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From:   Linus Torvalds <torvalds@linuxfoundation.org>
Date:   Fri, 3 Nov 2023 08:17:01 -1000
Message-ID: <CAHk-=wiGpX9+ajuRZeeX48C9ChwS0qNGj+Gkg_kDiuo6KJV00Q@mail.gmail.com>
Subject: Re: [PATCH 1/2] cleanup: Add conditional guard support
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Oleg Nesterov <oleg@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Jonathan Cameron <jic23@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Eric Biederman <ebiederm@xmission.com>
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On Thu, 2 Nov 2023 at 23:30, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Nov 02, 2023 at 03:40:11PM +0100, Oleg Nesterov wrote:
> >
> > To me
> >
> >       guard(rcu);
> >       guard(spinlock, &lock);
> >
> > looks better than
> >
> >       guard(rcu)();
> >       // doesn't match scoped_guard(spinlock, &lock)
> >       guard(spinlock)(&lock);
> >
> > And this will make guard() consistent with scoped_guard().
[...]
> That said; if we were to do this, then something like:
>
> #define __cond_guard(_name, _inst, _fail, args...) \
>         CLASS(_name, _inst)(args); \
>         if (!__guard_ptr(_name)(&_inst)) _fail
>
> #define cond_guard(_name, _fail, args...) \
>         __cond_guard(_name, __UNIQUE_ID(guard), _fail, args)
>
>   cond_guard(spinlock_try, return -EBUSY, &my_lock);
>
> Becomes possible.
>
> Linus, do you like that enough to suffer a flag day patch as proposed by
> Oleg?

I don't find myself caring too much whether we have that "double
grouping" of the guard type-vs-arguments or the "(type, arg...)"
syntax.

I honestly think that "guard(spinlock)(&lock)" makes it more visually
obvious that the first argument is the "type of guard", while
"guard(spinlock, &lock)" makes it look like the two arguments are
somehow at the same level, which they most definitely aren't.

But I also can't find it in myself to care too much about something
that is so purely syntactic, and that I suspect should be abstracted
away anyway to just become "guard_spinlock(&lock)" with a trivial
helper macro.

                Linus