linux-kernel - Re: [PATCH 3/4] locking: Introduce smp_cond

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Date:	Thu, 3 Dec 2015 21:26:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	mingo@...nel.org, oleg@...hat.com, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, boqun.feng@...il.com, corbet@....net,
	mhocko@...nel.org, dhowells@...hat.com,
	torvalds@...ux-foundation.org, waiman.long@....com, pjt@...gle.com
Subject: Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()

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?
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