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@xxxxxxxxxxxxxxx # v5.7+ > Reported-by: Josef <josef.grieb@xxxxxxxxx> > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> > --- > 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. > */ > if (ctx->flags & IORING_SETUP_SQPOLL) > notify = 0; > - 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.