Messages in this thread Patch in this message | | | Date | Thu, 12 Jan 2012 11:34:23 +0100 (CET) | From | Jan Engelhardt <> | Subject | Re: [PATCH] rcu: avoid checking for constant |
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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).
>In any case, __kfree_rcu has void return type, so how about just using >do { ... } while(0) and BUILD_BUG_ON instead? That seems significantly >clearer.
Ok. In that case, I'll allow myself to reintroduce the extra temporal typedef line that was there previously:
>Apart from that, I can live with this, though it seems horribly >backwards to replace an inline with a macro rather than the other way >around.
parent f9fab10bbd768b0e5254e53a4a8477a94bfc4b96 (v3.2-rc7-86-gf9fab10) commit 041ac1b39390b93d8db91075aefc56b585db75cd Author: Jan Engelhardt <jengelh@medozas.de> Date: Wed Jan 11 10:05:00 2012 +0100
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> --- include/linux/rcupdate.h | 31 +++++++++++++------------------ 1 files changed, 13 insertions(+), 18 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 2cf4226..7395ab6 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -795,24 +795,6 @@ static inline notrace void rcu_read_unlock_sched_notrace(void) #define RCU_INIT_POINTER(p, v) \ p = (typeof(*v) __force __rcu *)(v) -static __always_inline bool __is_kfree_rcu_offset(unsigned long offset) -{ - return offset < 4096; -} - -static __always_inline -void __kfree_rcu(struct rcu_head *head, unsigned long offset) -{ - typedef void (*rcu_callback)(struct rcu_head *); - - BUILD_BUG_ON(!__builtin_constant_p(offset)); - - /* See the kfree_rcu() header comment. */ - BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); - - call_rcu(head, (rcu_callback)offset); -} - /** * kfree_rcu() - kfree an object after a grace period. * @ptr: pointer to kfree @@ -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. */ +#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) + #define kfree_rcu(ptr, rcu_head) \ __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head)) --
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