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From: Hao Xu <haoxu@linux.alibaba.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: io-uring@vger.kernel.org, Pavel Begunkov <asml.silence@gmail.com>,
	Joseph Qi <joseph.qi@linux.alibaba.com>
Subject: [PATCH 3/3] io_uring: fix potential req refcount underflow
Date: Wed, 22 Sep 2021 18:12:38 +0800	[thread overview]
Message-ID: <20210922101238.7177-4-haoxu@linux.alibaba.com> (raw)
In-Reply-To: <20210922101238.7177-1-haoxu@linux.alibaba.com>

For multishot mode, there may be cases like:
    iowq                          original context
io_poll_add
  _arm_poll()
  mask = vfs_poll() is not 0
  if mask
(2)  io_poll_complete()
  compl_unlock
   (interruption happens
    tw queued to original
    context)
                                     io_poll_task_func()
                                     compl_lock
                                 (3) done = io_poll_complete() is true
                                     compl_unlock
                                     put req ref
(1) if (poll->flags & EPOLLONESHOT)
      put req ref

EPOLLONESHOT flag in (1) may be from (2) or (3), so there are multiple
combinations that can cause ref underfow.
Let's address it by:
- check the return value in (2) as done
- change (1) to if (done)
    in this way, we only do ref put in (1) if 'oneshot flag' is from
    (2)
- do poll.done check in io_poll_task_func(), so that we won't put ref
  for the second time.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
---
 fs/io_uring.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index b7c9fcce2de2..b008edccad46 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5370,6 +5370,10 @@ static void io_poll_task_func(struct io_kiocb *req, bool *locked)
 	} else {
 		bool done;
 
+		if (req->poll.done) {
+			spin_unlock(&ctx->completion_lock);
+			return;
+		}
 		done = __io_poll_complete(req, req->result);
 		if (done) {
 			io_poll_remove_double(req);
@@ -5833,6 +5837,7 @@ static int io_poll_add(struct io_kiocb *req, unsigned int issue_flags)
 	struct io_ring_ctx *ctx = req->ctx;
 	struct io_poll_table ipt;
 	__poll_t mask;
+	bool done;
 
 	ipt.pt._qproc = io_poll_queue_proc;
 
@@ -5841,13 +5846,13 @@ static int io_poll_add(struct io_kiocb *req, unsigned int issue_flags)
 
 	if (mask) { /* no async, we'd stolen it */
 		ipt.error = 0;
-		io_poll_complete(req, mask);
+		done = io_poll_complete(req, mask);
 	}
 	spin_unlock(&ctx->completion_lock);
 
 	if (mask) {
 		io_cqring_ev_posted(ctx);
-		if (poll->events & EPOLLONESHOT)
+		if (done)
 			io_put_req(req);
 	}
 	return ipt.error;
-- 
2.24.4


  parent reply	other threads:[~2021-09-22 10:13 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-22 10:12 [PATCH 0/3] poll fixes Hao Xu
2021-09-22 10:12 ` [PATCH 1/3] io_uring: fix race between poll completion and cancel_hash insertion Hao Xu
2021-09-22 10:12 ` [PATCH 2/3] io_uring: fix lacking of EPOLLONESHOT Hao Xu
2021-09-22 10:12 ` Hao Xu [this message]
2021-09-22 15:55 ` [PATCH 0/3] poll fixes Jens Axboe

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