On Wed, 30 Mar 2011, Arnd Bergmann wrote:

On Wednesday 30 March 2011, Thomas Gleixner wrote:

quoted

The problem is that you need cmpxchg in user space as well.

ARM does this with some syscall magic for machines which do not have
cmpxchg.

Ah, right. I wonder how sh does it.

I also remember that the original unicore32 ABI had support for a cmpxchg
system call, but that was removed with the move to the generic syscall
table.

It is of course possible to add it back using the __NR_arch_specific_syscall
entry point.

Either that or you take a trap. None of them is pretty. 

If the only purpose is futex support, then we can add an extra flag to
the futex ops and handle it there via asm-generic/futex.h. That would
avoid an extra syscall in the contended case.

Thanks,

	tglx