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 11:10:52 +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:
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 8089ca1..e6cfe4b3 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -228,6 +230,27 @@ 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_seqc 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_seqc;
> +	u64 gstate;

We typically name seqcount_t thingies _seq.