On Thu, Dec 03, 2015 at 09:26:27PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 03, 2015 at 04:37:26PM +0000, Will Deacon wrote:
> > > +#define smp_cond_acquire(cond) do { \
> > > + while (!(cond)) \
> > > + cpu_relax(); \
> > > + smp_rmb(); /* ctrl + rmb := acquire */ \
> > > +} while (0)
>
> > > + smp_cond_acquire(!((val = atomic_read(&lock->val)) &
> > > _Q_LOCKED_PENDING_MASK));
> >
> > I think we spoke about this before, but what would work really well for
> > arm64 here is if we could override smp_cond_acquire in such a way that
> > the atomic_read could be performed explicitly in the macro. That would
> > allow us to use an LDXR to set the exclusive monitor, which in turn
> > means we can issue a WFE and get a cheap wakeup when lock->val is
> > actually modified.
> >
> > With the current scheme, there's not enough information expressed in the
> > "cond" parameter to perform this optimisation.
>
> Right, but I'm having a hard time constructing something pretty that can
> do that. Lambda functions would be lovely, but we don't have those :/
>
> While we can easily pass a pointer to an arbitrary type, we need
> an expression to evaluate the result of the pointer load to act as our
> condition.
>
> smp_cond_acquire(&lock->val.counter,
> [](int val){ return !(val & _Q_LOCKED_PENDING_MASK); });
>
> Would be nice, but alas.
>
> The best we can do is hardcode a variable name; maybe something like:
>
> #define smp_cond_acquire(ptr, expr) do { \
> typeof(*ptr) val; \
> while ((val = READ_ONCE(*ptr)), expr) \
> cpu_relax(); \
> smp_rmb(); /* ctrl + rmb := acquire */ \
> } while (0)
>
> Which would let us write:
>
> smp_cond_acquire(&lock->val.counter, !(val & _Q_LOCKED_PENDING_MASK));
>
>
> Thoughts?
That would certainly work for me, but I appreciate it's not pretty. We
could have an extra macro parameter for the name of the temporary
variable, if you wanted to make it explicit.
Will
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