Messages in this thread Patch in this message | | | Date | Mon, 5 Sep 2016 04:13:30 -0700 | From | tip-bot for Johannes Berg <> | Subject | [tip:locking/urgent] locking/barriers: Don't use sizeof(void) in lockless_dereference() |
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Commit-ID: d7127b5e5fa0551be21b86640f1648b224e36d43 Gitweb: http://git.kernel.org/tip/d7127b5e5fa0551be21b86640f1648b224e36d43 Author: Johannes Berg <johannes.berg@intel.com> AuthorDate: Fri, 26 Aug 2016 08:16:00 +0200 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Mon, 5 Sep 2016 11:50:42 +0200
locking/barriers: Don't use sizeof(void) in lockless_dereference()
My previous commit:
112dc0c8069e ("locking/barriers: Suppress sparse warnings in lockless_dereference()")
caused sparse to complain that (in radix-tree.h) we use sizeof(void) since that rcu_dereference()s a void *.
Really, all we need is to have the expression *p in here somewhere to make sure p is a pointer type, and sizeof(*p) was the thing that came to my mind first to make sure that's done without really doing anything at runtime.
Another thing I had considered was using typeof(*p), but obviously we can't just declare a typeof(*p) variable either, since that may end up being void. Declaring a variable as typeof(*p)* gets around that, and still checks that typeof(*p) is valid, so do that. This type construction can't be done for _________p1 because that will actually be used and causes sparse address space warnings, so keep a separate unused variable for it.
Reported-by: Fengguang Wu <fengguang.wu@intel.com> Signed-off-by: Johannes Berg <johannes.berg@intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: kbuild-all@01.org Fixes: 112dc0c8069e ("locking/barriers: Suppress sparse warnings in lockless_dereference()") Link: http://lkml.kernel.org/r/1472192160-4049-1-git-send-email-johannes@sipsolutions.net Signed-off-by: Ingo Molnar <mingo@kernel.org> --- include/linux/compiler.h | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 436aa4e..6685698 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -527,13 +527,14 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s * object's lifetime is managed by something other than RCU. That * "something other" might be reference counting or simple immortality. * - * The seemingly unused size_t variable is to validate @p is indeed a pointer - * type by making sure it can be dereferenced. + * The seemingly unused variable ___typecheck_p validates that @p is + * indeed a pointer type by using a pointer to typeof(*p) as the type. + * Taking a pointer to typeof(*p) again is needed in case p is void *. */ #define lockless_dereference(p) \ ({ \ typeof(p) _________p1 = READ_ONCE(p); \ - size_t __maybe_unused __size_of_ptr = sizeof(*(p)); \ + typeof(*(p)) *___typecheck_p __maybe_unused; \ smp_read_barrier_depends(); /* Dependency order vs. p above. */ \ (_________p1); \ })
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