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Date:   Mon, 27 Apr 2020 17:36:01 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org, John Garry <john.garry@huawei.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Hannes Reinecke <hare@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>, will@kernel.org,
        peterz@infradead.org, paulmck@kernel.org
Subject: Re: [PATCH V8 07/11] blk-mq: stop to handle IO and drain IO before
 hctx becomes inactive
Message-ID: <20200427153601.GA7802@lst.de>
References: <20200424102351.475641-1-ming.lei@redhat.com> <20200424102351.475641-8-ming.lei@redhat.com> <20200424103851.GD28156@lst.de> <20200425031723.GC477579@T590> <20200425083224.GA5634@lst.de> <20200425093437.GA495669@T590> <20200425095351.GC495669@T590> <20200425154832.GA16004@lst.de> <20200426020621.GA511475@T590>
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On Sun, Apr 26, 2020 at 10:06:21AM +0800, Ming Lei wrote:
> On Sat, Apr 25, 2020 at 05:48:32PM +0200, Christoph Hellwig wrote:
> > FYI, here is what I think we should be doing (but the memory model
> > experts please correct me):
> > 
> >  - just drop the direct_issue flag and check for the CPU, which is
> >    cheap enough
> 
> That isn't correct because the CPU for running async queue may not be
> same with rq->mq_ctx->cpu since hctx->cpumask may include several CPUs
> and we run queue in RR style and it is really a normal case.

But in that case the memory barrier really doesn't matter anywaẏ.