linux-kernel - Re: [syzbot] [xfs?] INFO: task hung in __fdget

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Date:   Sun, 3 Sep 2023 20:57:23 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Aleksandr Nogikh <nogikh@...gle.com>
Cc:     Dave Chinner <david@...morbit.com>,
        syzbot <syzbot+e245f0516ee625aaa412@...kaller.appspotmail.com>,
        brauner@...nel.org, djwong@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, llvm@...ts.linux.dev, nathan@...nel.org,
        ndesaulniers@...gle.com, syzkaller-bugs@...glegroups.com,
        trix@...hat.com
Subject: Re: [syzbot] [xfs?] INFO: task hung in __fdget_pos (4)

On 9/3/23, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Sun, Sep 03, 2023 at 10:33:57AM +0200, Mateusz Guzik wrote:
>> On Sun, Sep 03, 2023 at 03:25:28PM +1000, Dave Chinner wrote:
>> > On Sat, Sep 02, 2023 at 09:11:34PM -0700, syzbot wrote:
>> > > Hello,
>> > >
>> > > syzbot found the following issue on:
>> > >
>> > > HEAD commit:    b97d64c72259 Merge tag
>> > > '6.6-rc-smb3-client-fixes-part1' of..
>> > > git tree:       upstream
>> > > console output:
>> > > https://syzkaller.appspot.com/x/log.txt?x=14136d8fa80000
>> > > kernel config:
>> > > https://syzkaller.appspot.com/x/.config?x=958c1fdc38118172
>> > > dashboard link:
>> > > https://syzkaller.appspot.com/bug?extid=e245f0516ee625aaa412
>> > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for
>> > > Debian) 2.40
>> > >
>> > > Unfortunately, I don't have any reproducer for this issue yet.
>> >
>> > Been happening for months, apparently, yet for some reason it now
>> > thinks a locking hang in __fdget_pos() is an XFS issue?
>> >
>> > #syz set subsystems: fs
>> >
>>
>> The report does not have info necessary to figure this out -- no
>> backtrace for whichever thread which holds f_pos_lock. I clicked on a
>> bunch of other reports and it is the same story.
>>
>> Can the kernel be configured to dump backtraces from *all* threads?
>>
>> If there is no feature like that I can hack it up.
>
> <break>t
>
> over serial console, or echo t >/proc/sysrq-trigger would do it...
>

This does not dump backtraces, just a list of tasks + some stats.

The closest to useful here I found are 'w' ("Dumps tasks that are in
uninterruptable (blocked) state.") and 'l' ("Shows a stack backtrace
for all active CPUs."), both of which can miss the task which matters
(e.g., stuck in a very much *interruptible* state with f_pos_lock
held).

Unless someone can point at a way to get all these stacks, I'm going
to hack something up in the upcoming week, if only for immediate
syzbot usage.

-- 
Mateusz Guzik <mjguzik gmail.com>