[dm-devel] [PATCH V2] md: don't unregister sync_thread with reconfig_mutex held

[Song Liu]

On Mon, Feb 15, 2021 at 3:08 AM Paul Menzel <pmenzel at molgen.mpg.de> wrote:
>
> [+cc Donald]
>
> Am 13.02.21 um 01:49 schrieb Guoqing Jiang:
> > Unregister sync_thread doesn't need to hold reconfig_mutex since it
> > doesn't reconfigure array.
> >
> > And it could cause deadlock problem for raid5 as follows:
> >
> > 1. process A tried to reap sync thread with reconfig_mutex held after echo
> >     idle to sync_action.
> > 2. raid5 sync thread was blocked if there were too many active stripes.
> > 3. SB_CHANGE_PENDING was set (because of write IO comes from upper layer)
> >     which causes the number of active stripes can't be decreased.
> > 4. SB_CHANGE_PENDING can't be cleared since md_check_recovery was not able
> >     to hold reconfig_mutex.
> >
> > More details in the link:
> > https://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t
> >
> > And add one parameter to md_reap_sync_thread since it could be called by
> > dm-raid which doesn't hold reconfig_mutex.
> >
> > Reported-and-tested-by: Donald Buczek <buczek at molgen.mpg.de>
> > Signed-off-by: Guoqing Jiang <guoqing.jiang at cloud.ionos.com>

I don't really like this fix. But I haven't got a better (and not too
complicated)
alternative.

> > ---
> > V2:
> > 1. add one parameter to md_reap_sync_thread per Jack's suggestion.
> >
> >   drivers/md/dm-raid.c |  2 +-
> >   drivers/md/md.c      | 14 +++++++++-----
> >   drivers/md/md.h      |  2 +-
> >   3 files changed, 11 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/md/dm-raid.c b/drivers/md/dm-raid.c
> > index cab12b2..0c4cbba 100644
> > --- a/drivers/md/dm-raid.c
> > +++ b/drivers/md/dm-raid.c
> > @@ -3668,7 +3668,7 @@ static int raid_message(struct dm_target *ti, unsigned int argc, char **argv,
> >       if (!strcasecmp(argv[0], "idle") || !strcasecmp(argv[0], "frozen")) {
> >               if (mddev->sync_thread) {
> >                       set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> > -                     md_reap_sync_thread(mddev);
> > +                     md_reap_sync_thread(mddev, false);

I think we can add mddev_lock() and mddev_unlock() here and then we don't
need the extra parameter?

Thanks,
Song