linux-kernel - Re: [PATCH] rcu: use try_cmpxchg in check_cpu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <E010E19D-94FD-4E23-BF46-A0F75734B8FB@joelfernandes.org>
Date:   Wed, 1 Mar 2023 14:45:05 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall



> On Mar 1, 2023, at 2:18 PM, Uros Bizjak <ubizjak@...il.com> wrote:
> 
> On Wed, Mar 1, 2023 at 7:52 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>> 
>>> On Wed, 1 Mar 2023 19:43:34 +0100
>>> Uros Bizjak <ubizjak@...il.com> wrote:
>>> 
>>> On Wed, Mar 1, 2023 at 5:38 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>>>> 
>>>> On Wed, 1 Mar 2023 11:28:47 +0100
>>>> Uros Bizjak <ubizjak@...il.com> 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.