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To:     Jens Axboe <axboe@kernel.dk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     io-uring <io-uring@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>
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From:   Stefan Metzmacher <metze@samba.org>
Subject: Re: [PATCH 0/2] Don't show PF_IO_WORKER in /proc/<pid>/task/
Message-ID: <5563d244-52c0-dafb-5839-e84990340765@samba.org>
Date:   Thu, 25 Mar 2021 22:57:20 +0100
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Am 25.03.21 um 22:44 schrieb Jens Axboe:
> On 3/25/21 2:40 PM, Jens Axboe wrote:
>> On 3/25/21 2:12 PM, Linus Torvalds wrote:
>>> On Thu, Mar 25, 2021 at 12:42 PM Linus Torvalds
>>> <torvalds@linux-foundation.org> wrote:
>>>>
>>>> On Thu, Mar 25, 2021 at 12:38 PM Linus Torvalds
>>>> <torvalds@linux-foundation.org> wrote:
>>>>>
>>>>> I don't know what the gdb logic is, but maybe there's some other
>>>>> option that makes gdb not react to them?
>>>>
>>>> .. maybe we could have a different name for them under the task/
>>>> subdirectory, for example (not  just the pid)? Although that probably
>>>> messes up 'ps' too..
>>>
>>> Actually, maybe the right model is to simply make all the io threads
>>> take signals, and get rid of all the special cases.
>>>
>>> Sure, the signals will never be delivered to user space, but if we
>>>
>>>  - just made the thread loop do "get_signal()" when there are pending signals
>>>
>>>  - allowed ptrace_attach on them
>>>
>>> they'd look pretty much like regular threads that just never do the
>>> user-space part of signal handling.
>>>
>>> The whole "signals are very special for IO threads" thing has caused
>>> so many problems, that maybe the solution is simply to _not_ make them
>>> special?
>>
>> Just to wrap up the previous one, yes it broke all sorts of things to
>> make the 'tid' directory different. They just end up being hidden anyway
>> through that, for both ps and top.
>>
>> Yes, I do think that maybe it's better to just embrace maybe just
>> embrace the signals, and have everything just work by default. It's
>> better than continually trying to make the threads special. I'll see
>> if there are some demons lurking down that path.
> 
> In the spirit of "let's just try it", I ran with the below patch. With
> that, I can gdb attach just fine to a test case that creates an io_uring
> and a regular thread with pthread_create(). The regular thread uses
> the ring, so you end up with two iou-mgr threads. Attach:
> 
> [root@archlinux ~]# gdb -p 360
> [snip gdb noise]
> Attaching to process 360
> [New LWP 361]
> [New LWP 362]
> [New LWP 363]
> 
> warning: Selected architecture i386:x86-64 is not compatible with reported target architecture i386
> 
> warning: Architecture rejected target-supplied description
> Error while reading shared library symbols for /usr/lib/libpthread.so.0:
> Cannot find user-level thread for LWP 363: generic error
> 0x00007f7aa526e125 in clock_nanosleep@GLIBC_2.2.5 () from /usr/lib/libc.so.6
> (gdb) info threads
>   Id   Target Id             Frame 
> * 1    LWP 360 "io_uring"    0x00007f7aa526e125 in clock_nanosleep@GLIBC_2.2.5 ()
>    from /usr/lib/libc.so.6
>   2    LWP 361 "iou-mgr-360" 0x0000000000000000 in ?? ()
>   3    LWP 362 "io_uring"    0x00007f7aa52a0a9d in syscall () from /usr/lib/libc.so.6
>   4    LWP 363 "iou-mgr-362" 0x0000000000000000 in ?? ()
> (gdb) thread 2
> [Switching to thread 2 (LWP 361)]
> #0  0x0000000000000000 in ?? ()
> (gdb) bt
> #0  0x0000000000000000 in ?? ()
> Backtrace stopped: Cannot access memory at address 0x0
> (gdb) cont
> Continuing.
> ^C
> Thread 1 "io_uring" received signal SIGINT, Interrupt.
> [Switching to LWP 360]
> 0x00007f7aa526e125 in clock_nanosleep@GLIBC_2.2.5 () from /usr/lib/libc.so.6
> (gdb) q
> A debugging session is active.
> 
> 	Inferior 1 [process 360] will be detached.
> 
> Quit anyway? (y or n) y
> Detaching from program: /root/git/fio/t/io_uring, process 360
> [Inferior 1 (process 360) detached]
> 
> The iou-mgr-x threads are stopped just fine, gdb obviously can't get any
> real info out of them. But it works... Regular test cases work fine too,
> just a sanity check. Didn't expect them not to.

I guess that's basically what I tried to describe when I said they should
look like a userspace process that is blocked in a syscall forever.

> Only thing that I dislike a bit, but I guess that's just a Linuxism, is
> that if can now kill an io_uring owning task by sending a signal to one
> of its IO thread workers.

Can't we just only allow SIGSTOP, which will be only delivered to
the iothread itself? And also SIGKILL should not be allowed from userspace.

And /proc/$iothread/ should be read only and owned by root with
"cmdline" and "exe" being empty.

Thanks!
metze