linux-kernel - [PATCH] 9p/trans_fd: p9_fd_request: kick rx thread if EPOLLIN

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Message-ID: <20250819161013.GB11345@redhat.com>
Date: Tue, 19 Aug 2025 18:10:13 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Dominique Martinet <asmadeus@...ewreck.org>,
	K Prateek Nayak <kprateek.nayak@....com>,
	syzbot <syzbot+d1b5dace43896bc386c3@...kaller.appspotmail.com>
Cc: akpm@...ux-foundation.org, brauner@...nel.org, dvyukov@...gle.com,
	elver@...gle.com, glider@...gle.com, jack@...e.cz,
	kasan-dev@...glegroups.com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
	willy@...radead.org, v9fs@...ts.linux.dev,
	David Howells <dhowells@...hat.com>
Subject: [PATCH] 9p/trans_fd: p9_fd_request: kick rx thread if EPOLLIN

p9_read_work() doesn't set Rworksched and doesn't do schedule_work(m->rq)
if list_empty(&m->req_list).

However, if the pipe is full, we need to read more data and this used to
work prior to commit aaec5a95d59615 ("pipe_read: don't wake up the writer
if the pipe is still full").

p9_read_work() does p9_fd_read() -> ... -> anon_pipe_read() which (before
the commit above) triggered the unnecessary wakeup. This wakeup calls
p9_pollwake() which kicks p9_poll_workfn() -> p9_poll_mux(), p9_poll_mux()
will notice EPOLLIN and schedule_work(&m->rq).

This no longer happens after the optimization above, change p9_fd_request()
to use p9_poll_mux() instead of only checking for EPOLLOUT.

Reported-by: syzbot+d1b5dace43896bc386c3@...kaller.appspotmail.com
Tested-by: syzbot+d1b5dace43896bc386c3@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/68a2de8f.050a0220.e29e5.0097.GAE@google.com/
Link: https://lore.kernel.org/all/67dedd2f.050a0220.31a16b.003f.GAE@google.com/
Co-developed-by: K Prateek Nayak <kprateek.nayak@....com>
Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 net/9p/trans_fd.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index 339ec4e54778..474fe67f72ac 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -666,7 +666,6 @@ static void p9_poll_mux(struct p9_conn *m)
 
 static int p9_fd_request(struct p9_client *client, struct p9_req_t *req)
 {
-	__poll_t n;
 	int err;
 	struct p9_trans_fd *ts = client->trans;
 	struct p9_conn *m = &ts->conn;
@@ -686,13 +685,7 @@ static int p9_fd_request(struct p9_client *client, struct p9_req_t *req)
 	list_add_tail(&req->req_list, &m->unsent_req_list);
 	spin_unlock(&m->req_lock);
 
-	if (test_and_clear_bit(Wpending, &m->wsched))
-		n = EPOLLOUT;
-	else
-		n = p9_fd_poll(m->client, NULL, NULL);
-
-	if (n & EPOLLOUT && !test_and_set_bit(Wworksched, &m->wsched))
-		schedule_work(&m->wq);
+	p9_poll_mux(m);
 
 	return 0;
 }
-- 
2.25.1.362.g51ebf55