io-wq: serialize hash clear with wakeup

commit d3e3c102d107bb84251455a298cf475f24bab995 upstream.

We need to ensure that we serialize the stalled and hash bits with the
wait_queue wait handler, or we could be racing with someone modifying
the hashed state after we find it busy, but before we then give up and
wait for it to be cleared. This can cause random delays or stalls when
handling buffered writes for many files, where some of these files cause
hash collisions between the worker threads.

Cc: stable@vger.kernel.org
Reported-by: Daniel Black <daniel@mariadb.org>
Fixes: e941894eae31 ("io-wq: make buffered file write hashed work map per-ctx")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 3ddad5f46114b59b4b35bbb3827df285c359c38e..0890d85ba285570751bff20da80c0701b5e19626 100644 (file)
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -401,9 +401,10 @@ static inline unsigned int io_get_work_hash(struct io_wq_work *work)
        return work->flags >> IO_WQ_HASH_SHIFT;
 }
 
-static void io_wait_on_hash(struct io_wqe *wqe, unsigned int hash)
+static bool io_wait_on_hash(struct io_wqe *wqe, unsigned int hash)
 {
        struct io_wq *wq = wqe->wq;
+       bool ret = false;
 
        spin_lock_irq(&wq->hash->wait.lock);
        if (list_empty(&wqe->wait.entry)) {
@@ -411,9 +412,11 @@ static void io_wait_on_hash(struct io_wqe *wqe, unsigned int hash)
                if (!test_bit(hash, &wq->hash->map)) {
                        __set_current_state(TASK_RUNNING);
                        list_del_init(&wqe->wait.entry);
+                       ret = true;
                }
        }
        spin_unlock_irq(&wq->hash->wait.lock);
+       return ret;
 }
 
 /*
@@ -474,14 +477,21 @@ static struct io_wq_work *io_get_next_work(struct io_wqe *wqe,
        }
 
        if (stall_hash != -1U) {
+               bool unstalled;
+
                /*
                 * Set this before dropping the lock to avoid racing with new
                 * work being added and clearing the stalled bit.
                 */
                wqe->flags |= IO_WQE_FLAG_STALLED;
                raw_spin_unlock(&wqe->lock);
-               io_wait_on_hash(wqe, stall_hash);
+               unstalled = io_wait_on_hash(wqe, stall_hash);
                raw_spin_lock(&wqe->lock);
+               if (unstalled) {
+                       wqe->flags &= ~IO_WQE_FLAG_STALLED;
+                       if (wq_has_sleeper(&wqe->wq->hash->wait))
+                               wake_up(&wqe->wq->hash->wait);
+               }
        }
 
        return NULL;
@@ -562,11 +572,14 @@ get_next:
                                io_wqe_enqueue(wqe, linked);
 
                        if (hash != -1U && !next_hashed) {
+                               /* serialize hash clear with wake_up() */
+                               spin_lock_irq(&wq->hash->wait.lock);
                                clear_bit(hash, &wq->hash->map);
+                               wqe->flags &= ~IO_WQE_FLAG_STALLED;
+                               spin_unlock_irq(&wq->hash->wait.lock);
                                if (wq_has_sleeper(&wq->hash->wait))
                                        wake_up(&wq->hash->wait);
                                raw_spin_lock_irq(&wqe->lock);
-                               wqe->flags &= ~IO_WQE_FLAG_STALLED;
                                /* skip unnecessary unlock-lock wqe->lock */
                                if (!work)
                                        goto get_next;