Hi Alasdair,
I recently fixed a deadlock for a customer we have with
the below patch. I have queued it up in my tree as not
to lose it. Can I have an ack from you, or do you want
to submit it yourself? I've marked it stable as well.
From 8ca211056519ac06bc96fb134dca1f8eb2141407 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Tue, 6 Nov 2012 12:24:26 +0100
Subject: [PATCH] dm: fix deadlock with request based dm and queue request_fn
recursion
Request based dm attempts to re-run the request queue off the
request completion path. If used with a driver that potentially does
end_io from its request_fn, we could deadlock trying to recurse
back into request dispatch. Fix this by punting the request queue
run to kblockd.
Tested to fix a quickly reproducible deadlock in such a scenario.
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
drivers/md/dm.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
(Cc to dm-devel)
On 11/06/12 20:29, Jens Axboe wrote:
I recently fixed a deadlock for a customer we have with
the below patch. I have queued it up in my tree as not
to lose it. Can I have an ack from you, or do you want
to submit it yourself? I've marked it stable as well.
Could you tell details about how the deadlock happened?
dm's end_io always throws the completion handling to softirq:
end_clone_request()
dm_complete_request()
blk_complete_request()
then it is processed in softirq context:
dm_softirq_done()
dm_done()
dm_end_request()
rq_completed(run_queue=true)
blk_run_queue()
Since queue_lock is always held with interrupt disabled,
I couldn't see why it could deadlock.
Request-based dm followed the completion model of scsi
mid layer. So similar code path exists in scsi, too.
For example:
scsi_request_fn()
scsi_kill_request()
blk_complete_request()
then:
scsi_softirq_done()
scsi_io_completion()
scsi_next_command()
scsi_run_queue()
spin_lock queue_lock
__blk_run_queue()
If calling run-queue from softirq_done_fn() can cause deadlock,
I'm afraid the problem is not limited to dm.
quoted hunk
From 8ca211056519ac06bc96fb134dca1f8eb2141407 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Tue, 6 Nov 2012 12:24:26 +0100
Subject: [PATCH] dm: fix deadlock with request based dm and queue request_fn
recursion
Request based dm attempts to re-run the request queue off the
request completion path. If used with a driver that potentially does
end_io from its request_fn, we could deadlock trying to recurse
back into request dispatch. Fix this by punting the request queue
run to kblockd.
Tested to fix a quickly reproducible deadlock in such a scenario.
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
drivers/md/dm.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
(Cc to dm-devel)
On 11/06/12 20:29, Jens Axboe wrote:
quoted
I recently fixed a deadlock for a customer we have with
the below patch. I have queued it up in my tree as not
to lose it. Can I have an ack from you, or do you want
to submit it yourself? I've marked it stable as well.
Could you tell details about how the deadlock happened?
dm's end_io always throws the completion handling to softirq:
end_clone_request()
dm_complete_request()
blk_complete_request()
then it is processed in softirq context:
dm_softirq_done()
dm_done()
dm_end_request()
rq_completed(run_queue=true)
blk_run_queue()
Since queue_lock is always held with interrupt disabled,
I couldn't see why it could deadlock.
Request-based dm followed the completion model of scsi
mid layer. So similar code path exists in scsi, too.
For example:
scsi_request_fn()
scsi_kill_request()
blk_complete_request()
then:
scsi_softirq_done()
scsi_io_completion()
scsi_next_command()
scsi_run_queue()
spin_lock queue_lock
__blk_run_queue()
If calling run-queue from softirq_done_fn() can cause deadlock,
I'm afraid the problem is not limited to dm.
dm_softirq_done()
rq_completed()
blk_run_queue()
dm_request_fn()
dm_dispatch_request()
blk_insert_cloned_request()
__elv_add_request()
elv_insert()
blk_run_queue()
...
Basically you recurse back into the request handler, since it ends up
running the queue. But I see I've been too focused on the older release,
since we don't actually do that anymore after the plugging rewrite. So
it should actually be safe in current kernels and hence the patch only
needed for the stable series where that is not the case.
--
Jens Axboe
(Cc to dm-devel)
On 11/06/12 20:29, Jens Axboe wrote:
quoted
I recently fixed a deadlock for a customer we have with
the below patch. I have queued it up in my tree as not
to lose it. Can I have an ack from you, or do you want
to submit it yourself? I've marked it stable as well.
Could you tell details about how the deadlock happened?
dm's end_io always throws the completion handling to softirq:
end_clone_request()
dm_complete_request()
blk_complete_request()
then it is processed in softirq context:
dm_softirq_done()
dm_done()
dm_end_request()
rq_completed(run_queue=true)
blk_run_queue()
Since queue_lock is always held with interrupt disabled,
I couldn't see why it could deadlock.
Request-based dm followed the completion model of scsi
mid layer. So similar code path exists in scsi, too.
For example:
scsi_request_fn()
scsi_kill_request()
blk_complete_request()
then:
scsi_softirq_done()
scsi_io_completion()
scsi_next_command()
scsi_run_queue()
spin_lock queue_lock
__blk_run_queue()
If calling run-queue from softirq_done_fn() can cause deadlock,
I'm afraid the problem is not limited to dm.
...
Basically you recurse back into the request handler, since it ends up
running the queue.
But the queues are different as commented inline above.
So it should be ok. (from deadlock point of view)
But I see I've been too focused on the older release,
since we don't actually do that anymore after the plugging rewrite. So
it should actually be safe in current kernels and hence the patch only
needed for the stable series where that is not the case.
BTW, using blk_run_queue_async() in softirq_done_fn() might be good
from performance point of view? It may reduce latency of the softirq
and have an effect of batching request_fn calls.
--
Jun'ichi Nomura, NEC Corporation
(Cc to dm-devel)
On 11/06/12 20:29, Jens Axboe wrote:
quoted
I recently fixed a deadlock for a customer we have with
the below patch. I have queued it up in my tree as not
to lose it. Can I have an ack from you, or do you want
to submit it yourself? I've marked it stable as well.
Could you tell details about how the deadlock happened?
dm's end_io always throws the completion handling to softirq:
end_clone_request()
dm_complete_request()
blk_complete_request()
then it is processed in softirq context:
dm_softirq_done()
dm_done()
dm_end_request()
rq_completed(run_queue=true)
blk_run_queue()
Since queue_lock is always held with interrupt disabled,
I couldn't see why it could deadlock.
Request-based dm followed the completion model of scsi
mid layer. So similar code path exists in scsi, too.
For example:
scsi_request_fn()
scsi_kill_request()
blk_complete_request()
then:
scsi_softirq_done()
scsi_io_completion()
scsi_next_command()
scsi_run_queue()
spin_lock queue_lock
__blk_run_queue()
If calling run-queue from softirq_done_fn() can cause deadlock,
I'm afraid the problem is not limited to dm.
...
Basically you recurse back into the request handler, since it ends up
running the queue.
But the queues are different as commented inline above.
So it should be ok. (from deadlock point of view)
It's still not OK, some drivers end up doing spin_unlock_irq() in their
request_fn. Running unknown request_fn from ipi/irq is a bad idea, imho,
it'll quickly cause problems.
quoted
But I see I've been too focused on the older release,
since we don't actually do that anymore after the plugging rewrite. So
it should actually be safe in current kernels and hence the patch only
needed for the stable series where that is not the case.
BTW, using blk_run_queue_async() in softirq_done_fn() might be good
from performance point of view? It may reduce latency of the softirq
and have an effect of batching request_fn calls.
Yes, I argued the same for the original people who saw the problem. It
is quite an extensive and expensive path to have off the IPI handler. So
from that point of view, I'd recommend the patch as well.
--
Jens Axboe
(Cc to dm-devel)
On 11/06/12 20:29, Jens Axboe wrote:
quoted
I recently fixed a deadlock for a customer we have with
the below patch. I have queued it up in my tree as not
to lose it. Can I have an ack from you, or do you want
to submit it yourself? I've marked it stable as well.
Could you tell details about how the deadlock happened?
dm's end_io always throws the completion handling to softirq:
end_clone_request()
dm_complete_request()
blk_complete_request()
then it is processed in softirq context:
dm_softirq_done()
dm_done()
dm_end_request()
rq_completed(run_queue=true)
blk_run_queue()
Since queue_lock is always held with interrupt disabled,
I couldn't see why it could deadlock.
Request-based dm followed the completion model of scsi
mid layer. So similar code path exists in scsi, too.
For example:
scsi_request_fn()
scsi_kill_request()
blk_complete_request()
then:
scsi_softirq_done()
scsi_io_completion()
scsi_next_command()
scsi_run_queue()
spin_lock queue_lock
__blk_run_queue()
If calling run-queue from softirq_done_fn() can cause deadlock,
I'm afraid the problem is not limited to dm.
...
Basically you recurse back into the request handler, since it ends up
running the queue.
But the queues are different as commented inline above.
So it should be ok. (from deadlock point of view)
It's still not OK, some drivers end up doing spin_unlock_irq() in their
request_fn. Running unknown request_fn from ipi/irq is a bad idea, imho,
it'll quickly cause problems.
Though I still don't think there is actual deadlock because
dm's queue lock is released before dm_dispatch_request(),
I see your point why it's potentially bad.
quoted
quoted
But I see I've been too focused on the older release,
since we don't actually do that anymore after the plugging rewrite. So
it should actually be safe in current kernels and hence the patch only
needed for the stable series where that is not the case.
BTW, using blk_run_queue_async() in softirq_done_fn() might be good
from performance point of view? It may reduce latency of the softirq
and have an effect of batching request_fn calls.
Yes, I argued the same for the original people who saw the problem. It
is quite an extensive and expensive path to have off the IPI handler. So
from that point of view, I'd recommend the patch as well.
Thank you for confirmation.
If it's good for performance, the patch is fine with me.
--
Jun'ichi Nomura, NEC Corporation