linux-kernel - Re: [PATCH v2 4/4] futex: Avoid taking hb lock if nothing to wakeup

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Date:	Tue, 10 Dec 2013 19:00:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	dvhart@...ux.intel.com, tglx@...utronix.de,
	paulmck@...ux.vnet.ibm.com, efault@....de, jeffm@...e.com,
	torvalds@...ux-foundation.org, scott.norton@...com,
	tom.vaden@...com, aswin@...com, Waiman.Long@...com,
	jason.low2@...com
Subject: Re: [PATCH v2 4/4] futex: Avoid taking hb lock if nothing to wakeup

On Tue, Dec 10, 2013 at 09:36:36AM -0800, Davidlohr Bueso wrote:
> On Tue, 2013-12-10 at 18:15 +0100, Peter Zijlstra wrote:
> > - * The length of the list is tracked with atomic ops (hb->waiters),
> > - * providing the necessary memory barriers for the waiters. For the
> > - * waker side, however, we rely on get_futex_key_refs(), using either
> > - * ihold() or the atomic_inc(), for shared futexes. The former provides
> > - * a full mb on all architectures. For architectures that do not have an
> > - * implicit barrier in atomic_inc/dec, we explicitly add it - please
> > - * refer to futex_get_mm() and hb_waiters_inc/dec().
> 
> IMHO this text gives a nice summary instead of documenting each function
> with this things like '... implies MB (B)'. Anyway, I'll resend this
> patch with your corrections.

Right, I didn't much care for that. Once you know that you need them and
what for, the actual finding of the barriers is usually a 'trivial'
matter.


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