linux-kernel - Re: [PATCH] rcu: Fix opposite might_sleep() check in rcu_blocking_is

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Message-ID: <20221218233056.GU4001@paulmck-ThinkPad-P17-Gen-1>
Date:   Sun, 18 Dec 2022 15:30:56 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Zhang, Qiang1" <qiang1.zhang@...el.com>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: Fix opposite might_sleep() check in
 rcu_blocking_is_gp()

On Sun, Dec 18, 2022 at 04:02:35PM -0500, Joel Fernandes wrote:
> On Sun, Dec 18, 2022 at 2:44 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> [...]
> > > > > If not, I would do something like this:
> > > > >
> > > > > ---8<-----------------------
> > > > >
> > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > index 79aea7df4345..23c2303de9f4 100644
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -3435,11 +3435,12 @@ static int rcu_blocking_is_gp(void)
> > > > >  {
> > > > >       int ret;
> > > > >
> > > > > +     might_sleep();  /* Check for RCU read-side critical section. */
> > > > > +
> > > > >       // Invoking preempt_model_*() too early gets a splat.
> > > > >       if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE ||
> > > > >           preempt_model_full() || preempt_model_rt())
> > > > >               return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
> >
> > If the scheduler is inactive (early boot with interrupts disabled),
> > we return here.
> >
> > > > > -     might_sleep();  /* Check for RCU read-side critical section. */
> >
> > We get here only if the scheduler has started, and even then only in
> > preemption-disabled kernels.
> >
> > Or is you concern that the might_sleep() never gets invoked in kernels
> > with preemption enabled?  Fixing that would require a slightly different
> > patch, though.
> >
> > Or should I have waited until tomorrow to respond to this email?  ;-)
> 
> No, I think you are quite right. I was not referring to
> rcu_sleep_check(), but rather the following prints in might_sleep(). I
> see an unconditional call to might_sleep()  from kvfree_call_rcu() but
> not one from synchronize_rcu() which can also sleep.
> 
> But I see your point, early boot code has interrupts disabled, but can
> still totally call synchronize_rcu() when the scheduler is INACTIVE.
> And might_sleep() might bitterly complain. Thanks for the
> clarification.
> 
> pr_err("BUG: sleeping function called from invalid context at %s:%d\n",
>       file, line);
> pr_err("in_atomic(): %d, irqs_disabled(): %d, non_block: %d, pid: %d,
> name: %s\n",
>       in_atomic(), irqs_disabled(), current->non_block_count,
>       current->pid, current->comm);
> pr_err("preempt_count: %x, expected: %x\n", preempt_count(),
>       offsets & MIGHT_RESCHED_PREEMPT_MASK);

And I do not believe that we have defined whether or not it is OK to
invoke single-argument kvfree_rcu() before the scheduler has started.  ;-)

							Thanx, Paul