6.10-stable review patch. If anyone has any objections, please let me know.
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From: Jens Axboe axboe@kernel.dk
commit 04beb6e0e08c30c6f845f50afb7d7953603d7a6f upstream.
If some part of the kernel adds task_work that needs executing, in terms of signaling it'll generally use TWA_SIGNAL or TWA_RESUME. Those two directly translate to TIF_NOTIFY_SIGNAL or TIF_NOTIFY_RESUME, and can be used for a variety of use case outside of task_work.
However, io_cqring_wait_schedule() only tests explicitly for TIF_NOTIFY_SIGNAL. This means it can miss if task_work got added for the task, but used a different kind of signaling mechanism (or none at all). Normally this doesn't matter as any task_work will be run once the task exits to userspace, except if:
1) The ring is setup with DEFER_TASKRUN 2) The local work item may generate normal task_work
For condition 2, this can happen when closing a file and it's the final put of that file, for example. This can cause stalls where a task is waiting to make progress inside io_cqring_wait(), but there's nothing else that will wake it up. Hence change the "should we schedule or loop around" check to check for the presence of task_work explicitly, rather than just TIF_NOTIFY_SIGNAL as the mechanism. While in there, also change the ordering of what type of task_work first in terms of ordering, to both make it consistent with other task_work runs in io_uring, but also to better handle the case of defer task_work generating normal task_work, like in the above example.
Reported-by: Jan Hendrik Farr kernel@jfarr.cc Link: https://github.com/axboe/liburing/issues/1235 Cc: stable@vger.kernel.org Fixes: 846072f16eed ("io_uring: mimimise io_cqring_wait_schedule") Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- io_uring/io_uring.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/io_uring/io_uring.c +++ b/io_uring/io_uring.c @@ -2401,7 +2401,7 @@ static inline int io_cqring_wait_schedul return 1; if (unlikely(!llist_empty(&ctx->work_llist))) return 1; - if (unlikely(test_thread_flag(TIF_NOTIFY_SIGNAL))) + if (unlikely(task_work_pending(current))) return 1; if (unlikely(task_sigpending(current))) return -EINTR; @@ -2502,9 +2502,9 @@ static int io_cqring_wait(struct io_ring * If we got woken because of task_work being processed, run it * now rather than let the caller do another wait loop. */ - io_run_task_work(); if (!llist_empty(&ctx->work_llist)) io_run_local_work(ctx, nr_wait); + io_run_task_work();
/* * Non-local task_work will be run on exit to userspace, but