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

[Mateusz Guzik <mjguzik@gmail.com>]

On 9/3/23, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Sun, Sep 03, 2023 at 08:57:23PM +0200, Mateusz Guzik wrote:
>
>> 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.
>
> Huh?  Sample of output here:
> 2023-09-03T15:34:36.271833-04:00 duke kernel: [87367.574459] task:ssh-agent
>      state:S stack:0     pid:3949  ppid:3947   flags:0x
> 00000002
> 2023-09-03T15:34:36.284796-04:00 duke kernel: [87367.582848] Call Trace:
> 2023-09-03T15:34:36.284797-04:00 duke kernel: [87367.585306]  <TASK>
> 2023-09-03T15:34:36.284797-04:00 duke kernel: [87367.587423]
> __schedule+0x222/0x630
> 2023-09-03T15:34:36.291459-04:00 duke kernel: [87367.590932]
> schedule+0x4b/0x90
> 2023-09-03T15:34:36.291460-04:00 duke kernel: [87367.594086]
> schedule_hrtimeout_range_clock+0xb1/0x110
> 2023-09-03T15:34:36.300477-04:00 duke kernel: [87367.599245]  ?
> __hrtimer_init+0xf0/0xf0
> 2023-09-03T15:34:36.300477-04:00 duke kernel: [87367.603103]
> do_sys_poll+0x489/0x580
> 2023-09-03T15:34:36.308971-04:00 duke kernel: [87367.606702]  ?
> _raw_spin_unlock_irqrestore+0x9/0x20
> 2023-09-03T15:34:36.308972-04:00 duke kernel: [87367.611598]  ?
> __alloc_pages+0x111/0x1a0
> 2023-09-03T15:34:36.317380-04:00 duke kernel: [87367.615544]  ?
> select_task_rq_fair+0x1c8/0xf70
> 2023-09-03T15:34:36.317381-04:00 duke kernel: [87367.620006]  ?
> _raw_spin_unlock+0x5/0x10
> 2023-09-03T15:34:36.325273-04:00 duke kernel: [87367.623953]  ?
> sched_clock_cpu+0x1c/0xd0
> 2023-09-03T15:34:36.325274-04:00 duke kernel: [87367.627899]  ?
> default_send_IPI_single_phys+0x21/0x30
> 2023-09-03T15:34:36.334812-04:00 duke kernel: [87367.632977]  ?
> ttwu_queue_wakelist+0x109/0x110
> 2023-09-03T15:34:36.334813-04:00 duke kernel: [87367.637439]  ?
> _raw_spin_unlock_irqrestore+0x9/0x20
> 2023-09-03T15:34:36.343753-04:00 duke kernel: [87367.642344]  ?
> try_to_wake_up+0x1eb/0x300
> 2023-09-03T15:34:36.343754-04:00 duke kernel: [87367.646380]  ?
> __pollwait+0x110/0x110
> 2023-09-03T15:34:36.351376-04:00 duke kernel: [87367.650063]  ?
> _raw_spin_unlock+0x5/0x10
> 2023-09-03T15:34:36.351377-04:00 duke kernel: [87367.654001]  ?
> unix_stream_read_generic+0x528/0xa90
> 2023-09-03T15:34:36.361179-04:00 duke kernel: [87367.658906]  ?
> _raw_spin_unlock_irqrestore+0x9/0x20
> 2023-09-03T15:34:36.361180-04:00 duke kernel: [87367.663805]  ?
> _raw_spin_unlock_irqrestore+0x9/0x20
> 2023-09-03T15:34:36.370988-04:00 duke kernel: [87367.668708]  ?
> __inode_wait_for_writeback+0x68/0xc0
> 2023-09-03T15:34:36.370989-04:00 duke kernel: [87367.673614]  ?
> fsnotify_grab_connector+0x49/0x90
> 2023-09-03T15:34:36.380274-04:00 duke kernel: [87367.678258]  ?
> fsnotify_destroy_marks+0x11/0x140
> 2023-09-03T15:34:36.380275-04:00 duke kernel: [87367.682901]  ?
> enqueue_task_fair+0x211/0x5f0
> 2023-09-03T15:34:36.389726-04:00 duke kernel: [87367.687196]  ?
> __rseq_handle_notify_resume+0x2b4/0x3a0
> 2023-09-03T15:34:36.389728-04:00 duke kernel: [87367.692353]  ?
> recalibrate_cpu_khz+0x10/0x10
> 2023-09-03T15:34:36.397884-04:00 duke kernel: [87367.696651]  ?
> ktime_get_ts64+0x47/0xe0
> 2023-09-03T15:34:36.397885-04:00 duke kernel: [87367.700509]
> __x64_sys_poll+0x93/0x120
> 2023-09-03T15:34:36.405254-04:00 duke kernel: [87367.704282]
> do_syscall_64+0x42/0x90
> 2023-09-03T15:34:36.405255-04:00 duke kernel: [87367.707880]
> entry_SYSCALL_64_after_hwframe+0x4b/0xb5
> 2023-09-03T15:34:36.413922-04:00 duke kernel: [87367.712959] RIP:
> 0033:0x7f451858f000
> 2023-09-03T15:34:36.413923-04:00 duke kernel: [87367.716548] RSP:
> 002b:00007ffd799cece8 EFLAGS: 00000202 ORIG_RAX: 0000000000000007
> 2023-09-03T15:34:36.428692-04:00 duke kernel: [87367.724154] RAX:
> ffffffffffffffda RBX: 00000000000001b0 RCX: 00007f451858f000
> 2023-09-03T15:34:36.428692-04:00 duke kernel: [87367.731317] RDX:
> 0000000000002710 RSI: 0000000000000001 RDI: 00005596fc603190
> 2023-09-03T15:34:36.443022-04:00 duke kernel: [87367.738485] RBP:
> 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
> 2023-09-03T15:34:36.443023-04:00 duke kernel: [87367.745649] R10:
> 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
> 2023-09-03T15:34:36.457354-04:00 duke kernel: [87367.752818] R13:
> 0000000000000001 R14: 000000000000000a R15: 00005596fc603190
> 2023-09-03T15:34:36.457355-04:00 duke kernel: [87367.759981]  </TASK>
>
> Looks like a stack trace to me; seeing one of the callers of fdget_pos()
> in that would tell you who's currently holding *some* ->f_pos_lock.
>
> That - on 6.1.42, with fairly bland .config (minimal debugging;
> I need that box for fast builds, among other things).  Enable
> lockdep and you'll get who's holding which logs in addition
> to those stack traces...
>

That's my bad, the only content which survived for me in terminal
buffer was a spew for the scheduler and a ps-like task list.
Stacktraces are indeed there. Sorry for the noise on this one. I
verified stack dumps are not gated by any defines either btw, so it's
all good on this front.

All this brainos aside, I added Aleksandr Nogikh to To: in my previous
e-mail. From short poking around I found he was responsive to some
queries concerning Linux vs syzbot and I figured would a good person
to ask.

So, Aleksandr, tl;dr would it be a problem to add the 't' sysrq to
syzbot in order to dump backtraces from all threads? Either break t
over a serial port or t written to /proc/sysrq-trigger. Lack of this
info blocks progress on the issue reported here
(https://syzkaller.appspot.com/bug?extid=e245f0516ee625aaa412)

-- 
Mateusz Guzik <mjguzik gmail.com>