> On Mon, Nov 11, 2013 at 09:17:52PM +0000, Tim Chen wrote:
>> An alternate implementation is
>> while (!ACCESS_ONCE(node->locked))
>> arch_mutex_cpu_relax();
>> smp_load_acquire(&node->locked);
>>
>> Leaving the smp_load_acquire at the end to provide appropriate barrier.
>> Will that be acceptable?
Will Deacon <will.deacon@xxxxxxx> wrote:
> It still doesn't solve my problem though: I want a way to avoid that busy
> loop by some architecture-specific manner. The arch_mutex_cpu_relax() hook
> is a start, but there is no corresponding hook on the unlock side to issue a
> wakeup. Given a sensible relax implementation, I don't have an issue with
> putting a load-acquire in a loop, since it shouldn't be aggresively spinning
> anymore.
So you want something like this?
/*
* This is a spin-wait with acquire semantics. That is, accesses after
* this are not allowed to be reordered before the load that meets
* the specified condition. This requires that it end with either a
* load-acquire or a full smp_mb(). The optimal way to do this is likely
* to be architecture-dependent. E.g. x86 MONITOR/MWAIT instructions.
*/
#ifndef smp_load_acquire_until
#define smp_load_acquire_until(addr, cond) \
while (!(smp_load_acquire(addr) cond)) { \
do { \
arch_mutex_cpu_relax(); \
} while (!(ACCESS_ONCE(*(addr)) cond)); \
}
#endif
smp_load_acquire_until(&node->locked, != 0);
Alternative implementations:
#define smp_load_acquire_until(addr, cond) { \
while (!(ACCESS_ONCE(*(addr)) cond)) \
arch_mutex_cpu_relax(); \
smp_mb(); }
#define smp_load_acquire_until(addr, cond) \
if (!(smp_load_acquire(addr) cond)) { \
do { \
arch_mutex_cpu_relax(); \
} while (!(ACCESS_ONCE(*(addr)) cond)); \
smp_mb(); \
}
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