Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

[Joel Fernandes]

> On Mar 1, 2023, at 2:18 PM, Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> 
> On Wed, Mar 1, 2023 at 7:52 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>> 
>>> On Wed, 1 Mar 2023 19:43:34 +0100
>>> Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>>> 
>>> On Wed, Mar 1, 2023 at 5:38 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>>>> 
>>>> On Wed, 1 Mar 2023 11:28:47 +0100
>>>> Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>>>> 
>>>>> These benefits are the reason the change to try_cmpxchg was accepted
>>>>> also in the linear code elsewhere in the linux kernel, e.g. [2,3] to
>>>>> name a few commits, with a thumbs-up and a claim that the new code is
>>>>> actually *clearer* at the merge commit [4].
>>>> 
>>>> I'll say it here too. I really like Joel's suggestion of having a
>>>> cmpxchg_success() that does not have the added side effect of modifying the
>>>> old variable.
>>>> 
>>>> I think that would allow for the arch optimizations that you are trying to
>>>> achieve, as well as remove the side effect that might cause issues down the
>>>> road.
>>> 
>>> Attached patch implements this suggestion.
>> 
>> I like it!

Me too :)

> Thanks!
> 
>> Anyway to make this more generic?
> 
> If we want to put the definition in generic headers, then we also need
> to define acquire/release/relaxed and 64bit variants. ATM, we have two
> sites that require this definition and I think that for now we could
> live with two instances of the same definition in two separate
> subsystems. But this would definitely be a good future addition. There
> is some code in the form of
> 
> if (cmpxchg (ptr, 0, 1) == 0)
> 
> that can not be converted to try_cmpxchg, but can use cmpxchg_success.

I would prefer if we can put it in generic headers instead of duplicating
across ftrace and RCU.

thanks,

 - Joel

> 
> Uros.