On Thu, Dec 13, 2018 at 10:49:49AM -0500, Alan Stern wrote: > On Wed, 12 Dec 2018, Paul E. McKenney wrote: > > > > 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? > > > > I am trying to check odd corner cases. Your sys_membarrier() model > > is quite nice and certainly fits nicely with the rest of the model, > > but where I come from, that is actually reason for suspicion. ;-) > > > > All kidding aside, your argument for the 6-CPU test was extremely > > valuable, as it showed me a way to think of that test from an > > implementation viewpoint. Then the question is whether or not that > > viewpoint actually matches the model, which seems to be the case thus far. > > It should, since I formulated the reasoning behind that viewpoint > directly from the model. The basic idea is this: > > By induction, show that whenever we have A ->rcu-fence B then > anything po-before A executes before anything po-after B, and > furthermore, any write which propagates to A's CPU before A > executes will propagate to every CPU before B finishes (i.e., > before anything po-after B executes). > > Using this, show that whenever X ->rb Y holds then X must > execute before Y. > > That's what the 6-CPU argument did. In that litmus test we have > mb2 ->rcu-fence mb23, Rc ->rb Re, mb1 ->rcu-fence mb14, Rb ->rb Rf, > mb0 ->rcu-fence mb05, and lastly Ra ->rb Ra. The last one is what > shows that the test is forbidden. I really am not trying to be difficult. Well, no more difficult than I normally am, anyway. Which admittedly isn't saying much. ;-) > > A good next step would be to automatically generate random tests along > > with an automatically generated prediction, like I did for RCU a few > > years back. I should be able to generalize my time-based cheat for RCU to > > also cover SRCU, though sys_membarrier() will require a bit more thought. > > (The time-based cheat was to have fixed duration RCU grace periods and > > RCU read-side critical sections, with the grace period duration being > > slightly longer than that of the critical sections. The number of > > processes is of course limited by the chosen durations, but that limit > > can easily be made insanely large.) > > Imagine that each sys_membarrier call takes a fixed duration and each > other instruction takes slightly less (the idea being that each > instruction is a critical section). Instructions can be reordered > (although not across a sys_membarrier call), but no matter how the > reordering is done, the result is disallowed. It gets a bit trickier with interleavings of different combinations of RCU, SRCU, and sys_membarrier(). Yes, your cat code very elegantly sorts this out, but my goal is to be able to explain a given example to someone. > > I guess that I still haven't gotten over being a bit surprised that the > > RCU counting rule also applies to sys_membarrier(). ;-) > > Why not? They are both synchronization mechanisms with heavy-weight > write sides and light-weight read sides, and most importantly, they > provide the same Guarantee. True, but I do feel the need to poke at it. The zero-size sys_membarrier() read-side critical sections do make things act a bit differently, for example, interchanging the accesses in an RCU read-side critical section has no effect, while doing so in a sys_membarrier() reader can cause the result to be allowed. One key point is that everything before the end of a read-side critical section of any type is ordered before any later grace period of that same type, and vice versa. This is why reordering accesses matters for sys_membarrier() readers but not for RCU and SRCU readers -- in the case of RCU and SRCU readers, the accesses are inside the read-side critical section, while for sys_membarrier() readers, the read-side critical sections don't have an inside. So yes, ordering also matters in the case of SRCU and RCU readers for accesses outside of the read-side critical sections. The reason sys_membarrier() seems surprising to me isn't because it is any different in theoretical structure, but rather because the practice is to put RCU and SRCU read-side accesses inside a read-side critical sections, which is impossible for sys_membarrier(). The other thing that took some time to get used to is the possibility of long delays during sys_membarrier() execution, allowing significant execution and reordering between different CPUs' IPIs. This was key to my understanding of the six-process example, and probably needs to be clearly called out, including in an example or two. The interleaving restrictions are straightforward for me, but the fixed-time approach does have some interesting cross-talk potential between sys_membarrier() and RCU read-side critical sections whose accesses have been reversed. I don't believe that it is possible to leverage this "order the other guy's read-side critical sections" effect in the general case, but I could be missing something. If you are claiming that I am worrying unnecessarily, you are probably right. But if I didn't worry unnecessarily, RCU wouldn't work at all! ;-) Thanx, Paul
