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

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Message-Id: <E068D091-F79C-4F56-974D-08ED95AD6F58@gmail.com>
Date: Wed, 23 Oct 2024 14:58:07 +0800
From: Alan Huang <mmpgouride@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>,
 Peter Zijlstra <peterz@...radead.org>,
 RCU <rcu@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>,
 kernel-team@...a.com,
 Steven Rostedt <rostedt@...dmis.org>,
 andrii@...nel.org
Subject: Re: [PATCH rcu] srcu: Guarantee non-negative return value from
 srcu_read_lock()

On Oct 22, 2024, at 22:26, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Tue, Oct 22, 2024 at 12:13:12AM -0700, Christoph Hellwig wrote:
>> On Tue, Oct 22, 2024 at 09:10:18AM +0200, Peter Zijlstra wrote:
>>> 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 :-)
>> 
>> Then please document that use.  Maybe even with a symolic name for
>> -1 that clearly describes these uses.
> 
> Would this work?
> 
> #define SRCU_INVALID_INDEX -1

Is there any similar guarantee of the return value of get_state_synchronize_rcu
or start_poll_synchronize_rcu, like invalid value?

> 
> Whatever the name, maybe Peter and Andrii define this under #ifndef
> right now, and we get it into include/linux/srcu.h over time.
> 
> Or is there a better way?  Or name, for that matter.
> 
> Thanx, Paul
>