On Tue, Oct 31, 2017 at 04:55:32PM +0300, Dmitry Vyukov wrote: > I noticed that for a simple 2 lock deadlock lockdep prints only 2 > stacks.
3, one of which is useless :-) For the AB-BA we print where we acquire A (#0) where we acquire B while holding A #1 and then where we acquire B while holding A the unwind at point of splat. The #0 trace is useless. > FWIW in user-space TSAN we print 4 stacks for such deadlocks, > namely where A was locked, where B was locked under A, where B was > locked, where A was locked under B. It makes it easier to figure out > what happens. However, for this report it seems to be 8 stacks this > way. So it's probably hard either way. Right, its a question of performance and overhead I suppose. Lockdep typically only saves a stack trace when it finds a new link. So only when we find the AB relation do we save the stacktrace; which reflects the location where we acquire B. But by that time we've lost where it was we acquire A. If we want to save those stacks; we have to save a stacktrace on _every_ lock acquire, simply because we never know ahead of time if there will be a new link. Doing this is _expensive_. Furthermore, the space into which we store stacktraces is limited; since memory allocators use locks we can't very well use dynamic memory for lockdep -- that would give recursive and robustness issues. Also, its usually not too hard to find the site where we took A if we know the site of AB.
