Re: [PATCH] unicore32 fix: remove arch-specific futex support

[Thomas Gleixner]

On Wed, 30 Mar 2011, Arnd Bergmann wrote:

> On Wednesday 30 March 2011, Thomas Gleixner wrote:
> > 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
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