A task that exits cancels its tracked requests and waits until they are gone. io_uring_cancel_generic() looks everywhere a request can be, and goes to sleep if it found nothing to cancel. It is woken up each time one of the requests of the task is completed by task work or freed. When io-wq is done with a request that has a link, there is a time in which the next request of the link can't be found: io_worker_handle_work() io_assign_current_work(worker, NULL) io_wq_free_work(work) io_req_find_next() // takes the link off the request io_free_req() // task work for the task io_assign_current_work(worker, linked) io_run_task_work() cond_resched() worker->cur_work = linked It is not pending, it is not the current work of a worker, and it is not yet on the list that it is about to be parked on. If the pass that the exiting task makes because of io_free_req() falls into that time, it finds nothing and the task goes to sleep. The worker then runs the link, which IO_WQ_BIT_EXIT doesn't prevent for bounded work. If that parks the request nothing is completed or freed, and nobody wakes up the task. If it is the process that exits it can't be killed any more either. NOP, IOSQE_ASYNC | IOSQE_IO_LINK FUTEX_WAIT, FUTEX2_PRIVATE exit of the thread, or of the process, up to 63us later With four of these per round 5 to 8 of 10,000 rounds hang (4 CPUs, KVM): io_uring_cancel_generic do_exit Commit f8b632e89a10 ("io_uring: tighten task exit cancellations") is about the same gap, and makes the task look again for whatever happens before io_free_req(). Free the request once the link is the current work of the worker, or pending. Whoever is woken up by that can find the link, until it is parked where cancellations look as well. To do so split io_wq_free_work() in two. Fixes: 768134d4f481 ("io_uring: don't do flush cancel under inflight_lock") Assisted-by: LLM Signed-off-by: Josef Bacik --- io_uring/io-wq.c | 18 +++++++++++++++--- io_uring/io_uring.c | 46 +++++++++++++++++++++++++++++----------------- io_uring/io_uring.h | 3 ++- 3 files changed, 46 insertions(+), 21 deletions(-) diff --git a/io_uring/io-wq.c b/io_uring/io-wq.c index 433c003a83e5..a5b3d2ba48bf 100644 --- a/io_uring/io-wq.c +++ b/io_uring/io-wq.c @@ -646,12 +646,13 @@ static void io_worker_handle_work(struct io_wq_acct *acct, /* handle a whole dependent link */ do { bool do_kill = test_bit(IO_WQ_BIT_EXIT, &wq->state); - struct io_wq_work *next_hashed, *linked; + struct io_wq_work *next_hashed, *linked, *done = work; unsigned int work_flags = atomic_read(&work->flags); unsigned int hash = __io_wq_is_hashed(work_flags) ? __io_get_work_hash(work_flags) : -1U; struct io_kiocb *req; + bool last; next_hashed = wq_next_work(work); @@ -670,7 +671,7 @@ static void io_worker_handle_work(struct io_wq_acct *acct, kcov_remote_stop(); io_assign_current_work(worker, NULL); - linked = io_wq_free_work(work); + last = io_wq_put_work(work, &linked); work = next_hashed; if (!work && linked && !io_wq_is_hashed(linked)) { work = linked; @@ -679,6 +680,13 @@ static void io_worker_handle_work(struct io_wq_acct *acct, io_assign_current_work(worker, work); if (linked) io_wq_enqueue(wq, linked); + /* + * Not before the link is current or pending work again. + * This may be the last time that a task is woken up + * which exits and looks for requests to cancel. + */ + if (last) + io_wq_free_work(done); if (hash != -1U && !next_hashed) { /* serialize hash clear with wake_up() */ @@ -1008,9 +1016,13 @@ void io_wq_set_exit_on_idle(struct io_wq *wq, bool enable) static void io_run_cancel(struct io_wq_work *work, struct io_wq *wq) { do { + struct io_wq_work *linked; + atomic_or(IO_WQ_WORK_CANCEL, &work->flags); io_wq_submit_work(work); - work = io_wq_free_work(work); + if (io_wq_put_work(work, &linked)) + io_wq_free_work(work); + work = linked; } while (work); } diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c index 0c91d466d825..464d9616234c 100644 --- a/io_uring/io_uring.c +++ b/io_uring/io_uring.c @@ -1486,28 +1486,40 @@ int io_poll_issue(struct io_kiocb *req, io_tw_token_t tw) return ret; } -struct io_wq_work *io_wq_free_work(struct io_wq_work *work) +/* + * io-wq is done with @work. Returns true if it held the last reference, and + * hands out the next request of the link then. The caller has to make sure + * that cancellations can find the latter before it calls io_wq_free_work(). + */ +bool io_wq_put_work(struct io_wq_work *work, struct io_wq_work **linked) { struct io_kiocb *req = container_of(work, struct io_kiocb, work); struct io_kiocb *nxt = NULL; - if (req_ref_put_and_test_atomic(req)) { - /* - * Only continue the link from here if the completion has been - * posted. If it's still pending, io_free_req() completes the - * request and the next link follows from there, after the CQE. - */ - if ((req->flags & IO_REQ_LINK_FLAGS) && - (req->flags & REQ_F_CQE_SKIP)) { - struct io_ring_ctx *ctx = req->ctx; + *linked = NULL; + if (!req_ref_put_and_test_atomic(req)) + return false; - mutex_lock(&ctx->uring_lock); - nxt = io_req_find_next(req); - mutex_unlock(&ctx->uring_lock); - } - io_free_req(req); + /* + * Only continue the link from here if the completion has been posted. + * If it's still pending, io_free_req() completes the request and the + * next link follows from there, after the CQE. + */ + if ((req->flags & IO_REQ_LINK_FLAGS) && (req->flags & REQ_F_CQE_SKIP)) { + struct io_ring_ctx *ctx = req->ctx; + + mutex_lock(&ctx->uring_lock); + nxt = io_req_find_next(req); + mutex_unlock(&ctx->uring_lock); } - return nxt ? &nxt->work : NULL; + if (nxt) + *linked = &nxt->work; + return true; +} + +void io_wq_free_work(struct io_wq_work *work) +{ + io_free_req(container_of(work, struct io_kiocb, work)); } void io_wq_submit_work(struct io_wq_work *work) @@ -1518,7 +1530,7 @@ void io_wq_submit_work(struct io_wq_work *work) bool needs_poll = false; int ret = 0, err = -ECANCELED; - /* one will be dropped by io_wq_free_work() after returning to io-wq */ + /* one will be dropped by io_wq_put_work() after returning to io-wq */ if (!(req->flags & REQ_F_REFCOUNT)) __io_req_set_refcount(req, 2); else diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h index 109261722ca7..c7b340e5d0a8 100644 --- a/io_uring/io_uring.h +++ b/io_uring/io_uring.h @@ -204,7 +204,8 @@ int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin); __cold void io_iopoll_try_reap_events(struct io_ring_ctx *ctx); void __io_submit_flush_completions(struct io_ring_ctx *ctx); -struct io_wq_work *io_wq_free_work(struct io_wq_work *work); +bool io_wq_put_work(struct io_wq_work *work, struct io_wq_work **linked); +void io_wq_free_work(struct io_wq_work *work); void io_wq_submit_work(struct io_wq_work *work); void io_free_req(struct io_kiocb *req); -- 2.55.0