I found a race condition in the way scrub interacts with btrfs workers:
Please carefully distinguish "workers" as struct btrfs_workers from "worker" as
struct btrfs_worker_thread.
Process A: umount
Process B: scrub-workers
Process C: genwork-worker
A: close_ctree()
A: scrub_cancel()
B: scrub_workers_put()
B: btrfs_stop_workers()
B: -> lock(workers->lock)
B: -> splice idle worker list into worker list
B: -> while (worker list)
B: -> worker = delete from worker list
B: -> unlock(workers->lock)
B: -> kthread_stop(worker)
C: __btrfs_start_workers()
C: -> kthread_run starts new scrub-worker
C: -> lock(workers->lock) /* from scrub-workers */
C: -> add to workers idle list
C: -> unlock(workers->lock)
B: -> lock(workers->lock)
A: stop genwork-worker
In that situation, the scrub-workers have one idle worker but return from
btrfs_stop_workers(). There are several strategies to fix this:
1. Let close_ctree call scrub_cancel() after stopping the generic worker.
2. Let btrfs_stop_workers be more careful on insertions to the idle list, like:
diff --git a/fs/btrfs/async-thread.c b/fs/btrfs/async-thread.c
index 58b7d14..2523781 100644
--- a/fs/btrfs/async-thread.c
+++ b/fs/btrfs/async-thread.c
@@ -431,6 +431,8 @@ void btrfs_stop_workers(struct btrfs_workers *workers)
kthread_stop(worker->task);
spin_lock_irq(&workers->lock);
put_worker(worker);
+ /* we dropped the lock above, check for new idle workers */
+ list_splice_init(&workers->idle_list, &workers->worker_list);
}
spin_unlock_irq(&workers->lock);
}
3. Find out why __btrfs_start_workers() gets called in the above scenario at
all.
Any thoughts? Otherwise, I'm going for option 3 and will call back.
-Jan
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