Re: [PATCH 02/36] scsi,rcu: convert call_rcu(fc_rport_free_rcu) tokfree_rcu()

[James Bottomley]

On Wed, 2011-03-23 at 15:24 -0700, Paul E. McKenney wrote:
> On Wed, Mar 23, 2011 at 09:05:51AM -0500, James Bottomley wrote:
> > On Tue, 2011-03-22 at 23:50 -0700, Paul E. McKenney wrote:
> > > The kfree_rcu() definition is as
> > > follows:
> > > 
> > > #define kfree_rcu(ptr, rcu_head)					\
> > > 	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
> > 
> > Isn't this one of those cases where the obvious use of the interface is
> > definitely wrong?
> > 
> > It's also another nasty pseudo C prototype.  I know we do this sort of
> > thing for container_of et al, but I don't really think we want to extend
> > it.
> > 
> > Why not make the interface take a pointer to the embedding structure and
> > one to the rcu_head ... that way all pointer mathematics can be
> > contained inside the RCU routines.
> 
> Hello, James,
> 
> If you pass in a pair of pointers, then it is difficult for RCU to detect
> bugs where the two pointers are unrelated.  Yes, you can do some sanity
> checks, but these get cumbersome and have corner cases where they can
> be fooled.  In contrast, Lai's interface allows the compiler to do the
> needed type checking -- unless the second argument is a field of type
> struct rcu_head in the structure pointed to by the first argument, the
> compiler will complain.
> 
> Either way, the pointer mathematics are buried in the RCU API.
> 
> Or am I missing something here?

No ... I like the utility ... I just dislike the inelegance of having to
name a structure element in what looks like a C prototype.

I can see this proliferating everywhere since most of our reference
counting release callbacks basically free the enclosing object ...

James


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