[Devel] Re: [PATCH] user-cr: invoke exit system call directly from ckpt_do_feeder

[Nathan Lynch]

On Thu, 2009-11-26 at 10:10 -0500, Oren Laadan wrote:
> 
> Nathan Lynch wrote:
> > The feeder thread can cause the restart process to fail by indirectly
> > calling exit_group, which sends SIGKILL to all other threads in the
> > process.  If the feeder thread "wins" the race, the restart is
> > disrupted.  A common symptom of this race is the coordinator task
> > returning from the wait_for_completion_interruptible in
> > wait_all_tasks_finish with a signal (the SIGKILL) pending.
> 
> So the clone mage page says:
>   ...
>   The main use of clone() is to implement threads: multiple threads
>   of control in a program that run concurrently in a shared memory
>   space.
>   ...
>   When the fn(arg) function application returns, the child process
>   terminates. The integer returned by fn is the exit code for the
>   child process.  The child process may also terminate explicitly by
>   calling exit(2) or after receiving a fatal signal.
>   ...
> (http://www.kernel.org/doc/man-pages/online/pages/man2/__clone2.2.html)
> 
> I expected "terminates" here to mean invoke the syscall _exit().
> Clearly this is desirable with CLONE_THREAD,

Calling _exit (as glibc's clone support code does) is clearly
undesirable for CLONE_THREAD users such as restart.c because _exit calls
exit_group, terminating the whole thread group.  That's kind of the
whole point of the patch :)


>  but not for regular
> processes that will want to proceed to the usual glibc exit path
> (e.g. process at_exit() and what-not). Then again, the last thread
> to exit should also call glibc's exit for the same reason. So
> that's probably why it's handled this way.
> 
> This matters for us because our user-space wrapper to eclone()
> should eventually do what the glibc's clone() wrapper does, instead
> of calling _exit() directly as it is today...

For compatibility's sake, the user-space eclone wrapper should
eventually do what glibc's clone support code does, yes -- branch to
_exit.  But I think you've stated the case backwards?  Currently the
eclone wrappers call sys_exit directly (e.g. "li r0,__NR_exit; sc" on
powerpc).


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