Subject: Re: [PATCH] Export PowerPC atomic operations to userspace

From: Brent Cook <[email-masked]>
Date: Mon, 15 May 2006 13:18:50 -0500

On Monday 15 May 2006 12:16, Tom Rini wrote:
> On Mon, May 15, 2006 at 11:54:52AM -0500, Brent Cook wrote:
> > The atomic operations in asm/atomic.h are really useful from userspace
> > too. Other architectures (i386, x86_64, mips) export these to userspace,
> > but the powerpc versions are guarded by __KERNEL__ for some reason. Can
> > we remove these if there is no good reason to guard them?
> >
> > Signed-off-by: Brent Cook <[email-masked]>
>
> NAK.  i386, x86_64 and mips are broken in this regard.  Please google up
> the previous threads that explain why you can't always get atomic
> operations on all architectures and why exporting this is bad.

OK, I see that possibly the only reason atomic_t is even exported is so that 
sem.h works. Hopefully sem.h will get fixed and people like me will not be 
even tempted by atomic.h.

What I really want is just a standard way to do atomic inc/dec in userspace; 
I'm sure that people are going to continue wanting to have atomic_t 
workalikes for their code.

After reading this thread: 
http://www.developerweb.net/forum/archive/index.php/t-3294.html
it appears that just wrapping an integer in a pthreads mutex on an NPTL 
machine wouldn't be much more overhead than using an atomic_t directly.

Looking at glibc's sources for powerpc nptl:

nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h

confirms that a futex lock is really similar to an atomic_t.

Thanks!

 - Brent