Messages in this thread | | | Date | Thu, 12 Jan 2012 03:58:30 -0800 | From | Josh Triplett <> | Subject | Re: [PATCH] rcu: avoid checking for constant |
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On Thu, Jan 12, 2012 at 11:34:23AM +0100, Jan Engelhardt wrote: > On Thursday 2012-01-12 10:52, Josh Triplett wrote: > >> +#define __kfree_rcu(head, offset) \ > >> + call_rcu(head, (void (*)(struct rcu_head *))(unsigned long)(offset) + \ > >> + BUILD_BUG_ON_ZERO((offset) >= 4096)) > >> + > > > >I had to stare at this for a while, and look up the definition of > >BUILD_BUG_ON_ZERO. Naturally I assumed that BUILD_BUG_ON_ZERO(arg) > >meant BUILT_BUG_ON((arg) == 0), which would have made the logic > >backwards here. However, per the definition it just provides a > >zero-returning version of BUILD_BUG_ON. Ow. > > Same impression here. BUILD_BUG_ON_ZERO was introduced by > > commit 4552d5dc08b79868829b4be8951b29b07284753f > Author: Jan Beulich <jbeulich@novell.com> > Date: Mon Jun 26 13:57:28 2006 +0200 > > while Rusty's CCAN archive calls it "BUILD_BUG_OR_ZERO" (since either > it's a bug, or returning neutral zero).
Sounds like a good target for a fix at some point.
> rcu: avoid checking for constant > > When compiling kernel or module code with -O0, "offset" is no longer > considered a constant, and therefore always triggers the build error > that BUILD_BUG_ON is defined to yield. > > Therefore, change the innards of kfree_rcu so that the offset is not > tunneled through a function argument before checking it.
The commit message looks good now.
> @@ -835,7 +817,20 @@ void __kfree_rcu(struct rcu_head *head, unsigned long offset) > * > * Note that the allowable offset might decrease in the future, for example, > * to allow something like kmem_cache_free_rcu(). > + * > + * The BUILD_BUG_ON check must not involve any function calls, hence the > + * checks are done in macros here. __is_kfree_rcu_offset is also used by > + * kernel/rcu.h.
The first sentence of that paragraph seems like a worthwhile addition. Please drop the second, though, since it'll inevitably become outdated.
> +#define __is_kfree_rcu_offset(offset) ((offset) < 4096) > + > +#define __kfree_rcu(head, offset) \ > + do { \ > + typedef void (*rcu_callback)(struct rcu_head *); \ > + BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \ > + call_rcu(head, (rcu_callback)(unsigned long)(offset)); \ > + } while (0)
No, you can't define that typedef here with that name. Unlike in the inline function, in a macro you could introduce a name conflict.
- Josh Triplett
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