Re: [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu critical sections

Peter Zijlstra
2016-04-08T06:41:54Z

On Thu, Apr 07, 2016 at 09:43:33AM -0700, Andy Lutomirski wrote:
> enter the critical section:
> 1:
> movq %[cpu], %%r12
> movq {address of counter for our cpu}, %%r13
> movq {some fresh value}, (%%r13)
> cmpq %[cpu], %%r12
> jne 1b

This is inherently racy; your forgot the detail of 'some fresh value',
but since you want to avoid collisions you really want an increment.

But load-store archs cannot do that. Or rather, they need to do:

load Rn, $event
add Rn, Rn, 1
store $event, Rn

But if they're preempted in the middle, two threads will collide and
generate the _same_ increment. Comparing CPU numbers will not fix that.