ptrace and thread

[Julian Elischer]

On Wed, 14 Jan 2004, David Xu wrote:

> I am current working on debug support for KSE thread program, however I 
> found
> ptrace interface is not thread-aware,  in a threaded program, I need to 
> get/set registers
> set for individual threads,  current ptrace can not support that 
> features, there are two
> ways to support these requirements:

Yes I tried to address this a bit around the time when I added the 
single ttreading code. Ialso made several posts looking for advice 
from gdb/ptrace experts but got very little response..
As you noticed, the ptrace facility is almost completely useless
WRT threads..

it is possible to imagine an extension where you select a single thread
of interest but you would have to decide whether you want all the other
threads to be left running or left suspended..
(you may need both possibilities to correctly debug a problem)

The problem is that the thread becomes invisible to the kernel when it
crosses over to userland so the UTS needs to take an active part,
(unless the kernel can recognise when the thread has yielded and the UTS
has been enterred. (possible I  guess) at which time 
single stepping would be turned off allowing the UTS to run at full
speed.

The UTS would hav eto co-operate by using a method of re-enterring the
thread that allows the kernel to re-start single stepping..

What the other threads are doing in teh meanwhile is unknown.



> 
> 1. keep current ptrace interface,  add a command for example 
> PT_SETDTHREAD to
>    set current thread for debug, and subsequent request for example 
> PT_SETREGS and
>    PT_GETREGS will work on the thread, for single thread process, the 
> default current
>    thread is always the first thread in the process, this way we needn't 
> change legacy debugger
>    code.

yes.. 

> 
> 2. introduce a second ptrace syscall, and accept a new parameter tid 
> (thread id),
>    the PT_SETREGS and PT_GETREGS will use the tid to operate on 
> corresponding
>    thread.
> 

as mentionned by others this is also what some other systems did.

should this be in -arch or -threads?


> For first method, I have a  patch  there:
> http://people.freebsd.org/~davidxu/kse/ptrace.diff
> The patch also includes some bits to support KSE debug,  not just for 
> pure 1:1 threading.
> 
> David Xu
> 
> 
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