linux-kernel - Re: [PATCH RFC v2 rcu 2/8] srcu: Create an srcu_read_lock_nmisafe() and srcu_read_unlock

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Message-ID: <20221002234655.GV4196@paulmck-ThinkPad-P17-Gen-1>
Date:   Sun, 2 Oct 2022 16:46:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH RFC v2 rcu 2/8] srcu: Create an srcu_read_lock_nmisafe()
 and srcu_read_unlock_nmisafe()

On Sun, Oct 02, 2022 at 11:47:10PM +0200, Frederic Weisbecker wrote:
> On Sun, Oct 02, 2022 at 09:09:57AM -0700, Paul E. McKenney wrote:
> > On Sun, Oct 02, 2022 at 05:55:16PM +0200, Frederic Weisbecker wrote:
> > > On Thu, Sep 29, 2022 at 11:07:25AM -0700, Paul E. McKenney wrote:
> > > > @@ -1090,7 +1121,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> > > >  	int ss_state;
> > > >  
> > > >  	check_init_srcu_struct(ssp);
> > > > -	idx = srcu_read_lock(ssp);
> > > > +	idx = __srcu_read_lock_nmisafe(ssp);
> > > 
> > > Why do we need to force the atomic based version here (even if CONFIG_NEED_SRCU_NMI_SAFE=y)?
> > 
> > In kernels built with CONFIG_NEED_SRCU_NMI_SAFE=n, we of course need it.
> > As you say, in kernels built with CONFIG_NEED_SRCU_NMI_SAFE=y, we don't.
> > But it doesn't hurt to always use __srcu_read_lock_nmisafe() here, and
> > this is nowhere near a fastpath, so there is little benefit to using
> > __srcu_read_lock() when it is safe to do so.
> > 
> > In addition, note that it is possible that a given srcu_struct structure's
> > first grace period is executed before its first reader.  In that
> > case, we have no way of knowing which of __srcu_read_lock_nmisafe()
> > or __srcu_read_lock() to choose.
> > 
> > So this code always does it the slow(ish) safe way.
> 
> But then srcu_read_lock_nmisafe() would work as well, right?

Almost.

The problem is that without the leading "__", this would convince SRCU
that this is an NMI-safe srcu_struct.  Which it might not be.  Worse yet,
if this srcu_struct had already done an srcu_read_lock(), it would splat.

> > > >  	ss_state = smp_load_acquire(&ssp->srcu_size_state);
> > > >  	if (ss_state < SRCU_SIZE_WAIT_CALL)
> > > >  		sdp = per_cpu_ptr(ssp->sda, 0);
> > > > @@ -1123,7 +1154,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> > > >  		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
> > > >  	else if (needexp)
> > > >  		srcu_funnel_exp_start(ssp, sdp_mynode, s);
> > > > -	srcu_read_unlock(ssp, idx);
> > > > +	__srcu_read_unlock_nmisafe(ssp, idx);
> > > >  	return s;
> > > >  }
> > > >  
> > > > @@ -1427,13 +1458,13 @@ void srcu_barrier(struct srcu_struct *ssp)
> > > >  	/* Initial count prevents reaching zero until all CBs are posted. */
> > > >  	atomic_set(&ssp->srcu_barrier_cpu_cnt, 1);
> > > >  
> > > > -	idx = srcu_read_lock(ssp);
> > > > +	idx = __srcu_read_lock_nmisafe(ssp);
> > > 
> > > And same here?
> > 
> > Yes, same here.  ;-)
> 
> Now bonus question: why do SRCU grace period starting/tracking
> need to be in an SRCU read side critical section? :o)

Because I am lazy and like to keep things simple?  ;-)

More seriously, take a look at srcu_gp_start_if_needed() and the functions
it calls and ask yourself what bad things could happen if they were
preempted for an arbitrarily long period of time.

							Thanx, Paul