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Date:   Tue, 6 Jun 2023 11:42:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     keescook@chromium.org, gregkh@linuxfoundation.org,
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Subject: Re: [PATCH v2 0/2] Lock and Pointer guards
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On Tue, May 30, 2023 at 11:23:42AM +0200, Peter Zijlstra wrote:

> Yes, it's a wee bit more involved, but I'm thinking it gives a fair
> amount of flexibility and we don't need to ret rid of
> -Wdeclaration-after-statement.

So I made all that work and .. Yes, you're absolutely right.

Busting -Wdeclaration-after-statement is the right thing to do for
guards.

So then I came up with:

#define __ptr_guard(_guard, _name)                                     \
       guard_##_guard##_t _name __cleanup(guard_##_guard##_cleanup)

#define ptr_guard(_guard, _name)                                       \
       __diag(push)                                                    \
       __diag(ignored "-Wdeclaration-after-statement")                 \
       __ptr_guard(_guard, _name)                                      \
       __diag(pop)

#define guard_init(_guard, _var...)                                    \
       guard_##_guard##_init(_var)

#define named_guard(_guard, _name, _var...)                            \
       ptr_guard(_guard, _name) = guard_init(_guard, _var)

#define guard(_guard, _var...)                                         \
       named_guard(_guard, __UNIQUE_ID(guard), _var)

#define scoped_guard(_guard, _var...)                                  \
       for (__ptr_guard(_guard, scope) = guard_init(_guard, _var),     \
            *done = NULL; !done; done = (void *)1)


And that all (mostly) works on clang, but not GCC :-( GCC refuses to
accept _Pragma() inside an expression.

So I now have that ptr_guard() with push/pop for clang but without for
GCC, which means that only clang has a fighting chance to report
-Wdeclaration-after-statement warns until such a time as that we can get
GCC 'fixed'.

  https://godbolt.org/z/5MPeq5W6K


FWIW: the work-in-progress patches I have are here:

  https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=core/guards