[PATCH] io_uring: prevent sq_thread from spinning when it should stop

[PATCH] io_uring: prevent sq_thread from spinning when it should stop

[Stefano Garzarella]

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>
---

Hi Jens,
I'm also sending a test to liburing for this case.

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

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5a826017ebb8..f902f77964ef 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5138,6 +5138,18 @@ static int io_sq_thread(void *data)
 		 * to enter the kernel to reap and flush events.
 		 */
 		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
@@ -5152,18 +5164,6 @@ 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);
 
-- 
2.24.1

Re: [PATCH] io_uring: prevent sq_thread from spinning when it should stop

[Jens Axboe]

On 2/21/20 8:42 AM, Stefano Garzarella wrote:
> 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.

Thanks, applied.

-- 
Jens Axboe