On Wed, Jun 20, 2018 at 11:31:55AM +0800, 陈华才 wrote:
> Loongson-3's Store Fill Buffer is nearly the same as your "Store Buffer",
> and it increases the memory ordering weakness. So, smp_cond_load_acquire()
> only need a __smp_mb() before the loop, not after every READ_ONCE(). In
> other word, the following code is just OK:
>
> #define smp_cond_load_acquire(ptr, cond_expr) \
> ({ \
> typeof(ptr) __PTR = (ptr); \
> typeof(*ptr) VAL; \
> __smp_mb(); \
> for (;;) { \
> VAL = READ_ONCE(*__PTR); \
> if (cond_expr) \
> break; \
> cpu_relax(); \
> } \
> __smp_mb(); \
> VAL; \
> })
>
> the __smp_mb() before loop is used to avoid "reads prioritised over
> writes", which is caused by SFB's weak ordering and similar to ARM11MPCore
> (mentioned by Will Deacon).
Sure, but smp_cond_load_acquire() isn't the only place you'll see this sort
of pattern in the kernel. In other places, the only existing arch hook is
cpu_relax(), so unless you want to audit all loops and add a special
MIPs-specific smp_mb() to those that are affected, I think your only option
is to stick it in cpu_relax().
I assume you don't have a control register that can disable this
prioritisation in the SFB?
Will
