linux-kernel - Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme

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Message-ID: <20171212180449.GJ3919388@devbig577.frc2.facebook.com>
Date:   Tue, 12 Dec 2017 10:04:49 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org, oleg@...hat.com,
        kernel-team@...com, osandov@...com
Subject: Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

Hello,

On Tue, Dec 12, 2017 at 12:56:57PM +0100, Peter Zijlstra wrote:
> On Sat, Dec 09, 2017 at 11:25:21AM -0800, Tejun Heo wrote:
> > +static inline void blk_mq_rq_update_state(struct request *rq,
> > +					  enum mq_rq_state state)
> > +{
> > +	u64 new_val = (rq->gstate & ~MQ_RQ_STATE_MASK) | state;
> > +
> > +	if (state == MQ_RQ_IN_FLIGHT) {
> > +		WARN_ON_ONCE(blk_mq_rq_state(rq) != MQ_RQ_IDLE);
> > +		new_val += MQ_RQ_GEN_INC;
> > +	}
> > +
> > +	/* avoid exposing interim values */
> 
> My paranoia would like to see READ_ONCE() on the rq->gstate load above
> as well, that makes it a fully explicit load-store operation.

Right now, only the request owner can update the field so there's no
data coherency issue but then again there's nothing to lose by adding
READ_ONCE there.  Will add it.

Thanks.

-- 
tejun