Messages in this thread | | | Subject | Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2] | From | Eric Dumazet <> | Date | Wed, 31 Mar 2010 20:32:46 +0200 |
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Le mercredi 31 mars 2010 à 18:37 +0100, David Howells a écrit : > Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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...
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