Re: arc_usr_cmpxchg and preemption

[Vineet Gupta]

On 03/16/2018 10:33 AM, Alexey Brodkin wrote:
> Hi Peter, Vineet,
>
> On Wed, 2018-03-14 at 18:53 +0100, Peter Zijlstra wrote:
> > On Wed, Mar 14, 2018 at 09:58:19AM -0700, Vineet Gupta wrote:
> >
> > > Well it is broken wrt the semantics the syscall is supposed to provide.
> > > Preemption disabling is what prevents a concurrent thread from coming in and
> > > modifying the same location (Imagine a variable which is being cmpxchg
> > > concurrently by 2 threads).
> > >
> > > One approach is to do it the MIPS way, emulate the llsc flag - set it under
> > > preemption disabled section and clear it in switch_to
> > *shudder*... just catch the -EFAULT, force the write fault and retry.
> More I look at this initially quite simple thing more it looks like
> a can of worms...

I'd say just bite the bullet, write the patch and we can refine it there !

-Vineet