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 <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Lechner <dlechner@baylibre.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/Z1LBnX9TpZLR5Dkf@gmail.com

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 966fcc5ff8ef09ee764feb4e12e5245b2634816f..ec00e3f7af2b3472f8c43c651402f3477fbb329c 100644 (file)
--- 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...) { <error handling> }:
- *     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