Re: [PATCH 6/9] pm/irq: make for_each_irq_desc() safe of irq_desc release

[Pingfan Liu <kernelfans@gmail.com>]

On Wed, Apr 20, 2022 at 06:23:48PM +0200, Rafael J. Wysocki wrote:
> On Wed, Apr 20, 2022 at 4:06 PM Pingfan Liu <kernelfans@gmail.com> wrote:
> >
> > The invloved context is no a RCU read section. Furthermore there may be
> > more than one task at this point. Hence it demands a measure to prevent
> > irq_desc from freeing. Use irq_lock_sparse to serve the protection
> > purpose.
> 
> Can you please describe an example scenario in which the added locking
> will prevent a failure from occurring?
> 

Sorry to forget mentioning that this is based on the code analysis.

Suppose the following scenario:
Two threads invloved
  threadA "hibernate" runs suspend_device_irqs()
  threadB "rcu_cpu_kthread" runs rcu_core()->rcu_do_batch(), which releases
  object, let's say irq_desc

Zoom in:
  threadA                                               threadB
  for_each_irq_desc(irq, desc) {
      get irq_descA which is under freeing
                                                    --->preempted by rcu_core()->rcu_do_batch()  which releases irq_descA
      raw_spin_lock_irqsave(&desc->lock, flags);
      //Oops

And since in the involved code piece, threadA runs in a preemptible
context, and there may be more than one thread at this stage. So the
preempted can happen.


Thanks,

	Pingfan


> > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > To: linux-kernel@vger.kernel.org
> > ---
> >  kernel/irq/pm.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/irq/pm.c b/kernel/irq/pm.c
> > index ca71123a6130..4b67a4c7de3c 100644
> > --- a/kernel/irq/pm.c
> > +++ b/kernel/irq/pm.c
> > @@ -133,6 +133,7 @@ void suspend_device_irqs(void)
> >         struct irq_desc *desc;
> >         int irq;
> >
> > +       irq_lock_sparse();
> >         for_each_irq_desc(irq, desc) {
> >                 unsigned long flags;
> >                 bool sync;
> > @@ -146,6 +147,7 @@ void suspend_device_irqs(void)
> >                 if (sync)
> >                         synchronize_irq(irq);
> >         }
> > +       irq_unlock_sparse();
> >  }
> >  EXPORT_SYMBOL_GPL(suspend_device_irqs);
> >
> > @@ -186,6 +188,7 @@ static void resume_irqs(bool want_early)
> >         struct irq_desc *desc;
> >         int irq;
> >
> > +       /* The early resume stage is free of irq_desc release */
> >         for_each_irq_desc(irq, desc) {
> >                 unsigned long flags;
> >                 bool is_early = desc->action &&
> > --
> > 2.31.1
> >