On 12/24/21 19:52, Eric W. Biederman wrote:

Pavel Begunkov [off-list ref] writes:

[...]

quoted

FWIW, I worked it around in io_uring back then by breaking the
dependency.

I am in the middle of untangling the dependencies between ptrace,
coredump, signal handling and maybe a few related things.

Sounds great

Do folks have a reproducer I can look at?  Pavel especially if you have
something that reproduces on the current kernels.

A syz reproducer was triggering it reliably, I'd try to revert the
commit below and test:
https://syzkaller.appspot.com/text?tag=ReproC&x=15d3600cb00000

It should hung a task. Syzbot report for reference:
https://syzkaller.appspot.com/bug?extid=27d62ee6f256b186883e


commit 1d5f5ea7cb7d15b9fb1cc82673ebb054f02cd7d2
Author: Pavel Begunkov [off-list ref]
Date:   Fri Oct 29 13:11:33 2021 +0100

     io-wq: remove worker to owner tw dependency

     INFO: task iou-wrk-6609:6612 blocked for more than 143 seconds.
           Not tainted 5.15.0-rc5-syzkaller #0
     "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
     task:iou-wrk-6609    state:D stack:27944 pid: 6612 ppid:  6526 flags:0x00004006
     Call Trace:
      context_switch kernel/sched/core.c:4940 [inline]
      __schedule+0xb44/0x5960 kernel/sched/core.c:6287
      schedule+0xd3/0x270 kernel/sched/core.c:6366
      schedule_timeout+0x1db/0x2a0 kernel/time/timer.c:1857
      do_wait_for_common kernel/sched/completion.c:85 [inline]
      __wait_for_common kernel/sched/completion.c:106 [inline]
      wait_for_common kernel/sched/completion.c:117 [inline]
      wait_for_completion+0x176/0x280 kernel/sched/completion.c:138
      io_worker_exit fs/io-wq.c:183 [inline]
      io_wqe_worker+0x66d/0xc40 fs/io-wq.c:597
      ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
      ...

As part of that I am in the process of guaranteeing all of the coredump
work happens in get_signal so nothing of io_uring or any cleanup
anywhere else runs until the coredump completes.

I haven't quite posted the code for review because it's the holidays.
But I am aiming at v5.17 or possibly v5.18, as the code is just about
ready.

-- 
Pavel Begunkov