Le mercredi 31 mars 2010 à 18:37 +0100, David Howells a écrit : > Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > Protected by something that the caller did, be it holding the the correct > > lock, operating on it during initialization before other CPUs have access > > to it, operating on it during cleanup after other CPUs' access has been > > revoked, or whatever. > > But the point I made very early this morning still stands: What if someone > simply wants to test the pointer, not actually to dereference it? > > NFS was using rcu_dereference() for this in a couple of places - which is > overkill. I suggested stripping this off and you countered with the > suggestion that it should be using rcu_dereference_check(). > If pointer has the rcu mark, and somehing access this pointer without proper locking, then automatic checkers (sparse...) will trigger a warning, this is what Paul said. Example of such checks, # define __percpu __attribute__((noderef, address_space(3))) If someone tries to manipulate a __percpu marked ptr without proper API, sparse loudly complains. > Why do I need anything at all? > If you dont own a lock, and test a pointer, what guarantee do you have this pointer doesnt change right after you tested it ? If *something* protects the pointer from being changed, then how can be expressed this fact ? If nothing protects the pointer, why test it then, as result of test is unreliable ? If NFS was using rcu_dereference(), it probably was for a reason, but if nobody can recall it, it was a wrong reason ? Sorry, too many questions and no answer I guess... -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
