linux-kernel - [PATCH 5.5 134/150] io_uring: prevent sq_thread from spinning when it should stop

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Date:   Thu, 27 Feb 2020 14:37:51 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Stefano Garzarella <sgarzare@...hat.com>,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 5.5 134/150] io_uring: prevent sq_thread from spinning when it should stop

From: Stefano Garzarella <sgarzare@...hat.com>

commit 7143b5ac5750f404ff3a594b34fdf3fc2f99f828 upstream.

This patch drops 'cur_mm' before calling cond_resched(), to prevent
the sq_thread from spinning even when the user process is finished.

Before this patch, if the user process ended without closing the
io_uring fd, the sq_thread continues to spin until the
'sq_thread_idle' timeout ends.

In the worst case where the 'sq_thread_idle' parameter is bigger than
INT_MAX, the sq_thread will spin forever.

Fixes: 6c271ce2f1d5 ("io_uring: add submission polling")
Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/io_uring.c |   24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3909,6 +3909,18 @@ static int io_sq_thread(void *data)
 		 */
 		if (!to_submit || ret == -EBUSY) {
 			/*
+			 * Drop cur_mm before scheduling, we can't hold it for
+			 * long periods (or over schedule()). Do this before
+			 * adding ourselves to the waitqueue, as the unuse/drop
+			 * may sleep.
+			 */
+			if (cur_mm) {
+				unuse_mm(cur_mm);
+				mmput(cur_mm);
+				cur_mm = NULL;
+			}
+
+			/*
 			 * We're polling. If we're within the defined idle
 			 * period, then let us spin without work before going
 			 * to sleep. The exception is if we got EBUSY doing
@@ -3922,18 +3934,6 @@ static int io_sq_thread(void *data)
 				continue;
 			}
 
-			/*
-			 * Drop cur_mm before scheduling, we can't hold it for
-			 * long periods (or over schedule()). Do this before
-			 * adding ourselves to the waitqueue, as the unuse/drop
-			 * may sleep.
-			 */
-			if (cur_mm) {
-				unuse_mm(cur_mm);
-				mmput(cur_mm);
-				cur_mm = NULL;
-			}
-
 			prepare_to_wait(&ctx->sqo_wait, &wait,
 						TASK_INTERRUPTIBLE);