Re: [PATCH -next v3 03/25] md: add new helpers to suspend/resume array

[Song Liu]

On Wed, Oct 4, 2023 at 7:59 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> 在 2023/09/29 2:45, Song Liu 写道:
> > On Wed, Sep 27, 2023 at 11:22 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> >>
> >> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >>
> >> Advantages for new apis:
> >>   - reconfig_mutex is not required;
> >>   - the weird logical that suspend array hold 'reconfig_mutex' for
> >>     mddev_check_recovery() to update superblock is not needed;
> >>   - the specail handling, 'pers->prepare_suspend', for raid456 is not
> >>     needed;
> >>   - It's safe to be called at any time once mddev is allocated, and it's
> >>     designed to be used from slow path where array configuration is changed;
> >>   - the new helpers is designed to be called before mddev_lock(), hence
> >>     it support to be interrupted by user as well.
> >>
> >> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> >> ---
> >>   drivers/md/md.c | 102 +++++++++++++++++++++++++++++++++++++++++++++++-
> >>   drivers/md/md.h |   3 ++
> >>   2 files changed, 103 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/md/md.c b/drivers/md/md.c
> >> index e460b380143d..a075d03d03d3 100644
> >> --- a/drivers/md/md.c
> >> +++ b/drivers/md/md.c
> >> @@ -443,12 +443,22 @@ void mddev_suspend(struct mddev *mddev)
> >>                          lockdep_is_held(&mddev->reconfig_mutex));
> >>
> >>          WARN_ON_ONCE(thread && current == thread->tsk);
> >> -       if (mddev->suspended++)
> >> +
> >> +       /* can't concurrent with __mddev_suspend() and __mddev_resume() */
> >> +       mutex_lock(&mddev->suspend_mutex);
> >> +       if (mddev->suspended++) {
> >> +               mutex_unlock(&mddev->suspend_mutex);
> >>                  return;
> >
> > Can we make mddev->suspended atomic_t, and use atomic_inc_return()
> > here?
>
> 'suspend_mutex' is needed, because concurrent caller of
> mddev_suspend() shound be ordered, they need to wait for the first
> mddev_suspend() to be done.
>
> Updating suspended is protected by 'suspend_mutex' in the new api, so I
> think it's not necessary to use atomic, WRITE/READ_ONCE() should be
> enough.

Thanks for the explanation.

Song