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From: Ming Lei <ming.lei@redhat.com>
To: David Jeffery <djeffery@redhat.com>
Cc: Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@kernel.dk>,
	Christoph Hellwig <hch@lst.de>,
	Bart Van Assche <bvanassche@acm.org>,
	stefanha@redhat.com, linux-block@vger.kernel.org,
	linux-scsi@vger.kernel.org,
	virtualization@lists.linux-foundation.org
Subject: Re: [Bug] double ->queue_rq() because of timeout in ->queue_rq()
Date: Sat, 22 Oct 2022 12:27:22 +0800	[thread overview]
Message-ID: <Y1NxKvq7MKcX6gTD@T590> (raw)
In-Reply-To: <CA+-xHTFp+gFVy6aKW2nj47+WY2+1vOLAE-X067C-hm4_8ngA6g@mail.gmail.com>

On Fri, Oct 21, 2022 at 02:33:21PM -0400, David Jeffery wrote:
> On Fri, Oct 21, 2022 at 11:22 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On Fri, Oct 21, 2022 at 08:32:31AM -0600, Keith Busch wrote:
> > >
> > > I agree with your idea that this is a lower level driver responsibility:
> > > it should reclaim all started requests before allowing new queuing.
> > > Perhaps the block layer should also raise a clear warning if it's
> > > queueing a request that's already started.
> >
> > The thing is that it is one generic issue, lots of VM drivers could be
> > affected, and it may not be easy for drivers to handle the race too.
> >
> 
> While virtual systems are a common source of the problem, fully
> preempt kernels (with or without real-time patches) can also trigger
> this condition rather simply with a poorly behaved real-time task. The
> involuntary preemption means the queue_rq call can be stopped to let
> another task run. Poorly behaving tasks claiming the CPU for longer
> than the request timeout when preempting a task in a queue_rq function
> could cause the condition on real or virtual hardware. So it's not
> just VM related drivers that are affected by the race.

In theory, yes. But ->queue_rq() is in rcu read critical area, and
usually CONFIG_RCU_BOOST is enabled for covering this problem otherwise
OOM can be triggered easily too.

I guess it is hard to trigger it in real hardware with preempt kernel.


Thanks,
Ming


WARNING: multiple messages have this Message-ID (diff)
From: Ming Lei <ming.lei@redhat.com>
To: David Jeffery <djeffery@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	Bart Van Assche <bvanassche@acm.org>,
	linux-scsi@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	linux-block@vger.kernel.org, stefanha@redhat.com,
	Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>
Subject: Re: [Bug] double ->queue_rq() because of timeout in ->queue_rq()
Date: Sat, 22 Oct 2022 12:27:22 +0800	[thread overview]
Message-ID: <Y1NxKvq7MKcX6gTD@T590> (raw)
In-Reply-To: <CA+-xHTFp+gFVy6aKW2nj47+WY2+1vOLAE-X067C-hm4_8ngA6g@mail.gmail.com>

On Fri, Oct 21, 2022 at 02:33:21PM -0400, David Jeffery wrote:
> On Fri, Oct 21, 2022 at 11:22 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On Fri, Oct 21, 2022 at 08:32:31AM -0600, Keith Busch wrote:
> > >
> > > I agree with your idea that this is a lower level driver responsibility:
> > > it should reclaim all started requests before allowing new queuing.
> > > Perhaps the block layer should also raise a clear warning if it's
> > > queueing a request that's already started.
> >
> > The thing is that it is one generic issue, lots of VM drivers could be
> > affected, and it may not be easy for drivers to handle the race too.
> >
> 
> While virtual systems are a common source of the problem, fully
> preempt kernels (with or without real-time patches) can also trigger
> this condition rather simply with a poorly behaved real-time task. The
> involuntary preemption means the queue_rq call can be stopped to let
> another task run. Poorly behaving tasks claiming the CPU for longer
> than the request timeout when preempting a task in a queue_rq function
> could cause the condition on real or virtual hardware. So it's not
> just VM related drivers that are affected by the race.

In theory, yes. But ->queue_rq() is in rcu read critical area, and
usually CONFIG_RCU_BOOST is enabled for covering this problem otherwise
OOM can be triggered easily too.

I guess it is hard to trigger it in real hardware with preempt kernel.


Thanks,
Ming

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

  reply	other threads:[~2022-10-22  4:28 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-20  9:10 [Bug] double ->queue_rq() because of timeout in ->queue_rq() Ming Lei
2022-10-20  9:10 ` Ming Lei
2022-10-20 20:01 ` Stefan Hajnoczi
2022-10-20 20:01   ` Stefan Hajnoczi
2022-10-21  2:23   ` Ming Lei
2022-10-21  2:23     ` Ming Lei
2022-10-24 15:30     ` Stefan Hajnoczi
2022-10-24 15:30       ` Stefan Hajnoczi
2022-10-24 15:41       ` Ming Lei
2022-10-24 15:41         ` Ming Lei
2022-10-20 20:26 ` Bart Van Assche
2022-10-20 20:26   ` Bart Van Assche
2022-10-21  0:57   ` Ming Lei
2022-10-21  0:57     ` Ming Lei
2022-10-21 14:32 ` Keith Busch
2022-10-21 15:22   ` Ming Lei
2022-10-21 15:22     ` Ming Lei
2022-10-21 18:33     ` David Jeffery
2022-10-22  4:27       ` Ming Lei [this message]
2022-10-22  4:27         ` Ming Lei
2022-10-21 18:21   ` David Jeffery

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