Re: [syzbot] [xfs?] INFO: task hung in __fdget_pos (4)

[Al Viro]

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...

"Locking hang in __fdget_pos()" would mean either something stuck inside
->read/->write/->read_iter/->write_iter/->llseek instance, making
any further syscalls in that bunch on the same open file to block,
or a lost fdput_pos() somewhere; AFAICS, we don't have anything of
the latter variety, but the former is too wide to tell anything
useful.