Messages in this thread | | | Date | Mon, 2 Nov 2015 17:43:48 +0000 | From | Will Deacon <> | Subject | Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() |
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On Mon, Nov 02, 2015 at 02:57:26PM +0100, Peter Zijlstra wrote: > 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?
Do we even know that Alpha needs a barrier for control-dependencies in the first place?
Will
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