Re: md_raid: mdX_raid6 looping after sync_action "check" to "idle" transition

[Donald Buczek <buczek@molgen.mpg.de>]

Dear Guoqing,

a colleague of mine was able to produce the issue inside a vm and were able to find a procedure to run the vm into the issue within minutes (not unreliably after hours on a physical system as before). This of course helped to pinpoint the problem.

My current theory of what is happening is:

- MD_SB_CHANGE_CLEAN + MD_SB_CHANGE_PENDING are set by md_write_start() when file-system I/O wants to do a write and the array transitions from "clean" to "active". (https://elixir.bootlin.com/linux/v5.4.57/source/drivers/md/md.c#L8308)

- Before raid5d gets to write the superblock (its busy processing active stripes because of the sync activity) , userspace wants to pause the check by `echo idle > /sys/block/mdX/md/sync_action`

- action_store() takes the reconfig_mutex before trying to stop the sync thread. (https://elixir.bootlin.com/linux/v5.4.57/source/drivers/md/md.c#L4689) Dump of struct mddev of email 1/19/21 confirms reconf_mutex non-zero.

- raid5d is running in its main loop. raid5d()->handle_active_stripes() returns a positive batch size ( https://elixir.bootlin.com/linux/v5.4.57/source/drivers/md/raid5.c#L6329 ) although raid5d()->handle_active_stripes()->handle_stripe() doesn't process any stripe because of MD_SB_CHANGE_PENDING. (https://elixir.bootlin.com/linux/v5.4.57/source/drivers/md/raid5.c#L4729 ). This is the reason, raid5d is busy looping.

- raid5d()->md_check_recovery() is called by the raid5d main loop. One of its duties is to write the superblock, if a change is pending. However to do so, it needs either MD_ALLOW_SB_UPDATE or must be able to take the reconfig_mutex. (https://elixir.bootlin.com/linux/v5.4.57/source/drivers/md/md.c#L8967 , https://elixir.bootlin.com/linux/v5.4.57/source/drivers/md/md.c#L9006) Both is not true, so the superblock is not written and MD_SB_CHANGE_PENDING is not cleared.

- (as discussed previously) the sync thread is waiting for the number of active stripes to go down and doesn't terminate. The userspace thread is waiting for the sync thread to terminate.

Does this make sense?

Just for reference, I add the procedure which triggers the issue on the vm (with /dev/md3 mounted on /mnt/raid_md3) and some debug output:

```
#! /bin/bash

(
         while true; do
                 echo "start check"
                 echo check > /sys/block/md3/md/sync_action
                 sleep 10
                 echo "stop check"
                 echo idle > /sys/block/md3/md/sync_action
                 sleep 2
         done
) &

(
         while true; do
                 dd bs=1k count=$((5*1024*1024)) if=/dev/zero of=/mnt/raid_md3/bigfile status=none
                 sync /mnt/raid_md3/bigfile
                 rm /mnt/raid_md3/bigfile
                 sleep .1
         done
) &

start="$(date +%s)"
cd /sys/block/md3/md
wp_count=0
while true; do
         array_state=$(cat array_state)
         if [ "$array_state" = write-pending ]; then
                 wp_count=$(($wp_count+1))
         else
                 wp_count=0
         fi
         echo $(($(date +%s)-$start)) $(cat sync_action) $(cat sync_completed) $array_state $(cat stripe_cache_active)
         if [ $wp_count -ge 3 ]; then
                 kill -- -$$
                 exit
         fi
         sleep 1
done
```

The time, this needs to trigger the issue, varies from under a minute to one hour with 5 minute being typical. The output ends like this:

     309 check 6283872 / 8378368 active-idle 4144
     310 check 6283872 / 8378368 active 1702
     311 check 6807528 / 8378368 active 4152
     312 check 7331184 / 8378368 clean 3021
     stop check
     313 check 7331184 / 8378368 write-pending 3905
     314 check 7331184 / 8378368 write-pending 3905
     315 check 7331184 / 8378368 write-pending 3905
     Terminated

If I add

     pr_debug("XXX batch_size %d release %d mdddev->sb_flags %lx\n", batch_size, released, mddev->sb_flags);

in raid5d after the call to handle_active_stripes and enable the debug location after the deadlock occurred, I get

     [ 3123.939143] [1223] raid5d:6332: XXX batch_size 8 release 0 mdddev->sb_flags 6
     [ 3123.939156] [1223] raid5d:6332: XXX batch_size 8 release 0 mdddev->sb_flags 6
     [ 3123.939170] [1223] raid5d:6332: XXX batch_size 8 release 0 mdddev->sb_flags 6
     [ 3123.939184] [1223] raid5d:6332: XXX batch_size 8 release 0 mdddev->sb_flags 6

If I add

     pr_debug("XXX 1 %s:%d mddev->flags %08lx mddev->sb_flags %08lx\n", __FILE__, __LINE__, mddev->flags, mddev->sb_flags);

at the head of md_check_recovery, I get:

     [  789.555462] [1191] md_check_recovery:8970: XXX 1 drivers/md/md.c:8970 mddev->flags 00000000 mddev->sb_flags 00000006
     [  789.555477] [1191] md_check_recovery:8970: XXX 1 drivers/md/md.c:8970 mddev->flags 00000000 mddev->sb_flags 00000006
     [  789.555491] [1191] md_check_recovery:8970: XXX 1 drivers/md/md.c:8970 mddev->flags 00000000 mddev->sb_flags 00000006
     [  789.555505] [1191] md_check_recovery:8970: XXX 1 drivers/md/md.c:8970 mddev->flags 00000000 mddev->sb_flags 00000006
     [  789.555520] [1191] md_check_recovery:8970: XXX 1 drivers/md/md.c:8970 mddev->flags 00000000 mddev->sb_flags 00000006

More debug lines in md_check_recovery confirm the control flow ( `if (mddev_trylock(mddev))` block not taken )

What approach would you suggest to fix this?

Best
   Donald