Re: [PATCH rcu] srcu: Guarantee non-negative return value from srcu_read_lock()

[Alan Huang]

On Oct 22, 2024, at 22:26, Paul E. McKenney <paulmck@xxxxxxxxxx> 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
>