Messages in this thread |  | | Date | Fri, 8 Apr 2016 08:41:36 +0200 | | From | Peter Zijlstra <> | | Subject | Re: [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu critical sections |
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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.
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