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

0 files changed, 0 insertions, 0 deletions