Date: Thu, 3 Dec 2015 21:26:27 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: mingo@kernel.org, oleg@redhat.com, linux-kernel@vger.kernel.org,
        paulmck@linux.vnet.ibm.com, boqun.feng@gmail.com, corbet@lwn.net,
        mhocko@kernel.org, dhowells@redhat.com, torvalds@linux-foundation.org,
        waiman.long@hpe.com, pjt@google.com
Subject: Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()
Message-ID: <20151203202627.GV17308@twins.programming.kicks-ass.net>
References: <20151203124010.627312076@infradead.org>
 <20151203124339.552838970@infradead.org>
 <20151203163725.GJ11337@arm.com>
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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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