linux-kernel - Re: [PATCH 3/8] blk-mq: replace timeout synchronization with a RCU and generation based scheme

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Date:   Mon, 8 Jan 2018 23:29:11 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "jbacik@...com" <jbacik@...com>, "tj@...nel.org" <tj@...nel.org>,
        "jack@...e.cz" <jack@...e.cz>, "clm@...com" <clm@...com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
CC:     "kernel-team@...com" <kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "hch@....de" <hch@....de>
Subject: Re: [PATCH 3/8] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

On Mon, 2018-01-08 at 11:15 -0800, Tejun Heo wrote:
> @@ -230,6 +232,27 @@ struct request {
>  
>  	unsigned short write_hint;
>  
> +	/*
> +	 * On blk-mq, the lower bits of ->gstate carry the MQ_RQ_* state
> +	 * value and the upper bits the generation number which is
> +	 * monotonically incremented and used to distinguish the reuse
> +	 * instances.
> +	 *
> +	 * ->gstate_seq allows updates to ->gstate and other fields
> +	 * (currently ->deadline) during request start to be read
> +	 * atomically from the timeout path, so that it can operate on a
> +	 * coherent set of information.
> +	 */
> +	seqcount_t gstate_seq;
> +	u64 gstate;
> +
> +	/*
> +	 * ->aborted_gstate is used by the timeout to claim a specific
> +	 * recycle instance of this request.  See blk_mq_timeout_work().
> +	 */
> +	struct u64_stats_sync aborted_gstate_sync;
> +	u64 aborted_gstate;
> +
>  	unsigned long deadline;
>  	struct list_head timeout_list;

Does "gstate" perhaps stand for "generation number and state"? If so, please
mention this in one of the above comments.

Thanks,

Bart.