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

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Date:	Fri, 04 Dec 2015 15:51:28 -0500
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Will Deacon <will.deacon@....com>, 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, pjt@...gle.com
Subject: Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()

On 12/03/2015 03:26 PM, 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?
Will the following work?

#define smp_cond_load_acquire(ptr, cond_expr, neg) ({   \
         typeof(*ptr) _val;                              \
         while (neg ((_val = READ_ONCE(*ptr)) cond_expr))\
                 cpu_relax();                            \
         smp_rmb();                                      \
         _val;                                           \
})

         val = smp_cond_load_acquire(&lock->val.counter, & 
_Q_LOCKED_PENDING_MASK, !);

Cheers,
Longman
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