Re: [syzbot] [netfs?] INFO: task hung in netfs_unbuffered_write_iter

[K Prateek Nayak]

Hello folks,

Some updates.

On 3/24/2025 6:49 PM, K Prateek Nayak wrote:
> > Per syzbot this attempt did not work out either.
> >
> > I think the blind stabs taken by everyone here are enough.
> >
> > The report does not provide the crucial bit: what are the other
> > threads doing. Presumably someone else is stuck somewhere, possibly
> > not even in pipe code and that stuck thread was supposed to wake up
> > the one which trips over hung task detector. Figuring out what that
> > thread is imo the next step.
> >
> > I failed to find a relevant command in
> > https://github.com/google/syzkaller/blob/master/docs/syzbot.md
> >
> > So if you guys know someone on syzkaller side, maybe you can ask them
> > to tweak the report *or* maybe syzbot can test a "fix" which makes
> > hung task detector also report all backtraces? I don't know if that
> > can work, the output may be long enough that it will get trimmed by
> > something.
> >
> > I don't have to time work on this for now, just throwing ideas.
>
> I got the reproducer running locally. Tracing stuff currently to see
> what is tripping. Will report back once I find something interesting.
> Might take a while since the 9p bits are so far spread out.

So far, with tracing, this is where I'm:

o Mainline + Oleg's optimization reverted:

    ...
    kworker/43:1-1723    [043] .....   115.309065: p9_read_work: Data read wait 55
    kworker/43:1-1723    [043] .....   115.309066: p9_read_work: Data read 55
    kworker/43:1-1723    [043] .....   115.309067: p9_read_work: Data read wait 7
    kworker/43:1-1723    [043] .....   115.309068: p9_read_work: Data read 7
           repro-4138    [043] .....   115.309084: netfs_wake_write_collector: Wake collector
           repro-4138    [043] .....   115.309085: netfs_wake_write_collector: Queuing collector work
           repro-4138    [043] .....   115.309088: netfs_unbuffered_write: netfs_unbuffered_write
           repro-4138    [043] .....   115.309088: netfs_end_issue_write: netfs_end_issue_write
           repro-4138    [043] .....   115.309089: netfs_end_issue_write: Write collector need poke 0
           repro-4138    [043] .....   115.309091: netfs_unbuffered_write_iter_locked: Waiting on NETFS_RREQ_IN_PROGRESS!
 kworker/u1030:1-1951    [168] .....   115.309096: netfs_wake_write_collector: Wake collector
 kworker/u1030:1-1951    [168] .....   115.309097: netfs_wake_write_collector: Queuing collector work
 kworker/u1030:1-1951    [168] .....   115.309102: netfs_write_collection_worker: Write collect clearing and waking up!
    ... (syzbot reproducer continues)

o Mainline:

   kworker/185:1-1767    [185] .....   109.485961: p9_read_work: Data read wait 7
   kworker/185:1-1767    [185] .....   109.485962: p9_read_work: Data read 7
   kworker/185:1-1767    [185] .....   109.485962: p9_read_work: Data read wait 55
   kworker/185:1-1767    [185] .....   109.485963: p9_read_work: Data read 55
           repro-4038    [185] .....   114.225717: netfs_wake_write_collector: Wake collector
           repro-4038    [185] .....   114.225723: netfs_wake_write_collector: Queuing collector work
           repro-4038    [185] .....   114.225727: netfs_unbuffered_write: netfs_unbuffered_write
           repro-4038    [185] .....   114.225727: netfs_end_issue_write: netfs_end_issue_write
           repro-4038    [185] .....   114.225728: netfs_end_issue_write: Write collector need poke 0
           repro-4038    [185] .....   114.225728: netfs_unbuffered_write_iter_locked: Waiting on NETFS_RREQ_IN_PROGRESS!
   ... (syzbot reproducer hangs)

There is a third "kworker/u1030" component that never gets woken up for
reasons currently unknown to me with Oleg's optimization. I'll keep
digging.

--
Thanks and Regards,
Prateek