Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()

[Will Deacon <will.deacon@arm.com>]

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