release/5.2.40/io_uring-prevent-sq_thread-from-spinning-when-it-sho.patch

From b2a3c63e90a060d4f65995385b5dffed034780f5 Mon Sep 17 00:00:00 2001
From: Stefano Garzarella <sgarzare@redhat.com>
Date: Fri, 21 Feb 2020 16:42:16 +0100
Subject: [PATCH] io_uring: prevent sq_thread from spinning when it should stop

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@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[PG: original moved cur_mm up; here polling block moves down instead.]
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 0761c28e8b2e..a6dd8cea8853 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2160,16 +2160,6 @@ static int io_sq_thread(void *data)
 
 		if (!io_get_sqring(ctx, &sqes[0])) {
 			/*
-			 * We're polling. If we're within the defined idle
-			 * period, then let us spin without work before going
-			 * to sleep.
-			 */
-			if (inflight || !time_after(jiffies, timeout)) {
-				cpu_relax();
-				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
@@ -2181,6 +2171,16 @@ static int io_sq_thread(void *data)
 				cur_mm = NULL;
 			}
 
+			/*
+			 * We're polling. If we're within the defined idle
+			 * period, then let us spin without work before going
+			 * to sleep.
+			 */
+			if (inflight || !time_after(jiffies, timeout)) {
+				cpu_relax();
+				continue;
+			}
+
 			prepare_to_wait(&ctx->sqo_wait, &wait,
 						TASK_INTERRUPTIBLE);
 
-- 
2.7.4