Re: [PATCH] MIPS: implement smp_cond_load_acquire() for Loongson-3

[Peter Zijlstra]

On Wed, Jun 20, 2018 at 09:17:16AM +0100, Will Deacon wrote:
> On Wed, Jun 20, 2018 at 11:31:55AM +0800, éåæ 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.