From: Jens Axboe axboe@kernel.dk
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 --- fs/io-wq.c | 19 ++++++++++++++++--- 1 file changed, 16 insertions(+), 3 deletions(-)
--- a/fs/io-wq.c +++ b/fs/io-wq.c @@ -401,9 +401,10 @@ static inline unsigned int io_get_work_h 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_wq 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_wo }
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;