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

[Peter Zijlstra <peterz@infradead.org>]

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.