FAILED: patch "[PATCH] io_uring: use signal based task_work running" failed to apply to 5.7-stable tree

FAILED: patch "[PATCH] io_uring: use signal based task_work running" failed to apply to 5.7-stable tree

[gregkh]

The patch below does not apply to the 5.7-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ce593a6c480a22acba08795be313c0c6d49dd35d Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Tue, 30 Jun 2020 12:39:05 -0600
Subject: [PATCH] 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: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index e507737f044e..700644a016a7 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -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);
 	}
-	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;
 }

Re: FAILED: patch "[PATCH] io_uring: use signal based task_work running" failed to apply to 5.7-stable tree

[Sasha Levin]

On Sun, Jul 05, 2020 at 01:26:56PM +0200, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 5.7-stable tree.
>If someone wants it applied there, or to any other stable or longterm
>tree, then please email the backport, including the original git commit
>id to <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From ce593a6c480a22acba08795be313c0c6d49dd35d Mon Sep 17 00:00:00 2001
>From: Jens Axboe <axboe@kernel.dk>
>Date: Tue, 30 Jun 2020 12:39:05 -0600
>Subject: [PATCH] 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: Jens Axboe <axboe@kernel.dk>

This patch depends on e91b48162332 ("task_work: teach task_work_add() to
do signal_wake_up()"), I've queued both for 5.7.

-- 
Thanks,
Sasha

Re: FAILED: patch "[PATCH] io_uring: use signal based task_work running" failed to apply to 5.7-stable tree

[Greg KH]

On Sun, Jul 05, 2020 at 09:49:10AM -0400, Sasha Levin wrote:
> On Sun, Jul 05, 2020 at 01:26:56PM +0200, gregkh@linuxfoundation.org wrote:
> > 
> > The patch below does not apply to the 5.7-stable tree.
> > If someone wants it applied there, or to any other stable or longterm
> > tree, then please email the backport, including the original git commit
> > id to <stable@vger.kernel.org>.
> > 
> > thanks,
> > 
> > greg k-h
> > 
> > ------------------ original commit in Linus's tree ------------------
> > 
> > > From ce593a6c480a22acba08795be313c0c6d49dd35d Mon Sep 17 00:00:00 2001
> > From: Jens Axboe <axboe@kernel.dk>
> > Date: Tue, 30 Jun 2020 12:39:05 -0600
> > Subject: [PATCH] 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: Jens Axboe <axboe@kernel.dk>
> 
> This patch depends on e91b48162332 ("task_work: teach task_work_add() to
> do signal_wake_up()"), I've queued both for 5.7.

Great, thanks for adding that.

greg k-h