linux-kernel - Re: [PATCH rcu] srcu: Guarantee non-negative return value from srcu_read

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Message-ID: <ZxiacAA9LIWv70Xp@infradead.org>
Date: Tue, 22 Oct 2024 23:40:48 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: paulmck@...nel.org, Christoph Hellwig <hch@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, andrii@...nel.org
Subject: Re: [PATCH rcu] srcu: Guarantee non-negative return value from
 srcu_read_lock()

On Tue, Oct 22, 2024 at 10:29:13AM -0700, Andrii Nakryiko wrote:
> >
> > Would this work?
> >
> > #define SRCU_INVALID_INDEX -1
> >
> 
> But why?

Becaue it very clearly documents what is going on.

>It's a nice property to have an int-returning API where valid
> values are only >= 0, so callers are free to use the entire negative
> range (not just -1) for whatever they need to store in case there is
> no srcu_idx value.

Well, if you have a concrete use case for that we can probably live
with it, but I'd rather have that use case extremely well documented,
as it will be very puzzling to the reader.