Re: [PATCH v2] io_thread/x86: setup io_threads more like normal user space threads

From: Jens Axboe
Date: Wed May 05 2021 - 17:24:23 EST


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!

--
Jens Axboe