linux-kernel - Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex

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Message-ID: <CA+55aFynzPDA2Ccrh=dKHJ3U25HSi+RrYsCs2x_wt2xTKyzXpg@mail.gmail.com>
Date:	Sun, 1 Dec 2013 13:46:11 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jason Low <jason.low2@...com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	Scott Norton <scott.norton@...com>,
	Tom Vaden <tom.vaden@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Waiman Long <Waiman.Long@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket
 plist in futex_wake()

On Sun, Dec 1, 2013 at 12:39 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> Last time I checked, private futexes were used when appropriate.
>
> "strace -e futex" mainly show _PRIVATE uses.

Yeah, pthread mutexes seem to do it. Sadly we don't do it for
mm_release(), so the case of clear_child_tid doesn't trigger it, and
that happened to be what I tested (I did "git diff" with the threaded
pre-population of the stat data).  And we can't change that because
it's ABI. Sad.

So we do have a couple of corner cases where we *could* use the
private ones but don't. But I guess that thread creation/exit is
heavy-weight enough that that particular one doesn't really matter.

               Linus
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