Re: [PATCH] RFC: futex: make futex_lock_pi interruptible

[Darren Hart]

Arnd Bergmann wrote:
> On Tuesday 27 October 2009, Darren Hart wrote:
> > The following C test case demonstrates how this patch could be used to
> > implement interruptible locking. There is an awful lot of debug code and
> > some other relics of a hacked together test in there now, but if anyone
> > wanted to test the futex changes, this will do the trick.
>
> Your test program uses a signal handler to interrupt the mutex. If you
> are using a signal handler already to implement a user space mutex_cancel,
> why can't you just do a longjmp out of the signal handler rather than
> modifying the kernel?

Interesting... so something like the following diff to the test case?

This works on an unmodfied kernel... although I certainly don't feel
like I understand what it's doing well enough to recommend someone use
this in conjunction with glibc pthread_mutexes and condvars. Seems like
their internal state could suffer from this sort of thing. Hrm... and
this warning in the man pages is worth heeding:

 setjmp() and sigsetjmp() make programs hard to understand and maintain.
 If possible an alternative should be used.

However, if that is the only concern, it's certainly worth risking to
avoid any further complication of the futex code (and glibc!). I'll take
a stab at a pure pthread_mutex implementation and see if anything breaks.
I also need to understand signal handling a bit better to decide if I need
the sigsetjmp and siglongjmp versions.

Thanks for the suggestion!

--
Darren