Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

[Leonid Yegoshin <Leonid.Yegoshin@xxxxxxxxxx>]

On 01/14/2016 01:29 PM, Paul E. McKenney wrote:
> > On 01/14/2016 12:34 PM, Paul E. McKenney wrote:
> > > The WRC+addr+addr is OK because data dependencies are not required to be
> > > transitive, in other words, they are not required to flow from one CPU to
> > > another without the help of an explicit memory barrier.
> > I don't see any reliable way to fit WRC+addr+addr into "DATA
> > DEPENDENCY BARRIERS" section recommendation to have data dependency
> > barrier between read of a shared pointer/index and read the shared
> > data based on that pointer. If you have this two reads, it doesn't
> > matter the rest of scenario, you should put the dependency barrier
> > in code anyway. If you don't do it in WRC+addr+addr scenario then
> > after years it can be easily changed to different scenario which
> > fits some of scenario in "DATA DEPENDENCY BARRIERS" section and
> > fails.
> The trick is that lockless_dereference() contains an
> smp_read_barrier_depends():
>
> #define lockless_dereference(p) \
> ({ \
>         typeof(p) _________p1 = READ_ONCE(p); \
>         smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
>         (_________p1); \
> })
>
> Or am I missing your point?
WRC+addr+addr has no any barrier. lockless_dereference() has a barrier. 
I don't see a common points between this and that in your answer, sorry.
- Leonid.


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