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

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Message-ID: <314522ae-05a4-4dfb-af99-6bb3901a5522@amd.com>
Date: Fri, 28 Mar 2025 23:26:20 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Oleg Nesterov <oleg@...hat.com>, syzbot
	<syzbot+62262fdc0e01d99573fc@...kaller.appspotmail.com>
CC: <asmadeus@...ewreck.org>, <brauner@...nel.org>, <dhowells@...hat.com>,
	<ericvh@...nel.org>, <jack@...e.cz>, <jlayton@...nel.org>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux_oss@...debyte.com>, <lucho@...kov.net>, <mjguzik@...il.com>,
	<netfs@...ts.linux.dev>, <swapnil.sapkal@....com>,
	<syzkaller-bugs@...glegroups.com>, <v9fs@...ts.linux.dev>,
	<viro@...iv.linux.org.uk>
Subject: Re: [syzbot] [netfs?] INFO: task hung in netfs_unbuffered_write_iter

Hello Oleg,

On 3/28/2025 10:30 PM, Oleg Nesterov wrote:
> Dear syzbot,
> 
> On 03/28, syzbot wrote:
>>
>> syzbot has tested the proposed patch and the reproducer did not trigger any issue:
>>
>> Reported-by: syzbot+62262fdc0e01d99573fc@...kaller.appspotmail.com
>> Tested-by: syzbot+62262fdc0e01d99573fc@...kaller.appspotmail.com
>

Yours is the right approach. I was searching for p9_poll_mux()
which would wake the read collector to free pipe buffers but I
didn't dig enough and I was too lazy to open code it (guilty!)
If this gets picked, feel free to add:

Reviewed-and-tested-by: K Prateek Nayak <kprateek.nayak@....com>
  
> Thanks. so the previous
> 
> 	syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> 	unregister_netdevice: waiting for DEV to become free
> 
> 	unregister_netdevice: waiting for batadv0 to become free. Usage count = 3
> 
> report suggests that upstream/master has another/unrelated issue(s).

I could not reproduce this bit on mainline at commit acb4f33713b9
("Merge tag 'm68knommu-for-v6.15' of
git://git.kernel.org/pub/scm/linux/kernel/git/gerg/m68knommu") so I too
second that it is an unrelated issue.

> 
> As for the patches from me or Prateek (thanks again!), I think that
> the maintainers should take a look.
> 
> But at this point I am mostly confident that the bisected commit aaec5a95d5961
> ("pipe_read: don't wake up the writer if the pipe is still full") is innocent,
> it just reveals yet another problem.
> 
> I guess (I hope ;) Prateek agrees.

I agree aaec5a95d5961 is not guilty! (unlike me) :)

> 
> Oleg.
> 

-- 
Thanks and Regards,
Prateek