On Wed, 12 Dec 2018, Paul E. McKenney wrote: > > > I believe that this ordering forbids the cycle: > > > > > > Wa=1 > membs -> [m01] -> Rc=0 -> Wc=2 -> rcu_read_unlock() -> > > > return from synchronize_rcu() -> Ra > > > > > > Does this make sense, or am I missing something? > > > > It's hard to tell. What you have written here isn't justified by the > > litmus test source code, since the position of m01 in P1's program > > order is undetermined. How do you justify m01 -> Rc, for example? > > ... justifies Rc=0 following [m01]. > > > Write it this way instead, using the relations defined in the > > sys_membarrier patch for linux-kernel.cat: > > > > memb ->memb-gp memb ->rcu-link Rc ->memb-rscsi Rc ->rcu-link > > > > rcu_read_unlock ->rcu-rscsi rcu_read_lock ->rcu-link > > > > synchronize_rcu ->rcu-gp synchronize_rcu ->rcu-link memb > > > > Recall that: > > > > memb-gp is the identity relation on sys_membarrier events, > > > > rcu-link includes (po? ; fre ; po), > > > > memb-rscsi is the identity relation on all events, > > > > rcu-rscsi links unlocks to their corresponding locks, and > > > > rcu-gp is the identity relation on synchronize_rcu events. > > > > These facts justify the cycle above. > > > > Leaving off the final rcu-link step, the sequence matches the > > definition of rcu-fence (the relations are memb-gp, memb-rscsi, > > rcu-rscsi, rcu-gp with rcu-links in between). Therefore the cycle is > > forbidden. > > Understood, but that would be using the model to check the model. ;-) Well, what are you trying to accomplish? Do you want to find an argument similar to the one I posted for the 6-CPU test to show that this test should be forbidden? Alan
