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SubjectRe: [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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