Messages in this thread Patch in this message | | | Date | Thu, 19 Apr 2012 11:57:18 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu: avoid checking for constant |
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On Thu, Jan 12, 2012 at 10:41:50AM -0800, Josh Triplett wrote: > On Thu, Jan 12, 2012 at 04:38:52PM +0100, Jan Engelhardt wrote: > > On Thursday 2012-01-12 12:58, Josh Triplett wrote: > > >> 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. > > > > What names do you propose? I have BUILD_BUG_ON_EXPR for some of my code, > > though in the kernel, it has both _ON_ZERO and _ON_NULL. > > BUILD_BUG_OR_ZERO (and BUILD_BUG_OR_NULL) seem like improvements over > _ON_ZERO and _ON_NULL; that would remove the ambiguity we both tripped > over. > > > 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. > > > > Signed-off-by: Jan Engelhardt <jengelh@medozas.de> > > Reviewed-by: Josh Triplett <josh@joshtriplett.org>
OK, a similar run-in with copy_from_user() convince me that we need this. (And I was even using default optimization levels!)
Queue for 3.5, with reworked comments and commit log.
Thanx, Paul
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rcu: Make __kfree_rcu() less dependent on compiler choices
Currently, __kfree_rcu() is implemented as an inline function, and contains a BUILD_BUG_ON() that malfunctions if __kfree_rcu() is compiled as an out-of-line function. Unfortunately, there are compiler settings (e.g., -O0) that can result in __kfree_rcu() being compiled out of line, resulting in annoying build breakage. This commit therefore converts both __kfree_rcu() and __is_kfree_rcu_offset() from inline functions to macros to prevent such misbehavior on the part of the compiler.
Signed-off-by: Jan Engelhardt <jengelh@medozas.de> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 20fb776..d5dfb10 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -922,6 +922,21 @@ void __kfree_rcu(struct rcu_head *head, unsigned long offset) kfree_call_rcu(head, (rcu_callback)offset); } +/* + * Does the specified offset indicate that the corresponding rcu_head + * structure can be handled by kfree_rcu()? + */ +#define __is_kfree_rcu_offset(offset) ((offset) < 4096) + +/* + * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain. + */ +#define __kfree_rcu(head, offset) \ + do { \ + BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \ + call_rcu(head, (void (*)(struct rcu_head *))(unsigned long)(offset)); \ + } while (0) + /** * kfree_rcu() - kfree an object after a grace period. * @ptr: pointer to kfree @@ -944,6 +959,9 @@ 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. */ #define kfree_rcu(ptr, rcu_head) \ __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
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