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

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

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