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Date:	Sun, 09 Nov 2014 20:16:30 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 0/2] blk-mq: switch from preempt_disable/enable to get/put_cpu

On 2014-11-07 15:03, Paolo Bonzini wrote:
> blk-mq is using preempt_disable/enable in order to ensure that the
> queue runners are placed on the right CPU.  This does not work with
> the RT patches, because __blk_mq_run_hw_queue takes a non-raw
> spinlock with the preemption-disabled region.  If there is contention
> on the lock, this violates the rules for preemption-disabled regions.
>
> While this could be fixed easily within the RT patches just by doing
> migrate_disable/enable (note: this was not tested :)), we can do
> better.  The first patch concentrates the preempt_disable/enable in
> a single place in blk_mq_run_hw_queue, and also avoids useless calls
> when the caller wants to start __blk_mq_run_hw_queue asynchronously.
> (There is already a call to __blk_mq_run_hw_queue that does not disable
> preemption in blk-flush.c; not coincidentially, it passes async=true).
>
> Once this is done, it is trivial to use get/put_cpu instead of
> preempt_disable/smp_processor_id/preempt_enable, which is what the
> second patch does.  The RT patches then can change this to use
> get_cpu_light.
>
> With these changes (and the additional switch to get_cpu_light),
> virtio-blk can be used again with RT kernels.

These both look good. I'll double check and run through some testing, 
then apply for 3.19 if it all passes.


-- 
Jens Axboe

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