On Wed, Mar 23, 2011 at 05:45:32PM -0500, James Bottomley wrote: > 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 ... Indeed! Improvements are welcome -- it is just that I am not convinced that the dual-pointer approach is really an improvement. The C preprocessor... It is ugly, inelegant, painful, annoying, and should have been strangled at birth -- but it is always there when you need it! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
