Commit ce593a6c authored by Jens Axboe's avatar Jens Axboe
Browse files

io_uring: use signal based task_work running 



Since 5.7, we've been using task_work to trigger async running of
requests in the context of the original task. This generally works
great, but there's a case where if the task is currently blocked
in the kernel waiting on a condition to become true, it won't process
task_work. Even though the task is woken, it just checks whatever
condition it's waiting on, and goes back to sleep if it's still false.

This is a problem if that very condition only becomes true when that
task_work is run. An example of that is the task registering an eventfd
with io_uring, and it's now blocked waiting on an eventfd read. That
read could depend on a completion event, and that completion event
won't get trigged until task_work has been run.

Use the TWA_SIGNAL notification for task_work, so that we ensure that
the task always runs the work when queued.

Cc: stable@vger.kernel.org # v5.7
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent e91b4816

fs/io_uring.c

+24 −8
Original line number Diff line number Diff line
@@ -4072,6 +4072,21 @@ struct io_poll_table { 
	int error;

};



static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb,

				int notify)

{

	struct task_struct *tsk = req->task;

	int ret;



	if (req->ctx->flags & IORING_SETUP_SQPOLL)

		notify = 0;



	ret = task_work_add(tsk, cb, notify);

	if (!ret)

		wake_up_process(tsk);

	return ret;

}



static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,

			   __poll_t mask, task_work_func_t func)

{
@@ -4095,13 +4110,13 @@ static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll, 
	 * of executing it. We can't safely execute it anyway, as we may not

	 * have the needed state needed for it anyway.

	 */

	ret = task_work_add(tsk, &req->task_work, true);

	ret = io_req_task_work_add(req, &req->task_work, TWA_SIGNAL);

	if (unlikely(ret)) {

		WRITE_ONCE(poll->canceled, true);

		tsk = io_wq_get_task(req->ctx->io_wq);

		task_work_add(tsk, &req->task_work, true);

	}

		task_work_add(tsk, &req->task_work, 0);

		wake_up_process(tsk);

	}

	return 1;

}
@@ -6182,19 +6197,20 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events, 
	do {

		prepare_to_wait_exclusive(&ctx->wait, &iowq.wq,

						TASK_INTERRUPTIBLE);

		/* make sure we run task_work before checking for signals */

		if (current->task_works)

			task_work_run();

		if (io_should_wake(&iowq, false))

			break;

		schedule();

		if (signal_pending(current)) {

			ret = -EINTR;

			ret = -ERESTARTSYS;

			break;

		}

		if (io_should_wake(&iowq, false))

			break;

		schedule();

	} while (1);

	finish_wait(&ctx->wait, &iowq.wq);



	restore_saved_sigmask_unless(ret == -EINTR);

	restore_saved_sigmask_unless(ret == -ERESTARTSYS);



	return READ_ONCE(rings->cq.head) == READ_ONCE(rings->cq.tail) ? ret : 0;

}

CRA Git | Maintained and supported by SUSTech CRA and CCSE