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Subject: Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es'
 registers for io_threads
From:   Olivier Langlois <olivier@trillion01.com>
To:     Jens Axboe <axboe@kernel.dk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Stefan Metzmacher <metze@samba.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        io-uring <io-uring@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>
Date:   Wed, 12 May 2021 00:24:32 -0400
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On Mon, 2021-05-03 at 20:50 -0600, Jens Axboe wrote:
> 
> I tested the below, which is the two combined, with a case that
> deliberately has two types of io threads - one for SQPOLL submission,
> and one that was created due to async work being needed. gdb attaches
> just fine to the creator, with a slight complaint:
> 
> Attaching to process 370
> [New LWP 371]
> [New LWP 372]
> Error while reading shared library symbols for
> /usr/lib/libpthread.so.0:
> Cannot find user-level thread for LWP 372: generic error
> 0x00007f1a74675125 in clock_nanosleep@GLIBC_2.2.5 () from
> /usr/lib/libc.so.6
> (gdb) info threads
> ? Id?? Target Id???????????? Frame 
> * 1??? LWP 370 "io_uring"??? 0x00007f1a74675125 in
> clock_nanosleep@GLIBC_2.2.5 ()
> ?? from /usr/lib/libc.so.6
> ? 2??? LWP 371 "iou-sqp-370" 0x00007f1a746a7a9d in syscall () from
> /usr/lib/libc.so.6
> ? 3??? LWP 372 "io_uring"??? 0x00007f1a74675125 in
> clock_nanosleep@GLIBC_2.2.5 ()
> ?? from /usr/lib/libc.so.6
> 
> (gdb) thread 2
> [Switching to thread 2 (LWP 371)]
> #0? 0x00007f1a746a7a9d in syscall () from /usr/lib/libc.so.6
> (gdb) bt
> #0? 0x00007f1a746a7a9d in syscall () from /usr/lib/libc.so.6
> Backtrace stopped: Cannot access memory at address 0x0
> 
> (gdb) thread 1
> [Switching to thread 1 (LWP 370)]
> #0? 0x00007f1a74675125 in clock_nanosleep@GLIBC_2.2.5 () from
> /usr/lib/libc.so.6
> (gdb) bt
> #0? 0x00007f1a74675125 in clock_nanosleep@GLIBC_2.2.5 () from
> /usr/lib/libc.so.6
> #1? 0x00007f1a7467a357 in nanosleep () from /usr/lib/libc.so.6
> #2? 0x00007f1a7467a28e in sleep () from /usr/lib/libc.so.6
> #3? 0x000055bd41e929ba in main (argc=<optimized out>, argv=<optimized
> out>)
> ??? at t/io_uring.c:658
> 
> which looks very reasonable to me - no backtraces for the io threads,
> and
> no arch complaints.
> 
I have reported an issue that I have with a user process using io_uring
where when it core dumps, the dump fails to be generated.
https://github.com/axboe/liburing/issues/346

Pavel did comment to my report and he did point out this thread as
possibly a related issue.

I'm far from being 100% convinced that Stefan patch can help but I am
going to give it a try and report back here if it does help.

Greetings,
Olivier