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

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Message-Id: <181BFA9D-92C8-4ACC-BE50-8C752EB9B8E8@gmail.com>
Date: Thu, 24 Oct 2024 00:56:19 +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 24, 2024, at 00:40, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Wed, Oct 23, 2024 at 02:58:07PM +0800, Alan Huang wrote:
>> 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?
> 
> Yes, there is a get_completed_synchronize_rcu() function that returns a
> value that causes poll_state_synchronize_rcu() to always return true.
> There is also a get_completed_synchronize_rcu_full() function that
> returns a value that causes poll_state_synchronize_rcu_full() to always
> return true.

This is exactly the API I was searching for, didn’t read the doc thoroughly : )

Thanks!

> 
> There has been some discussion of another set of values that cause
> poll_state_synchronize_rcu() and poll_state_synchronize_rcu_full() to
> always return false, but there is not yet a use case for this.  Easy to
> provide if required, but why further explode the RCU API unless it really
> is required?
> 
> Thanx, Paul
> 
>>> 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
>>> 
>>