Messages in this thread Patch in this message | | | Date | Thu, 12 Jan 2012 10:25:23 +0100 (CET) | From | Jan Engelhardt <> | Subject | Re: [PATCH] rcu: avoid checking for constant |
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On Thursday 2012-01-12 08:14, Josh Triplett wrote:
>On Thu, Jan 12, 2012 at 06:11:44AM +0100, Jan Engelhardt wrote: >> 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. >[...] >> --- a/include/linux/rcupdate.h >> +++ b/include/linux/rcupdate.h >> @@ -805,8 +805,6 @@ 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)); > >The very next check after the one you removed will break if the >compiler can't check the offset at compile time.
Thanks for catching that. How about the following approach? :
parent f9fab10bbd768b0e5254e53a4a8477a94bfc4b96 (v3.2-rc7-86-gf9fab10) commit a4145604f1e40ce93a25a053f0f49341324b0077 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.
What is the rationale between the forced constant check, introduced in 9ab1544eb4196ca8d05c433b2eb56f74496b1ee3?
Signed-off-by: Jan Engelhardt <jengelh@medozas.de> --- include/linux/rcupdate.h | 22 ++++------------------ 1 files changed, 4 insertions(+), 18 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 2cf4226..5e7286d 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 @@ -836,6 +818,10 @@ 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(). */ +#define __kfree_rcu(head, offset) \ + call_rcu(head, (void (*)(struct rcu_head *))(unsigned long)(offset) + \ + BUILD_BUG_ON_ZERO((offset) >= 4096)) + #define kfree_rcu(ptr, rcu_head) \ __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head)) -- # Created with git-export-patch
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