Re: [PATCH v2] md: recheck spare changes before starting sync

[yu kuai]

在 2026/7/8 19:20, Abd-Alrhman Masalkhi 写道:

> remove_spares() and remove_and_add_spares() modify the array's rdev
> configuration. These operations are only safe after the array has been
> suspended.
>
> md_start_sync() checks whether spare configuration changes are needed
> before taking reconfig_mutex. However, the rdev state can change before
> the mutex is acquired, so the initial check can become stale. In that
> case, md_choose_sync_action() may remove or replace rdevs while normal
> I/O is still accessing them.
>
> The race can occur as follows:
>
> raid10d          Worker                      Normal IO
> ____________     _______________________     ______________________
>
>                                               raid10_write_request()
>                                               wait_blocked_dev()
> set Blocked
> set Faulty
>                                               Skip Faulty rdev
>                                               rrdev->nr_pending++
>                                               .repl_bio = bio
>                   removeable_rdev = false     .
>                   array not suspended         .
> lock mddev                                   goto err_handle
>                   lock mddev (wait)
>                   .
> update sb        .
> clear Blocked    .
>                   .
> unlock mddev     .
>                   lock mddev (acquires)
>                   remove_spares()
>                   removeable_rdev = true
>
>                   raid10_remove_disk()
>                   rdev = replacement
>                   replacement = NULL
>                                               rdev_dec_pending(NULL)
>                   unlock mddev                (NULL)->nr_pending--
>
> In this case, rdev_dec_pending() is called with a NULL pointer,
> resulting in a NULL pointer dereference when attempting to decrement
> nr_pending.
>
> Fix this by suspending the array when spare configuration changes are
> needed, including for non-read-write arrays, and checking again after
> taking reconfig_mutex. If the array was not already suspended and a
> change is now needed, release the mutex, suspend the array, and
> reacquire the mutex before continuing.
>
> Fixes: bc08041b32ab ("md: suspend array in md_start_sync() if array need reconfiguration")
> Reported-by: sashiko-bot<sashiko-bot@xxxxxxxxxx>
> Closes:https://sashiko.dev/#/patchset/20260628142420.1051027-1-abd.masalkhi@xxxxxxxxx?part=3
> Signed-off-by: Abd-Alrhman Masalkhi<abd.masalkhi@xxxxxxxxx>
> ---
> Changes in v2:
>   - Recheck whether spare configuration changes are needed after taking
>     reconfig_mutex, and suspend the array before continuing if necessary.
>   - Account for read only arrays, where reshape_position may not be
>     MaxSector even though md_start_sync() can still modify the spare
>     configuration.
>   - Link to v1:https://lore.kernel.org/linux-raid/20260630075640.1081634-1-abd.masalkhi@xxxxxxxxx/
> ---
>   drivers/md/md.c | 14 +++++++++++++-
>   1 file changed, 13 insertions(+), 1 deletion(-)

Reviewed-by: Yu Kuai <yukuai@xxxxxxx>

-- 
Thanks,
Kuai