Re: [MD] Crash with 4.12+ kernel and high disk load -- bisected to 4ad23a976413: MD: use per-cpu counter for writes_pending

[David R <david@unsolicited.net>]

Ignore me. The increment and decrement of sync_checkers should protect  
switch_to_percpu(). Sigh.


Quoting David R <david@unsolicited.net>:

> Quoting Shaohua Li <shli@kernel.org>:
>
>> Spent some time to check this one, unfortunately I can't find how that patch
>> makes rcu stall. the percpu part looks good to me too. Can you  
>> double check if
>> reverting 4ad23a976413aa57 makes the issue go away? When the rcu  
>> stall happens,
>> what the /sys/block/md/md0/array_state? please also attach  
>> /proc/mdstat. When
>> you say the mdx_raid1 threads are in 'R' state, can you double check if the
>> /proc/pid/stack always 0xffffffffff?
>>
>> Thanks,
>> Shaohua
>
> I confess to knowing absolutely nothing about the md code, so please  
> don't be too hard on me. However
> :-
>
> static bool set_in_sync(struct mddev *mddev)
> {
>     WARN_ON_ONCE(!spin_is_locked(&mddev->lock));
>     if (!mddev->in_sync) {
>         mddev->sync_checkers++;
>         spin_unlock(&mddev->lock);
>         percpu_ref_switch_to_atomic_sync(&mddev->writes_pending);
>         spin_lock(&mddev->lock);
>         if (!mddev->in_sync &&
>             percpu_ref_is_zero(&mddev->writes_pending)) {
>             mddev->in_sync = 1;
>             /*
>              * Ensure ->in_sync is visible before we clear
>              * ->sync_checkers.
>              */
>             smp_mb();
>             set_bit(MD_SB_CHANGE_CLEAN, &mddev->sb_flags);
>             sysfs_notify_dirent_safe(mddev->sysfs_state);
>         }
>         if (--mddev->sync_checkers == 0)
>             percpu_ref_switch_to_percpu(&mddev->writes_pending);
>
>
> The switch_to_percpu() takes place under mddev->lock however  
> switch_to_atomic_sync() does not. A thread can be in the middle of  
> (or about to execute) switch_to_atomic_sync() at the same time as  
> another is calling switch_to_percpu(). This can't be correct surely?
>
> Cheers
> David