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

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Message-ID: <20241022071018.GA16066@noisy.programming.kicks-ass.net>
Date: Tue, 22 Oct 2024 09:10:18 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...nel.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 12:07:35AM -0700, Christoph Hellwig wrote:
> On Tue, Oct 22, 2024 at 09:06:35AM +0200, Peter Zijlstra wrote:
> > What is returned is an array index -- and SRCU is currently built using
> > an array of size 2. Using larger arrays is conceivable (IIRC some
> > versions of preemptible RCU used up to 4 or something).
> > 
> > So while the values 0,1 are possible inside bool, that does not reflect
> > the nature of the numbers, which is an array index. Mapping that onto
> > bool would be slightly confusing (and limit possible future extention of
> > using larger arrays for SRCU).
> 
> Ok, make sense.  Maybe add this to the comment if we're updating іt.
> But using an unsigned return value might still be useful.

Ah, well, the thing that got us here is that we (Andrii and me) wanted
to use -1 as an 'invalid' value to indicate SRCU is not currently in
use.

So it all being int is really rather convenient :-)