linux-kernel - Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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Message-ID: <20150916150131.GC3604@twins.programming.kicks-ass.net>
Date:	Wed, 16 Sep 2015 17:01:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

On Tue, Sep 15, 2015 at 11:29:14AM -0400, Waiman Long wrote:

> Only the queue head vCPU will be in pv_wait_head() spinning to acquire the
> lock.

But what will guarantee fwd progress for the lock that is the head?

Suppose CPU0 becomes head and enters the /* claim the lock */ loop.

Then CPU1 comes in, steals it in pv_wait_head(). CPU1 releases, CPU1
re-acquires and _again_ steals in pv_wait_head(), etc..

All the while CPU0 doesn't go anywhere.

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