On Sat, Aug 08, 2020 at 12:34:39PM -0600, Jens Axboe wrote:
An earlier commit:
b7db41c9e03b ("io_uring: fix regression with always ignoring signals in io_cqring_wait()")
ensured that we didn't get stuck waiting for eventfd reads when it's registered with the io_uring ring for event notification, but we still have a gap where the task can be waiting on other events in the kernel and need a bigger nudge to make forward progress.
Ensure that we use signaled notifications for a task that isn't currently running, to be certain the work is seen and processed immediately.
Cc: stable@vger.kernel.org # v5.7+ Reported-by: Josef josef.grieb@gmail.com Signed-off-by: Jens Axboe axboe@kernel.dk
fs/io_uring.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c index e9b27cdaa735..443eecdfeda9 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -1712,21 +1712,27 @@ static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb) struct io_ring_ctx *ctx = req->ctx; int ret, notify = TWA_RESUME;
- ret = __task_work_add(tsk, cb);
- if (unlikely(ret))
return ret;- /*
- SQPOLL kernel thread doesn't need notification, just a wakeup.
* If we're not using an eventfd, then TWA_RESUME is always fine,* as we won't have dependencies between request completions for* other kernel wait conditions.
* For any other work, use signaled wakeups if the task isn't* running to avoid dependencies between tasks or threads. If* the issuing task is currently waiting in the kernel on a thread,* and same thread is waiting for a completion event, then we need* to ensure that the issuing task processes task_work. TWA_SIGNAL* is needed for that.
- else if (ctx->cq_ev_fd)
- else if (READ_ONCE(tsk->state) != TASK_RUNNING) notify = TWA_SIGNAL;
- ret = task_work_add(tsk, cb, notify);
- if (!ret)
wake_up_process(tsk);- return ret;
- __task_work_notify(tsk, notify);
- wake_up_process(tsk);
- return 0;
}
Wait.. so the only change here is that you look at tsk->state, _after_ doing __task_work_add(), but nothing, not the Changelog nor the comment explains this.
So you're relying on __task_work_add() being an smp_mb() vs the add, and you order this against the smp_mb() in set_current_state() ?
This really needs spelling out.