linux-kernel - Re: [syzbot] [netfs?] INFO: task hung in netfs_unbuffered_write

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Message-ID: <20250326124402.GD30181@redhat.com>
Date: Wed, 26 Mar 2025 13:44:03 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Dominique Martinet <asmadeus@...ewreck.org>,
	K Prateek Nayak <kprateek.nayak@....com>
Cc: Eric Van Hensbergen <ericvh@...nel.org>,
	Latchesar Ionkov <lucho@...kov.net>,
	Christian Schoenebeck <linux_oss@...debyte.com>,
	Mateusz Guzik <mjguzik@...il.com>,
	syzbot <syzbot+62262fdc0e01d99573fc@...kaller.appspotmail.com>,
	brauner@...nel.org, dhowells@...hat.com, jack@...e.cz,
	jlayton@...nel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, netfs@...ts.linux.dev,
	swapnil.sapkal@....com, syzkaller-bugs@...glegroups.com,
	viro@...iv.linux.org.uk, v9fs@...ts.linux.dev
Subject: Re: [syzbot] [netfs?] INFO: task hung in netfs_unbuffered_write_iter

On 03/26, Oleg Nesterov wrote:
>
> Hmm... I don't understand why the 2nd vfs_poll(ts->wr) depends on the
> ret from vfs_poll(ts->rd), but I assume this is correct.

I meant, if pt != NULL and ts->rd != ts->wr we need both
vfs_poll(ts->rd) and vfs_poll(ts->wr) ?

and the reproducer writes to the pipe before it mounts 9p...

Prateek, this is just a shot in the dark but since you can reproduce,
can you check if the patch below makes any difference?

Oleg.

--- x/net/9p/trans_fd.c
+++ x/net/9p/trans_fd.c
@@ -234,8 +234,10 @@ p9_fd_poll(struct p9_client *client, str
 	}
 
 	ret = vfs_poll(ts->rd, pt);
-	if (ts->rd != ts->wr)
+	if (ts->rd != ts->wr) {
+		if (pt != NULL) vfs_poll(ts->wr, pt);
 		ret = (ret & ~EPOLLOUT) | (vfs_poll(ts->wr, pt) & ~EPOLLIN);
+	}
 	return ret;
 }