On Wed, Jun 20, 2018 at 09:17:16AM +0100, Will Deacon wrote:
> 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?
Right, I think we also want to clarify that this 'feature' is not
supported by the Linux kernel in general and LKMM in specific.
It really is a CPU bug. And the cpu_relax() change is a best effort
work-around.
