linux-kernel - Re: [PATCH 20/21] context_tracking: Convert state to atomic

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Date:   Mon, 23 May 2022 13:59:48 +0200
From:   nicolas saenz julienne <nsaenz@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu Liao <liaoyu15@...wei.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 20/21] context_tracking: Convert state to atomic_t

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:
> > 
> > [...]
> > 
> > > +/**
> > > + * ct_state() - return the current context tracking state if known
> > > + *
> > > + * Returns the current cpu's context tracking state if context tracking
> > > + * 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 = __ct_state();
> > > +	preempt_enable();
> > > +
> > > +	return ret;
> > > +}
> > > +
> > 
> > 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 future.
> > Wouldn't it make more sense to move the burden into the callers? They all DTRT,
> > plus, this_cpu_ptr() will spew warnings if someone shows up and doesn't comply.
> 
> 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 applies
> on any CPU.

Fair enough.

-- 
Nicolás Sáenz