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Subject: Re: [PATCH v2] io_thread/x86: setup io_threads more like normal user
 space threads
To:     Thomas Gleixner <tglx@linutronix.de>,
        Stefan Metzmacher <metze@samba.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@kernel.org>, linux-kernel@vger.kernel.org,
        io-uring@vger.kernel.org, x86@kernel.org
References: <20210411152705.2448053-1-metze@samba.org>
 <20210505110310.237537-1-metze@samba.org>
 <df4b116a-3324-87b7-ff40-67d134b4e55c@kernel.dk>
 <878s4soncx.ffs@nanos.tec.linutronix.de>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <4d79811e-7b71-009d-ef31-d6af045b25fd@kernel.dk>
Date:   Wed, 5 May 2021 17:35:19 -0600
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On 5/5/21 3:57 PM, Thomas Gleixner wrote:
> On Wed, May 05 2021 at 15:24, Jens Axboe wrote:
>> On 5/5/21 5:03 AM, Stefan Metzmacher wrote:
>>> As io_threads are fully set up USER threads it's clearer to
>>> separate the code path from the KTHREAD logic.
>>>
>>> The only remaining difference to user space threads is that
>>> io_threads never return to user space again.
>>> Instead they loop within the given worker function.
>>>
>>> The fact that they never return to user space means they
>>> don't have an user space thread stack. In order to
>>> indicate that to tools like gdb we reset the stack and instruction
>>> pointers to 0.
>>>
>>> This allows gdb attach to user space processes using io-uring,
>>> which like means that they have io_threads, without printing worrying
>>> message like this:
>>>
>>>   warning: Selected architecture i386:x86-64 is not compatible with reported target architecture i386
>>>
>>>   warning: Architecture rejected target-supplied description
>>>
>>> The output will be something like this:
>>>
>>>   (gdb) info threads
>>>     Id   Target Id                  Frame
>>>   * 1    LWP 4863 "io_uring-cp-for" syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
>>>     2    LWP 4864 "iou-mgr-4863"    0x0000000000000000 in ?? ()
>>>     3    LWP 4865 "iou-wrk-4863"    0x0000000000000000 in ?? ()
>>>   (gdb) thread 3
>>>   [Switching to thread 3 (LWP 4865)]
>>>   #0  0x0000000000000000 in ?? ()
>>>   (gdb) bt
>>>   #0  0x0000000000000000 in ?? ()
>>>   Backtrace stopped: Cannot access memory at address 0x0
>>
>> I have queued this one up in the io_uring branch, also happy to drop it if
>> the x86 folks want to take it instead. Let me know!
> 
> I have no objections, but heck what's the rush here?
> 
> Waiting a day for the x86 people to respond it not too much asked for
> right?

There's no rush. I just said I've queued it up, and to object if you
want to take it through the tip tree. It's not going out before end of
week anyway, so there's plenty of time. Then I know I won't forget...

-- 
Jens Axboe