[PATCH v3 2/4] io_uring: Fix missing smp_mb() in io_cancel_async_work()

[Muchun Song]

The store to req->flags and load req->work_task should not be
reordering in io_cancel_async_work(). We should make sure that
either we store REQ_F_CANCE flag to req->flags or we see the
req->work_task setted in io_sq_wq_submit_work().

Fixes: 1c4404efcf2c ("io_uring: make sure async workqueue is canceled on exit")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 fs/io_uring.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2f46def7f5832..5d9583e3d0d25 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2252,6 +2252,12 @@ static void io_sq_wq_submit_work(struct work_struct *work)
 
 		if (!ret) {
 			req->work_task = current;
+
+			/*
+			 * Pairs with the smp_store_mb() (B) in
+			 * io_cancel_async_work().
+			 */
+			smp_mb(); /* A */
 			if (req->flags & REQ_F_CANCEL) {
 				ret = -ECANCELED;
 				goto end_req;
@@ -3730,7 +3736,15 @@ static void io_cancel_async_work(struct io_ring_ctx *ctx,
 
 		req = list_first_entry(&ctx->task_list, struct io_kiocb, task_list);
 		list_del_init(&req->task_list);
-		req->flags |= REQ_F_CANCEL;
+
+		/*
+		 * The below executes an smp_mb(), which matches with the
+		 * smp_mb() (A) in io_sq_wq_submit_work() such that either
+		 * we store REQ_F_CANCEL flag to req->flags or we see the
+		 * req->work_task setted in io_sq_wq_submit_work().
+		 */
+		smp_store_mb(req->flags, req->flags | REQ_F_CANCEL); /* B */
+
 		if (req->work_task && (!files || req->files == files))
 			send_sig(SIGINT, req->work_task, 1);
 	}
-- 
2.11.0