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

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Message-ID: <Z-LEsPFE4e7TTMiY@codewreck.org>
Date: Tue, 25 Mar 2025 23:58:56 +0900
From: Dominique Martinet <asmadeus@...ewreck.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Oleg Nesterov <oleg@...hat.com>,
	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

Thanks for the traces.

w/ revert
K Prateek Nayak wrote on Tue, Mar 25, 2025 at 08:19:26PM +0530:
>    kworker/100:1-1803    [100] .....   286.618822: p9_fd_poll: p9_fd_poll rd poll
>    kworker/100:1-1803    [100] .....   286.618822: p9_fd_poll: p9_fd_request wr poll
>    kworker/100:1-1803    [100] .....   286.618823: p9_read_work: Data read wait 7

new behavior
>            repro-4076    [031] .....    95.011394: p9_fd_poll: p9_fd_poll rd poll
>            repro-4076    [031] .....    95.011394: p9_fd_poll: p9_fd_request wr poll
>            repro-4076    [031] .....    99.731970: p9_client_rpc: Wait event killable (-512)

For me the problem isn't so much that this gets ERESTARTSYS but that it
nevers gets to read the 7 bytes that are available?

If the repro has already written the bytes in both cases then there's no
reason to wait 5 seconds here...

OTOH syzbot 9p code is silly and might have been depending on something
that's not supposed to work e.g. they might be missing a flush or
equivalent for all I know (I still haven't looked at the repro)

-- 
Dominique