Re: [PATCH -next] md: split MD_RECOVERY_NEEDED out of mddev_resume

[Song Liu]

On Wed, Dec 6, 2023 at 3:36 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> 在 2023/12/06 16:30, Song Liu 写道:
> > On Sun, Dec 3, 2023 at 7:18 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> >>
> >> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >>
> >> New mddev_resume() calls are added to synchroniza IO with array
> >> reconfiguration, however, this introduce a regression while adding it in
> >> md_start_sync():
> >>
> >> 1) someone set MD_RECOVERY_NEEDED first;
> >> 2) daemon thread grab reconfig_mutex, then clear MD_RECOVERY_NEEDED and
> >>     queue a new sync work;
> >> 3) daemon thread release reconfig_mutex;
> >> 4) in md_start_sync
> >>     a) check that there are spares that can be added/removed, then suspend
> >>        the array;
> >>     b) remove_and_add_spares may not be called, or called without really
> >>        add/remove spares;
> >>     c) resume the array, then set MD_RECOVERY_NEEDED again!
> >>
> >> Loop between 2 - 4, then mddev_suspend() will be called quite often, for
> >> consequence, normal IO will be quite slow.
> >>
> >> Fix this problem by spliting MD_RECOVERY_NEEDED out of mddev_resume(), so
> >> that md_start_sync() won't set such flag and hence the loop will be broken.
> >
> > I hope we don't leak set_bit MD_RECOVERY_NEEDED to all call
> > sites of mddev_resume().
>
> There are also some other mddev_resume() that is added later and don't
> need recovery, so md_start_sync() is not the only place:
>
>   - md_setup_drive
>   - rdev_attr_store
>   - suspend_lo_store
>   - suspend_hi_store
>   - autorun_devices
>   - md_ioct
>   - r5c_disable_writeback_async
>   - error path from new_dev_store(), ...
>
> I'm not sure add a new helper is a good idea, because all above apis
> should use new helper as well.

I think for most of these call sites, it is OK to set MD_RECOVERY_NEEDED
(although it is not needed), and md_start_sync() is the only one that may
trigger "loop between 2 - 4" scenario. Did I miss something?

It is already rc4, so we need to send the fix soon.

Thanks,
Song