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From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall
Date:   Wed, 1 Mar 2023 14:45:05 -0500
Message-Id: <E010E19D-94FD-4E23-BF46-A0F75734B8FB@joelfernandes.org>
References: <CAFULd4YPP_5koKN5ZbdgOueXcpUgap97UkaxxyzzGcQ=GEV__Q@mail.gmail.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@kernel.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
In-Reply-To: <CAFULd4YPP_5koKN5ZbdgOueXcpUgap97UkaxxyzzGcQ=GEV__Q@mail.gmail.com>
To:     Uros Bizjak <ubizjak@gmail.com>
Precedence: bulk


> On Mar 1, 2023, at 2:18 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
>=20
> =EF=BB=BFOn Wed, Mar 1, 2023 at 7:52=E2=80=AFPM Steven Rostedt <rostedt@go=
odmis.org> wrote:
>>=20
>>> On Wed, 1 Mar 2023 19:43:34 +0100
>>> Uros Bizjak <ubizjak@gmail.com> wrote:
>>>=20
>>> On Wed, Mar 1, 2023 at 5:38=E2=80=AFPM Steven Rostedt <rostedt@goodmis.o=
rg> wrote:
>>>>=20
>>>> On Wed, 1 Mar 2023 11:28:47 +0100
>>>> Uros Bizjak <ubizjak@gmail.com> wrote:
>>>>=20
>>>>> 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].
>>>>=20
>>>> 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.
>>>>=20
>>>> 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.
>>>=20
>>> Attached patch implements this suggestion.
>>=20
>> I like it!

Me too :)

> Thanks!
>=20
>> Anyway to make this more generic?
>=20
> 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
>=20
> if (cmpxchg (ptr, 0, 1) =3D=3D 0)
>=20
> 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

>=20
> Uros.