Re: [PATCH v5] futex: Remove requirement for lock_page in get_futex_key

From	Davidlohr Bueso <dave@stgolabs.net>
Newsgroups	linux.kernel
Subject	Re: [PATCH v5] futex: Remove requirement for lock_page in get_futex_key
Date	2016-02-05 19:10 +0100
Message-ID	<qYSZr-3G7-1@gated-at.bofh.it> (permalink)
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On Fri, 05 Feb 2016, Ingo Molnar wrote:

>So I too didn't understand that sentence at first, because the capitalization
>really throws off quick parsing of that comment, as 'MB' ususally denotes
>megabytes.

Sure, fair enough.

>
>So please change it to "mb(); (A)" or so - and I think all of these comments
>should be changed to use a standard API name for the barrier they imply, as the
>head of futex.c does:
>
> *   waiters++; (a)
> *   mb(); (A) <-- paired with -.
> *                              |
> *   lock(hash_bucket(futex));  |
> *                              |
> *   uval = *futex;             |
> *                              |        *futex = newval;
> *                              |        sys_futex(WAKE, futex);
> *                              |          futex_wake(futex);
> *                              |
> *                              `------->  mb(); (B)
>
>Btw., pedantic: shouldn't that be smp_mb()? Futexes don't operate on IO spaces, so
>on UP they only need compiler barriers.

Right, but we do in fact use smp barriers in this cases in the real code, that mb() is
just in the comments, I guess it would be desirable to change it to smp_mb nonetheless.

However, could these changes be in a followup? Mainly because the barrier B references
will be updated across all futex.c... unless there are still concerns about this particular
patch, of course.

Thanks,
Davidlohr

Thread

[PATCH v5] futex: Remove requirement for lock_page in get_futex_key Davidlohr Bueso <dave@stgolabs.net> - 2016-02-04 07:40 +0100
  Re: [PATCH v5] futex: Remove requirement for lock_page in  get_futex_key Thomas Gleixner <tglx@linutronix.de> - 2016-02-04 10:10 +0100
    Re: [PATCH v5] futex: Remove requirement for lock_page in  get_futex_key Davidlohr Bueso <dave@stgolabs.net> - 2016-02-04 18:50 +0100
      Re: [PATCH v5] futex: Remove requirement for lock_page in  get_futex_key Thomas Gleixner <tglx@linutronix.de> - 2016-02-04 19:00 +0100
        Re: [PATCH v5] futex: Remove requirement for lock_page in  get_futex_key Ingo Molnar <mingo@kernel.org> - 2016-02-05 10:50 +0100
          Re: [PATCH v5] futex: Remove requirement for lock_page in  get_futex_key Davidlohr Bueso <dave@stgolabs.net> - 2016-02-05 19:10 +0100
            Re: [PATCH v5] futex: Remove requirement for lock_page in  get_futex_key Ingo Molnar <mingo@kernel.org> - 2016-02-06 09:30 +0100

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