continues processing
the requeued request again.
This map and requeue loop can be done with interrupt disabled,
so 1 CPU system can be stalled if this situation happens.
For example, commands below can stall my 1 CPU box within 1 minute or so:
# dmsetup table mp
mp: 0 2097152 multipath 1 queue_if_no_path 0 1 1 service-time 0 1 2 8:144 1 1
# while true; do dd if=/dev/mapper/mp of=/dev/null bs=1M count=100; done &
# while true; do \
> dmsetup message mp 0 "fail_path 8:144" \
> dmsetup suspend --noflush mp \
> dmsetup resume mp \
> dmsetup message mp 0 "reinstate_path 8:144" \
> done
To fix the problem above, this patch changes dm_request_fn() to exit
the I/O dispatching loop once if a request is requeued in map_request().
Signed-off-by: Kiyoshi Ueda <k-ueda@...jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@...jp.nec.com>
Cc: Alasdair G Kergon <agk@...hat.com>
Cc: Greg Kroah-Hartman <gregkh@...e.de>
---
drivers/md/dm.c | 21 +++++++++++++++++----
1 file changed, 17 insertions(+), 4 deletions(-)
Index: 2.6.32.8/drivers/md/dm.c
===================================================================
--- 2.6.32.8.orig/drivers/md/dm.c
+++ 2.6.32.8/drivers/md/dm.c
@@ -1487,10 +1487,15 @@ static int dm_prep_fn(struct request_que
return BLKPREP_OK;
}
-static void map_request(struct dm_target *ti, struct request *rq,
- struct mapped_device *md)
+/*
+ * Returns:
+ * 0 : the request has been processed (not requeued)
+ * !0 : the request has been requeued
+ */
+static int map_request(struct dm_target *ti, struct request *rq,
+ struct mapped_device *md)
{
- int r;
+ int r, requeued = 0;
struct request *clone = rq->special;
struct dm_rq_target_io *tio = clone->end_io_data;
@@ -1516,6 +1521,7 @@ static void map_request(struct dm_target
case DM_MAPIO_REQUEUE:
/* The target wants to requeue the I/O */
dm_requeue_unmapped_request(clone);
+ requeued = 1;
break;
default:
if (r > 0) {
@@ -1527,6 +1533,8 @@ static void map_request(struct dm_target
dm_kill_unmapped_request(clone, r);
break;
}
+
+ return requeued;
}
/*
@@ -1568,12 +1576,17 @@ static void dm_request_fn(struct request
blk_start_request(rq);
spin_unlock(q->queue_lock);
- map_request(ti, rq, md);
+ if (map_request(ti, rq, md))
+ goto requeued;
+
spin_lock_irq(q->queue_lock);
}
goto out;
+requeued:
+ spin_lock_irq(q->queue_lock);
+
plug_and_out:
if (!elv_queue_empty(q))
/* Some requests still remain, retry later */
--
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