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From:   Nadav Amit <nadav.amit@gmail.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     io-uring@vger.kernel.org, linux-kernel@vger.kernel.org,
        Nadav Amit <namit@vmware.com>,
        Pavel Begunkov <asml.silence@gmail.com>
Subject: [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task work
Date:   Sat,  7 Aug 2021 17:13:41 -0700
Message-Id: <20210808001342.964634-2-namit@vmware.com>
In-Reply-To: <20210808001342.964634-1-namit@vmware.com>
References: <20210808001342.964634-1-namit@vmware.com>
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From: Nadav Amit <namit@vmware.com>

When using SQPOLL, the submission queue polling thread calls
task_work_run() to run queued work. However, when work is added with
TWA_SIGNAL - as done by io_uring itself - the TIF_NOTIFY_SIGNAL remains
set afterwards and is never cleared.

Consequently, when the submission queue polling thread checks whether
signal_pending(), it may always find a pending signal, if
task_work_add() was ever called before.

The impact of this bug might be different on different kernel versions.
It appears that on 5.14 it would only cause unnecessary calculation and
prevent the polling thread from sleeping. On 5.13, where the bug was
found, it stops the polling thread from finding newly submitted work.

Instead of task_work_run(), use tracehook_notify_signal() that clears
TIF_NOTIFY_SIGNAL. Test for TIF_NOTIFY_SIGNAL in addition to
current->task_works to avoid a race in which task_works is cleared but
the TIF_NOTIFY_SIGNAL is set.

Fixes: 685fe7feedb96 ("io-wq: eliminate the need for a manager thread")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 fs/io_uring.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5a0fd6bcd318..f39244d35f90 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -78,6 +78,7 @@
 #include <linux/task_work.h>
 #include <linux/pagemap.h>
 #include <linux/io_uring.h>
+#include <linux/tracehook.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/io_uring.h>
@@ -2203,9 +2204,9 @@ static inline unsigned int io_put_rw_kbuf(struct io_kiocb *req)
 
 static inline bool io_run_task_work(void)
 {
-	if (current->task_works) {
+	if (test_thread_flag(TIF_NOTIFY_SIGNAL) || current->task_works) {
 		__set_current_state(TASK_RUNNING);
-		task_work_run();
+		tracehook_notify_signal();
 		return true;
 	}
 
-- 
2.25.1