linux-kernel - Re: [RFC][PATCH 2/3] locking/qspinlock: Rework some comments

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Message-ID: <20181002134358.GD26858@hirez.programming.kicks-ass.net>
Date:   Tue, 2 Oct 2018 15:43:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, longman@...hat.com,
        andrea.parri@...rulasolutions.com, tglx@...utronix.de
Subject: Re: [RFC][PATCH 2/3] locking/qspinlock: Rework some comments

On Tue, Oct 02, 2018 at 02:20:05PM +0100, Will Deacon wrote:
> > Ah, so the reason I write it like so is because when we get here,
> > val.locked_pending == 0, per the atomic_cond_read_acquire() condition.
> 
> Ah, and I vaguely remember discussing this before. The way I read these
> transition diagrams, I find it most useful if they correspond to the lock
> word in memory. That way, it makes it clear about exactly which fields are
> stable, and which can be concurrently modified. So in the comment above,
> saying:
> 
> 	 *,*,0 -> *,*,1 : lock, contended
> 
> is really helpful, because it clearly says "we're taking the lock, but the
> rest of the lock word could be modified by others at the same time", whereas
> saying:
> 
> 	 *,0,0 -> *,0,1 : lock, contended
> 
> implies to me that pending is stable and cannot be set concurrently.

Fair enough, will restore.