linux-kernel - Re: [PATCH v5] futex: Remove requirement for lock_page in get_futex

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Message-ID: <20160205180239.GB12375@linux-uzut.site>
Date:	Fri, 5 Feb 2016 10:02:39 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Chris Mason <clm@...com>, Darren Hart <dvhart@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dbueso@...e.de>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v5] futex: Remove requirement for lock_page in
 get_futex_key

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