Re: [PATCH -next v2 1/6] Revert "md: unlock mddev before reap sync_thread in action_store"

[Xiao Ni]

在 2023/5/29 下午9:20, Yu Kuai 写道:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> This reverts commit 9dfbdafda3b34e262e43e786077bab8e476a89d1.
>
> Because it will introduce a defect that sync_thread can be running while
> MD_RECOVERY_RUNNING is cleared, which will cause some unexpected problems,
> for example:
>
> list_add corruption. prev->next should be next (ffff0001ac1daba0), but was ffff0000ce1a02a0. (prev=ffff0000ce1a02a0).
> Call trace:
>   __list_add_valid+0xfc/0x140
>   insert_work+0x78/0x1a0
>   __queue_work+0x500/0xcf4
>   queue_work_on+0xe8/0x12c
>   md_check_recovery+0xa34/0xf30
>   raid10d+0xb8/0x900 [raid10]
>   md_thread+0x16c/0x2cc
>   kthread+0x1a4/0x1ec
>   ret_from_fork+0x10/0x18
>
> This is because work is requeued while it's still inside workqueue:
>
> t1:			t2:
> action_store
>   mddev_lock
>    if (mddev->sync_thread)
>     mddev_unlock
>     md_unregister_thread
>     // first sync_thread is done
> 			md_check_recovery
> 			 mddev_try_lock
> 			 /*
> 			  * once MD_RECOVERY_DONE is set, new sync_thread
> 			  * can start.
> 			  */
> 			 set_bit(MD_RECOVERY_RUNNING, &mddev->recovery)
> 			 INIT_WORK(&mddev->del_work, md_start_sync)
> 			 queue_work(md_misc_wq, &mddev->del_work)
> 			  test_and_set_bit(WORK_STRUCT_PENDING_BIT, ...)
> 			  // set pending bit
> 			  insert_work
> 			   list_add_tail
> 			 mddev_unlock
>     mddev_lock_nointr
>     md_reap_sync_thread
>     // MD_RECOVERY_RUNNING is cleared
>   mddev_unlock
>
> t3:
>
> // before queued work started from t2
> md_check_recovery
>   // MD_RECOVERY_RUNNING is not set, a new sync_thread can be started
>   INIT_WORK(&mddev->del_work, md_start_sync)
>    work->data = 0
>    // work pending bit is cleared
>   queue_work(md_misc_wq, &mddev->del_work)
>    insert_work
>     list_add_tail
>     // list is corrupted
>
> The above commit is reverted to fix the problem, the deadlock this
> commit tries to fix will be fixed in following patches.
>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>   drivers/md/dm-raid.c |  1 -
>   drivers/md/md.c      | 19 ++-----------------
>   2 files changed, 2 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/md/dm-raid.c b/drivers/md/dm-raid.c
> index 8846bf510a35..1f22bef27841 100644
> --- a/drivers/md/dm-raid.c
> +++ b/drivers/md/dm-raid.c
> @@ -3725,7 +3725,6 @@ 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_unregister_thread(&mddev->sync_thread);
>   			md_reap_sync_thread(mddev);
>   		}
>   	} else if (decipher_sync_action(mddev, mddev->recovery) != st_idle)
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index a5a7af2f4e59..9b97731e1fe4 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -4772,19 +4772,6 @@ action_store(struct mddev *mddev, const char *page, size_t len)
>   			if (work_pending(&mddev->del_work))
>   				flush_workqueue(md_misc_wq);
>   			if (mddev->sync_thread) {
> -				sector_t save_rp = mddev->reshape_position;
> -
> -				mddev_unlock(mddev);
> -				set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> -				md_unregister_thread(&mddev->sync_thread);
> -				mddev_lock_nointr(mddev);
> -				/*
> -				 * set RECOVERY_INTR again and restore reshape
> -				 * position in case others changed them after
> -				 * got lock, eg, reshape_position_store and
> -				 * md_check_recovery.
> -				 */
> -				mddev->reshape_position = save_rp;
>   				set_bit(MD_RECOVERY_INTR, &mddev->recovery);
>   				md_reap_sync_thread(mddev);
>   			}
> @@ -6184,7 +6171,6 @@ static void __md_stop_writes(struct mddev *mddev)
>   		flush_workqueue(md_misc_wq);
>   	if (mddev->sync_thread) {
>   		set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> -		md_unregister_thread(&mddev->sync_thread);
>   		md_reap_sync_thread(mddev);
>   	}
>   
> @@ -9336,7 +9322,6 @@ void md_check_recovery(struct mddev *mddev)
>   			 * ->spare_active and clear saved_raid_disk
>   			 */
>   			set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> -			md_unregister_thread(&mddev->sync_thread);
>   			md_reap_sync_thread(mddev);
>   			clear_bit(MD_RECOVERY_RECOVER, &mddev->recovery);
>   			clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
> @@ -9372,7 +9357,6 @@ void md_check_recovery(struct mddev *mddev)
>   			goto unlock;
>   		}
>   		if (mddev->sync_thread) {
> -			md_unregister_thread(&mddev->sync_thread);
>   			md_reap_sync_thread(mddev);
>   			goto unlock;
>   		}
> @@ -9452,7 +9436,8 @@ void md_reap_sync_thread(struct mddev *mddev)
>   	sector_t old_dev_sectors = mddev->dev_sectors;
>   	bool is_reshaped = false;
>   
> -	/* sync_thread should be unregistered, collect result */
> +	/* resync has finished, collect result */
> +	md_unregister_thread(&mddev->sync_thread);
>   	if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
>   	    !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery) &&
>   	    mddev->degraded != mddev->raid_disks) {


Hi Kuai

Thanks for the patch and the explanation in V1. In version1, I took much 
time to try to understand the problem. Maybe we can use the problem

itself as the subject. Something like "Don't allow two sync processes 
running at the same time"? And could you add the test steps which talked 
in v1

in the patch? It can help to understand the problem very much.

Best Regards

Xiao