Re: [PATCH v3 5/5] md/raid10: convert resync_lock to use seqlock

[Guoqing Jiang <guoqing.jiang@linux.dev>]

On 9/16/22 7:34 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
>
> Currently, wait_barrier() will hold 'resync_lock' to read 'conf->barrier',
> and io can't be dispatched until 'barrier' is dropped.
>
> Since holding the 'barrier' is not common, convert 'resync_lock' to use
> seqlock so that holding lock can be avoided in fast path.
>
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>   drivers/md/raid10.c | 87 ++++++++++++++++++++++++++++++---------------
>   drivers/md/raid10.h |  2 +-
>   2 files changed, 59 insertions(+), 30 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 9a28abd19709..2daa7d57034c 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -79,6 +79,21 @@ static void end_reshape(struct r10conf *conf);
>   
>   #include "raid1-10.c"
>   
> +#define NULL_CMD
> +#define cmd_before(conf, cmd) \
> +	do { \
> +		write_sequnlock_irq(&(conf)->resync_lock); \
> +		cmd; \
> +	} while (0)
> +#define cmd_after(conf) write_seqlock_irq(&(conf)->resync_lock)

The two is not paired well given only cmd_before needs the 'cmd'.

> +
> +#define wait_event_barrier_cmd(conf, cond, cmd) \
> +	wait_event_cmd((conf)->wait_barrier, cond, cmd_before(conf, cmd), \
> +		       cmd_after(conf))
> +
> +#define wait_event_barrier(conf, cond) \
> +	wait_event_barrier_cmd(conf, cond, NULL_CMD)

What is the issue without define NULL_CMD?

Thanks,
Guoqing