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

[Guoqing Jiang]

On 9/19/22 9:08 AM, Yu Kuai wrote:
> Hi,
>
> 在 2022/09/18 19:36, Guoqing Jiang 写道:
> > On 9/16/22 7:34 PM, Yu Kuai wrote:
> > > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > ---
> > >   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'.
>
> May be should I just remove cmd_after?

I'd remove it but just my personal flavor.

> > > +
> > > +#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?
>
> Checkpatch will complain this:
>
> ERROR: space prohibited before that close parenthesis ')'
> #38: FILE: drivers/md/raid10.c:94:
> +       wait_event_barrier_cmd(conf, cond, )
>
> total: 1 errors, 0 warnings, 169 lines checked

Hmm, freeze_array has a different usage for it, so two cmds before sleep
and one cmd after sleep, perhaps it is the best way for now.

Thanks,
Guoqing