linux-kernel - Re: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL

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Message-Id: <201004281406.04080.arnd@arndb.de>
Date:	Wed, 28 Apr 2010 14:06:03 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tony Breeds <tonyb@....ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL

On Wednesday 28 April 2010, Benjamin Herrenschmidt wrote:
> Now, we -could- make it a bit smarter about the BKL by introducing a
> contention counter and only go out if we own the BKL and it is contended,
> but I didn't feel like this was worth the effort, time is better spent
> removing the BKL from sensitive code path instead.

Agreed.

> -       for (;;) {
> +       for (timeout = jiffies + 2; jiffies < timeout;) {
> [...]
> -       for (;;) {
> +       while (jiffies < timeout) {

This needs to use time_before() to avoid problems on jiffies wraparound.

	Arnd
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