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SubjectRe: [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@kernel.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

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