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.
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