linux-kernel - Re: [PATCH RFC 1/2] qrwlock: A queue read/write lock implementation

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Message-ID: <20130719211148.10251.qmail@science.horizon.com>
Date:	19 Jul 2013 17:11:48 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, waiman.long@...com
Cc:	JBeulich@...ell.com, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...nel.org, tglx@...utronix.de
Subject: Re: [PATCH RFC 1/2] qrwlock: A queue read/write lock implementation

> What I have in mind is to have 2 separate rwlock initializers - one for 
> fair and one for reader-bias behavior. So the lock owners can decide 
> what behavior do they want with a one line change.

That's definitely a nicer patch, if it will work.  I was imagining that,
even for a single (type of) lock, only a few uses require reader bias
(because they might be recursive, or are in an interrupt), but you'd
want most read_lock sites to be fair.

Deciding on a per-lock basis means that one potentially recursive call
means you can't use fair queueing anywhere.

I was hoping that the number of necessary unfair calls would
be small enough that making the read_lock default fair and
only marking the unfair call sites would be enough.

But I don't really know until doing a survey of the calls.
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