From b4d83c8323b0c4a899a996fed919cfe10720d289 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Fri, 6 Dec 2024 10:19:31 +0100 Subject: headers/cleanup.h: Remove the if_not_guard() facility Linus noticed that the new if_not_guard() definition is fragile: "This macro generates actively wrong code if it happens to be inside an if-statement or a loop without a block. IOW, code like this: for (iterate-over-something) if_not_guard(a) return -BUSY; looks like will build fine, but will generate completely incorrect code." The reason is that the __if_not_guard() macro is multi-statement, so while most kernel developers expect macros to be simple or at least compound statements - but for __if_not_guard() it is not so: #define __if_not_guard(_name, _id, args...) \ BUILD_BUG_ON(!__is_cond_ptr(_name)); \ CLASS(_name, _id)(args); \ if (!__guard_ptr(_name)(&_id)) To add insult to injury, the placement of the BUILD_BUG_ON() line makes the macro appear to compile fine, but it will generate incorrect code as Linus reported, for example if used within iteration or conditional statements that will use the first statement of a macro as a loop body or conditional statement body. [ I'd also like to note that the original submission by David Lechner did not contain the BUILD_BUG_ON() line, so it was safer than what we ended up committing. Mea culpa. ] It doesn't appear to be possible to turn this macro into a robust single or compound statement that could be used in single statements, due to the necessity to define an auto scope variable with an open scope and the necessity of it having to expand to a partial 'if' statement with no body. Instead of trying to work around this fragility, just remove the construct before it gets used. Reported-by: Linus Torvalds Signed-off-by: Ingo Molnar Cc: David Lechner Cc: Peter Zijlstra Link: https://lore.kernel.org/r/Z1LBnX9TpZLR5Dkf@gmail.com --- include/linux/cleanup.h | 14 -------------- 1 file changed, 14 deletions(-) diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h index 966fcc5ff8ef..ec00e3f7af2b 100644 --- a/include/linux/cleanup.h +++ b/include/linux/cleanup.h @@ -273,12 +273,6 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \ * an anonymous instance of the (guard) class, not recommended for * conditional locks. * - * if_not_guard(name, args...) { }: - * convenience macro for conditional guards that calls the statement that - * follows only if the lock was not acquired (typically an error return). - * - * Only for conditional locks. - * * scoped_guard (name, args...) { }: * similar to CLASS(name, scope)(args), except the variable (with the * explicit name 'scope') is declard in a for-loop such that its scope is @@ -350,14 +344,6 @@ _label: \ #define scoped_cond_guard(_name, _fail, args...) \ __scoped_cond_guard(_name, _fail, __UNIQUE_ID(label), args) -#define __if_not_guard(_name, _id, args...) \ - BUILD_BUG_ON(!__is_cond_ptr(_name)); \ - CLASS(_name, _id)(args); \ - if (!__guard_ptr(_name)(&_id)) - -#define if_not_guard(_name, args...) \ - __if_not_guard(_name, __UNIQUE_ID(guard), args) - /* * Additional helper macros for generating lock guards with types, either for * locks that don't have a native type (eg. RCU, preempt) or those that need a -- cgit v1.2.3-70-g09d2