On Thu, Feb 22, 2018 at 11:12:10PM +0800, Boqun Feng wrote: > On Thu, Feb 22, 2018 at 03:26:14PM +0100, Peter Zijlstra wrote: > > On Thu, Feb 22, 2018 at 03:08:52PM +0800, Boqun Feng wrote: > > > Now we have four kinds of dependencies in the dependency graph, and not > > > all the pathes carry strong dependencies, for example: > > > > > > Given lock A, B, C, if we have: > > > > > > CPU1 CPU2 > > > ============= ============== > > > write_lock(A); read_lock(B); > > > read_lock(B); write_lock(C); > > > > > > then we have dependencies A--(NR)-->B, and B--(RN)-->C, (NR and > > > RN are to indicate the dependency kind), A actually doesn't have > > > strong dependency to C(IOW, C doesn't depend on A), to see this, > > > let's say we have a third CPU3 doing: > > > > > > CPU3: > > > ============= > > > write_lock(C); > > > write_lock(A); > > > > > > , this is not a deadlock. However if we change the read_lock() > > > on CPU2 to a write_lock(), it's a deadlock then. > > > > > > So A --(NR)--> B --(RN)--> C is not a strong dependency path but > > > A --(NR)--> B --(NN)-->C is a strong dependency path. > > > > I'm not really satisfied with the above reasoning. I don't disagree, but > > if possible it would be nice to have something a little more solid. > > > > What do you mean by "solid"? You mean "A --(NR)--> B --(NN)-->C" is too > abstract, and want something like the below instead:
The above description mostly leaves it as an exercise to the reader to 'proof' ignoring *R -> R* is both safe and complete while that is the main argument.
