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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>,
        Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 5.4 121/135] io_uring: fix __io_iopoll_check deadlock in io_sq_thread
Date:   Thu, 27 Feb 2020 14:37:41 +0100
Message-Id: <20200227132247.269204084@linuxfoundation.org>
In-Reply-To: <20200227132228.710492098@linuxfoundation.org>
References: <20200227132228.710492098@linuxfoundation.org>
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From: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

commit c7849be9cc2dd2754c48ddbaca27c2de6d80a95d upstream.

Since commit a3a0e43fd770 ("io_uring: don't enter poll loop if we have
CQEs pending"), if we already events pending, we won't enter poll loop.
In case SETUP_IOPOLL and SETUP_SQPOLL are both enabled, if app has
been terminated and don't reap pending events which are already in cq
ring, and there are some reqs in poll_list, io_sq_thread will enter
__io_iopoll_check(), and find pending events, then return, this loop
will never have a chance to exit.

I have seen this issue in fio stress tests, to fix this issue, let
io_sq_thread call io_iopoll_getevents() with argument 'min' being zero,
and remove __io_iopoll_check().

Fixes: a3a0e43fd770 ("io_uring: don't enter poll loop if we have CQEs pending")
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/io_uring.c |   27 +++++++++------------------
 1 file changed, 9 insertions(+), 18 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -882,11 +882,17 @@ static void io_iopoll_reap_events(struct
 	mutex_unlock(&ctx->uring_lock);
 }
 
-static int __io_iopoll_check(struct io_ring_ctx *ctx, unsigned *nr_events,
-			    long min)
+static int io_iopoll_check(struct io_ring_ctx *ctx, unsigned *nr_events,
+			   long min)
 {
 	int iters = 0, ret = 0;
 
+	/*
+	 * We disallow the app entering submit/complete with polling, but we
+	 * still need to lock the ring to prevent racing with polled issue
+	 * that got punted to a workqueue.
+	 */
+	mutex_lock(&ctx->uring_lock);
 	do {
 		int tmin = 0;
 
@@ -922,21 +928,6 @@ static int __io_iopoll_check(struct io_r
 		ret = 0;
 	} while (min && !*nr_events && !need_resched());
 
-	return ret;
-}
-
-static int io_iopoll_check(struct io_ring_ctx *ctx, unsigned *nr_events,
-			   long min)
-{
-	int ret;
-
-	/*
-	 * We disallow the app entering submit/complete with polling, but we
-	 * still need to lock the ring to prevent racing with polled issue
-	 * that got punted to a workqueue.
-	 */
-	mutex_lock(&ctx->uring_lock);
-	ret = __io_iopoll_check(ctx, nr_events, min);
 	mutex_unlock(&ctx->uring_lock);
 	return ret;
 }
@@ -2721,7 +2712,7 @@ static int io_sq_thread(void *data)
 				 */
 				mutex_lock(&ctx->uring_lock);
 				if (!list_empty(&ctx->poll_list))
-					__io_iopoll_check(ctx, &nr_events, 0);
+					io_iopoll_getevents(ctx, &nr_events, 0);
 				else
 					inflight = 0;
 				mutex_unlock(&ctx->uring_lock);