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Message-ID: <3dd156e4de15693f76cbad1cd8980aee827b2f20.camel@kernel.org>
Subject: Re: [PATCH 20/21] context_tracking: Convert state to atomic_t
From:   nicolas saenz julienne <nsaenz@kernel.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Phil Auld <pauld@redhat.com>,
        Alex Belits <abelits@marvell.com>,
        Xiongfeng Wang <wangxiongfeng2@huawei.com>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Yu Liao <liaoyu15@huawei.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Uladzislau Rezki <uladzislau.rezki@sony.com>,
        Joel Fernandes <joel@joelfernandes.org>
Date:   Mon, 23 May 2022 13:59:48 +0200
In-Reply-To: <20220519143738.GA612967@lothringen>
References: <20220503100051.2799723-1-frederic@kernel.org>
         <20220503100051.2799723-21-frederic@kernel.org>
         <372364b33b8d4b93908c3822e18f7d295de2ede9.camel@kernel.org>
         <20220519143738.GA612967@lothringen>
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On Thu, 2022-05-19 at 16:37 +0200, Frederic Weisbecker wrote:
> On Wed, May 18, 2022 at 05:09:55PM +0200, nicolas saenz julienne wrote:
> > On Tue, 2022-05-03 at 12:00 +0200, Frederic Weisbecker wrote:
> >=20
> > [...]
> >=20
> > > +/**
> > > + * ct_state() - return the current context tracking state if known
> > > + *
> > > + * Returns the current cpu's context tracking state if context track=
ing
> > > + * is enabled.  If context tracking is disabled, returns
> > > + * CONTEXT_DISABLED.  This should be used primarily for debugging.
> > > + */
> > > +static __always_inline int ct_state(void)
> > > +{
> > > +	int ret;
> > > +
> > > +	if (!context_tracking_enabled())
> > > +		return CONTEXT_DISABLED;
> > > +
> > > +	preempt_disable();
> > > +	ret =3D __ct_state();
> > > +	preempt_enable();
> > > +
> > > +	return ret;
> > > +}
> > > +
> >=20
> > I can't see any use for this function with preemption enabled. You can'=
t trust
> > the data due to CPU migration and it could be a source of bugs in the f=
uture.
> > Wouldn't it make more sense to move the burden into the callers? They a=
ll DTRT,
> > plus, this_cpu_ptr() will spew warnings if someone shows up and doesn't=
 comply.
>=20
> I believe syscall_exit_to_user_mode_prepare() has preemption enabled.
> Then it's ok if we are scheduled away right before the check, it still ap=
plies
> on any CPU.

Fair enough.

--=20
Nicol=C3=A1s S=C3=A1enz