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From: Jan Kara <jack@suse.cz>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jan Kara <jack@suse.cz>, Khazhy Kumykov <khazhy@google.com>,
	Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org,
	Paolo Valente <paolo.valente@linaro.org>
Subject: Re: [PATCH 2/2] blk: Fix lock inversion between ioc lock and bfqd lock
Date: Fri, 21 May 2021 15:47:13 +0200	[thread overview]
Message-ID: <20210521134713.GO18952@quack2.suse.cz> (raw)
In-Reply-To: <YKe3RXOjaw4Lm2dL@T590>

On Fri 21-05-21 21:36:05, Ming Lei wrote:
> On Fri, May 21, 2021 at 02:05:51PM +0200, Jan Kara wrote:
> > On Fri 21-05-21 14:54:09, Ming Lei wrote:
> > > On Thu, May 20, 2021 at 08:29:49PM -0700, Khazhy Kumykov wrote:
> > > > On Thu, May 20, 2021 at 5:57 PM Ming Lei <ming.lei@redhat.com> wrote:
> > > > >
> > > > > On Fri, May 21, 2021 at 12:33:53AM +0200, Jan Kara wrote:
> > > > > > Lockdep complains about lock inversion between ioc->lock and bfqd->lock:
> > > > > >
> > > > > > bfqd -> ioc:
> > > > > >  put_io_context+0x33/0x90 -> ioc->lock grabbed
> > > > > >  blk_mq_free_request+0x51/0x140
> > > > > >  blk_put_request+0xe/0x10
> > > > > >  blk_attempt_req_merge+0x1d/0x30
> > > > > >  elv_attempt_insert_merge+0x56/0xa0
> > > > > >  blk_mq_sched_try_insert_merge+0x4b/0x60
> > > > > >  bfq_insert_requests+0x9e/0x18c0 -> bfqd->lock grabbed
> > > > >
> > > > > We could move blk_put_request() into scheduler code, then the lock
> > > > > inversion is avoided. So far only mq-deadline and bfq calls into
> > > > > blk_mq_sched_try_insert_merge(), and this change should be small.
> > > > 
> > > > We'd potentially be putting multiple requests if we keep the recursive merge.
> > > 
> > > Oh, we still can pass a list to hold all requests to be freed, then free
> > > them all outside in scheduler code.
> > 
> > If we cannot really get rid of the recursive merge (not yet convinced),
> > this is also an option I've considered. I was afraid what can we use in
> > struct request to attach request to a list but it seems .merged_requests
> > handlers remove the request from the queuelist already so we should be fine
> > using that.
> 
> The request has been removed from scheduler queue, and safe to free,
> so it is safe to be held in one temporary list.

Not quite, there's still ->finish_request hook that will be called from
blk_mq_free_request() on the request and e.g. BFQ performs quite a lot of
cleanup there. But yes, at least queuelist seems to be available for reuse
here.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

  reply	other threads:[~2021-05-21 13:47 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-20 22:33 [PATCH 0/2] block: Fix deadlock when merging requests with BFQ Jan Kara
2021-05-20 22:33 ` [PATCH 1/2] block: Do not merge recursively in elv_attempt_insert_merge() Jan Kara
2021-05-21  0:42   ` Ming Lei
2021-05-21 11:53     ` Jan Kara
2021-05-21 13:12       ` Ming Lei
2021-05-21 13:44         ` Jan Kara
2021-05-20 22:33 ` [PATCH 2/2] blk: Fix lock inversion between ioc lock and bfqd lock Jan Kara
2021-05-21  0:57   ` Ming Lei
2021-05-21  3:29     ` Khazhy Kumykov
2021-05-21  6:54       ` Ming Lei
2021-05-21 12:05         ` Jan Kara
2021-05-21 13:36           ` Ming Lei
2021-05-21 13:47             ` Jan Kara [this message]
2021-05-24 10:04 [PATCH 0/2 v2] block: Fix deadlock when merging requests with BFQ Jan Kara
2021-05-24 10:04 ` [PATCH 2/2] blk: Fix lock inversion between ioc lock and bfqd lock Jan Kara
2021-05-25  0:29   ` Ming Lei
2021-05-28  9:33   ` Paolo Valente
2021-05-28 12:30 [PATCH 0/2 v3] block: Fix deadlock when merging requests with BFQ Jan Kara
2021-05-28 12:30 ` [PATCH 2/2] blk: Fix lock inversion between ioc lock and bfqd lock Jan Kara
2021-06-23  9:36 [PATCH 0/2 v4] block: Fix deadlock when merging requests with BFQ Jan Kara
2021-06-23  9:36 ` [PATCH 2/2] blk: Fix lock inversion between ioc lock and bfqd lock Jan Kara

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