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md: recheck spare changes before starting sync

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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: bc08041b32 ("md: suspend array in md_start_sync() if array need reconfiguration")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260628142420.1051027-1-abd.masalkhi@gmail.com?part=3
Signed-off-by: Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/20260708112003.474537-1-abd.masalkhi@gmail.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>
This commit is contained in:
Abd-Alrhman Masalkhi
2026-07-08 11:20:03 +00:00
committed by Yu Kuai
parent 3fe5b7c9fb
commit c7d34d17ea
1 changed files with 13 additions and 1 deletions
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14
drivers/md/md.c
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@@ -10179,13 +10179,25 @@ static void md_start_sync(struct work_struct *ws)
* If reshape is still in progress, spares won't be added or removed
* from conf until reshape is done.
*/
if (mddev->reshape_position == MaxSector &&
if ((mddev->reshape_position == MaxSector || !md_is_rdwr(mddev)) &&
md_spares_need_change(mddev)) {
suspend = true;
mddev_suspend(mddev, false);
}
mddev_lock_nointr(mddev);
/*
* The spare configuration can change before reconfig_mutex is acquired.
* Recheck while holding the lock and suspend if needed.
*/
if (!suspend && (mddev->reshape_position == MaxSector || !md_is_rdwr(mddev)) &&
md_spares_need_change(mddev)) {
mddev_unlock(mddev);
mddev_suspend_and_lock_nointr(mddev);
suspend = true;
}
if (!md_is_rdwr(mddev)) {
/*
* On a read-only array we can: