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io-wq: fix race around io_worker grabbing

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There's a small window between lookup dropping the reference to the
worker and calling wake_up_process() on the worker task, where the worker
itself could have exited. We ensure that the worker struct itself is
valid, but worker->task may very well be gone by the time we issue the
wakeup.

Fix the race by using a completion triggered by the reference going to
zero, and having exit wait for that completion before proceeding.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2021-02-23 19:59:06 -07:00
parent 8b3e78b595
commit eb2de9418d
1 changed files with 9 additions and 9 deletions
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18
fs/io-wq.c
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@ -56,6 +56,8 @@ struct io_worker {
const struct cred *cur_creds;
const struct cred *saved_creds;
struct completion ref_done;
struct rcu_head rcu;
};
@ -129,7 +131,7 @@ static bool io_worker_get(struct io_worker *worker)
static void io_worker_release(struct io_worker *worker)
{
if (refcount_dec_and_test(&worker->ref))
wake_up_process(worker->task);
complete(&worker->ref_done);
}
static inline struct io_wqe_acct *io_work_get_acct(struct io_wqe *wqe,
@ -157,14 +159,9 @@ static void io_worker_exit(struct io_worker *worker)
struct io_wqe_acct *acct = io_wqe_get_acct(worker);
unsigned flags;
/*
* If we're not at zero, someone else is holding a brief reference
* to the worker. Wait for that to go away.
*/
set_current_state(TASK_INTERRUPTIBLE);
if (!refcount_dec_and_test(&worker->ref))
schedule();
__set_current_state(TASK_RUNNING);
if (refcount_dec_and_test(&worker->ref))
complete(&worker->ref_done);
wait_for_completion(&worker->ref_done);
preempt_disable();
current->flags &= ~PF_IO_WORKER;
@ -615,6 +612,7 @@ static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index)
worker->nulls_node.pprev = NULL;
worker->wqe = wqe;
spin_lock_init(&worker->lock);
init_completion(&worker->ref_done);
refcount_inc(&wq->refs);
@ -724,6 +722,7 @@ static int io_wq_manager(void *data)
io_wq_check_workers(wq);
if (refcount_dec_and_test(&wq->refs)) {
wq->manager = NULL;
complete(&wq->done);
do_exit(0);
}
@ -734,6 +733,7 @@ static int io_wq_manager(void *data)
io_wq_for_each_worker(wq->wqes[node], io_wq_worker_wake, NULL);
rcu_read_unlock();
}
wq->manager = NULL;
do_exit(0);
}