Using __kuser_cmpxchg

[Nicolas Pitre]

On Thu, 15 Nov 2007, Andrew Haley wrote:

> Andrew Haley writes:
>  > Paul Brook writes:
>  >  > For recent kernels there is a kernel helper you should use.
>  >  > See __kernel_cmpxchg in linux/arch/arm/kernel/entry-armv.S
>  >  >
>  >  > Any kernel capable of running on armv6/v7 hardware should have this helper.
>  >  > So should all targets on targets that define __ARM_EABI__
>  >  >
>  >  > So I suggest using the kernel helper for EABI and v6/v7 libraries, everyone
>  >  > else will have to make do with the crappy old sequence.
>  >
>  > OK, I'll investigate this as soon as I get some time.
> 
> It seems to be impossible to use this kernel helper because
> 
>  *    - A failure might be transient, i.e. it is possible, although unlikely,
>  *      that "failure" be returned even if *ptr == oldval.
> 
> If this were to happen, the logic used to block and unblock threads
> wouldn't work.

Why?


Nicolas