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

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Date:   Tue, 12 Dec 2017 12:56:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.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

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.

> +	WRITE_ONCE(rq->gstate, new_val);
> +}