linux-kernel - Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs before GP kthread is running

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Date:   Sat, 15 Feb 2020 03:01:11 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs
 before GP kthread is running

On Fri, Feb 14, 2020 at 10:53:05PM -0500, Steven Rostedt wrote:
> On Fri, 14 Feb 2020 15:55:59 -0800
> paulmck@...nel.org wrote:
> 
> > @@ -1252,10 +1252,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
> >   */
> >  static void rcu_gp_kthread_wake(void)
> >  {
> > -	if ((current == rcu_state.gp_kthread &&
> > +	if ((current == READ_ONCE(rcu_state.gp_kthread) &&
> >  	     !in_irq() && !in_serving_softirq()) ||
> >  	    !READ_ONCE(rcu_state.gp_flags) ||
> > -	    !rcu_state.gp_kthread)
> > +	    !READ_ONCE(rcu_state.gp_kthread))
> >  		return;
> 
> This looks buggy. You have two instances of
> READ_ONCE(rcu_state.gp_thread), which means they can be different. Is
> that intentional?

It might well be a bug, but let's see...

The rcu_state.gp_kthread field is initially NULL and transitions only once
to the non-NULL pointer to the RCU grace-period kthread's task_struct
structure.  So yes, this does work, courtesy of the compiler not being
allowed to change the order of READ_ONCE() instances and conherence-order
rules for READ_ONCE() and WRITE_ONCE().

But it would clearly be way better to do just one READ_ONCE() into a
local variable and test that local variable twice.

I will make this change, and thank you for calling my attention to it!

						Thanx, Paul