Messages in this thread | | | Date | Mon, 2 Nov 2015 14:57:26 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() |
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On Mon, Nov 02, 2015 at 02:29:05PM +0100, Peter Zijlstra wrote:
> Note that while smp_cond_acquire() has an explicit > smp_read_barrier_depends() for Alpha, neither sites it gets used in > were actually buggy on Alpha for their lack of it. The first uses > smp_rmb(), which on Alpha is a full barrier too and therefore serves > its purpose. The second had an explicit full barrier.
> +/** > + * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering > + * @cond: boolean expression to wait for > + * > + * Equivalent to using smp_load_acquire() on the condition variable but employs > + * the control dependency of the wait to reduce the barrier on many platforms. > + * > + * The control dependency provides a LOAD->STORE order, the additional RMB > + * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order, > + * aka. ACQUIRE. > + */ > +#define smp_cond_acquire(cond) do { \ > + while (!(cond)) \ > + cpu_relax(); \ > + smp_read_barrier_depends(); /* ctrl */ \ > + smp_rmb(); /* ctrl + rmb := acquire */ \ > +} while (0)
So per the above argument we could leave out the smp_read_barrier_depends() for Alpha, although that would break consistency with all the other control dependency primitives we have. It would avoid issuing a double barrier.
Thoughts?
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