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Btrfs: eliminate races in worker stopping code

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The current implementation of worker threads in Btrfs has races in
worker stopping code, which cause all kinds of panics and lockups when
running btrfs/011 xfstest in a loop.  The problem is that
btrfs_stop_workers is unsynchronized with respect to check_idle_worker,
check_busy_worker and __btrfs_start_workers.

E.g., check_idle_worker race flow:

       btrfs_stop_workers():            check_idle_worker(aworker):
- grabs the lock
- splices the idle list into the
  working list
- removes the first worker from the
  working list
- releases the lock to wait for
  its kthread's completion
                                  - grabs the lock
                                  - if aworker is on the working list,
                                    moves aworker from the working list
                                    to the idle list
                                  - releases the lock
- grabs the lock
- puts the worker
- removes the second worker from the
  working list
                              ......
        btrfs_stop_workers returns, aworker is on the idle list
                 FS is umounted, memory is freed
                              ......
              aworker is waken up, fireworks ensue

With this applied, I wasn't able to trigger the problem in 48 hours,
whereas previously I could reliably reproduce at least one of these
races within an hour.

Reported-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
This commit is contained in:
Ilya Dryomov 2013-10-02 19:39:50 +03:00 committed by Josef Bacik
parent 385fe0bede
commit 964fb15acf
2 changed files with 21 additions and 6 deletions
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25
fs/btrfs/async-thread.c
View File

@ -107,7 +107,8 @@ static void check_idle_worker(struct btrfs_worker_thread *worker)
worker->idle = 1;
/* the list may be empty if the worker is just starting */
if (!list_empty(&worker->worker_list)) {
if (!list_empty(&worker->worker_list) &&
!worker->workers->stopping) {
list_move(&worker->worker_list,
&worker->workers->idle_list);
}
@ -127,7 +128,8 @@ static void check_busy_worker(struct btrfs_worker_thread *worker)
spin_lock_irqsave(&worker->workers->lock, flags);
worker->idle = 0;
if (!list_empty(&worker->worker_list)) {
if (!list_empty(&worker->worker_list) &&
!worker->workers->stopping) {
list_move_tail(&worker->worker_list,
&worker->workers->worker_list);
}
@ -412,6 +414,7 @@ void btrfs_stop_workers(struct btrfs_workers *workers)
int can_stop;
spin_lock_irq(&workers->lock);
workers->stopping = 1;
list_splice_init(&workers->idle_list, &workers->worker_list);
while (!list_empty(&workers->worker_list)) {
cur = workers->worker_list.next;
@ -455,6 +458,7 @@ void btrfs_init_workers(struct btrfs_workers *workers, char *name, int max,
workers->ordered = 0;
workers->atomic_start_pending = 0;
workers->atomic_worker_start = async_helper;
workers->stopping = 0;
}
/*
@ -480,15 +484,19 @@ static int __btrfs_start_workers(struct btrfs_workers *workers)
atomic_set(&worker->num_pending, 0);
atomic_set(&worker->refs, 1);
worker->workers = workers;
worker->task = kthread_run(worker_loop, worker,
"btrfs-%s-%d", workers->name,
workers->num_workers + 1);
worker->task = kthread_create(worker_loop, worker,
"btrfs-%s-%d", workers->name,
workers->num_workers + 1);
if (IS_ERR(worker->task)) {
ret = PTR_ERR(worker->task);
kfree(worker);
goto fail;
}
spin_lock_irq(&workers->lock);
if (workers->stopping) {
spin_unlock_irq(&workers->lock);
goto fail_kthread;
}
list_add_tail(&worker->worker_list, &workers->idle_list);
worker->idle = 1;
workers->num_workers++;
@ -496,8 +504,13 @@ static int __btrfs_start_workers(struct btrfs_workers *workers)
WARN_ON(workers->num_workers_starting < 0);
spin_unlock_irq(&workers->lock);
wake_up_process(worker->task);
return 0;
fail_kthread:
kthread_stop(worker->task);
fail:
kfree(worker);
spin_lock_irq(&workers->lock);
workers->num_workers_starting--;
spin_unlock_irq(&workers->lock);

2
fs/btrfs/async-thread.h
View File

@ -107,6 +107,8 @@ struct btrfs_workers {
/* extra name for this worker, used for current->name */
char *name;
int stopping;
};
void btrfs_queue_worker(struct btrfs_workers *workers, struct btrfs_work *work);