md: fix a potential deadlock of raid5/raid10 reshape

There is a potential deadlock if mount/umount happens when
raid5_finish_reshape() tries to grow the size of emulated disk.

How the deadlock happens?
1) The raid5 resync thread finished reshape (expanding array).
2) The mount or umount thread holds VFS sb->s_umount lock and tries to
   write through critical data into raid5 emulated block device. So it
   waits for raid5 kernel thread handling stripes in order to finish it
   I/Os.
3) In the routine of raid5 kernel thread, md_check_recovery() will be
   called first in order to reap the raid5 resync thread. That is,
   raid5_finish_reshape() will be called. In this function, it will try
   to update conf and call VFS revalidate_disk() to grow the raid5
   emulated block device. It will try to acquire VFS sb->s_umount lock.
The raid5 kernel thread cannot continue, so no one can handle mount/
umount I/Os (stripes). Once the write-through I/Os cannot be finished,
mount/umount will not release sb->s_umount lock. The deadlock happens.

The raid5 kernel thread is an emulated block device. It is responible to
handle I/Os (stripes) from upper layers. The emulated block device
should not request any I/Os on itself. That is, it should not call VFS
layer functions. (If it did, it will try to acquire VFS locks to
guarantee the I/Os sequence.) So we have the resync thread to send
resync I/O requests and to wait for the results.

For solving this potential deadlock, we can put the size growth of the
emulated block device as the final step of reshape thread.

2017/12/29:
Thanks to Guoqing Jiang <gqjiang@suse.com>,
we confirmed that there is the same deadlock issue in raid10. It's
reproducible and can be fixed by this patch. For raid10.c, we can remove
the similar code to prevent deadlock as well since they has been called
before.

Reported-by: Alex Wu <alexwu@synology.com>
Reviewed-by: Alex Wu <alexwu@synology.com>
Reviewed-by: Chung-Chiang Cheng <cccheng@synology.com>
Signed-off-by: BingJing Chang <bingjingc@synology.com>
Signed-off-by: Shaohua Li <sh.li@alibaba-inc.com>

diff --git a/drivers/md/md.c b/drivers/md/md.c
index ba152dddaaa339e8491e0d9e1bdf21f664369aab..254e44e44668f5fff8cc2d95bdb3b682450a204f 100644 (file)
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8569,6 +8569,19 @@ void md_do_sync(struct md_thread *thread)
        set_mask_bits(&mddev->sb_flags, 0,
                      BIT(MD_SB_CHANGE_PENDING) | BIT(MD_SB_CHANGE_DEVS));
 
+       if (test_bit(MD_RECOVERY_RESHAPE, &mddev->recovery) &&
+                       !test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
+                       mddev->delta_disks > 0 &&
+                       mddev->pers->finish_reshape &&
+                       mddev->pers->size &&
+                       mddev->queue) {
+               mddev_lock_nointr(mddev);
+               md_set_array_sectors(mddev, mddev->pers->size(mddev, 0, 0));
+               mddev_unlock(mddev);
+               set_capacity(mddev->gendisk, mddev->array_sectors);
+               revalidate_disk(mddev->gendisk);
+       }
+
        spin_lock(&mddev->lock);
        if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery)) {
                /* We completed so min/max setting can be forgotten if used. */

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 93fa947fef22cf95ed6ea25e1621f09f78218f14..c5e6c60fc0d41b53a578874087ae87259e3ebcce 100644 (file)
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -4832,17 +4832,11 @@ static void raid10_finish_reshape(struct mddev *mddev)
                return;
 
        if (mddev->delta_disks > 0) {
-               sector_t size = raid10_size(mddev, 0, 0);
-               md_set_array_sectors(mddev, size);
                if (mddev->recovery_cp > mddev->resync_max_sectors) {
                        mddev->recovery_cp = mddev->resync_max_sectors;
                        set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
                }
-               mddev->resync_max_sectors = size;
-               if (mddev->queue) {
-                       set_capacity(mddev->gendisk, mddev->array_sectors);
-                       revalidate_disk(mddev->gendisk);
-               }
+               mddev->resync_max_sectors = mddev->array_sectors;
        } else {
                int d;
                rcu_read_lock();

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index e3b0f799fbfae4cf985e421161002227d13bfd47..b5d2601483e34fa97fc8cfb7faeff6f014c0e035 100644 (file)
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -8000,13 +8000,7 @@ static void raid5_finish_reshape(struct mddev *mddev)
 
        if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery)) {
 
-               if (mddev->delta_disks > 0) {
-                       md_set_array_sectors(mddev, raid5_size(mddev, 0, 0));
-                       if (mddev->queue) {
-                               set_capacity(mddev->gendisk, mddev->array_sectors);
-                               revalidate_disk(mddev->gendisk);
-                       }
-               } else {
+               if (mddev->delta_disks <= 0) {
                        int d;
                        spin_lock_irq(&conf->device_lock);
                        mddev->degraded = raid5_calc_degraded(conf);