linux-kernel - Re: [PATCH 6/9] pm/irq: make for_each_irq_desc() safe of irq

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Date:   Fri, 22 Apr 2022 18:43:07 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 6/9] pm/irq: make for_each_irq_desc() safe of irq_desc
 release

On Thu, Apr 21, 2022 at 12:57:28PM +0200, Rafael J. Wysocki wrote:
> On Thu, Apr 21, 2022 at 5:31 AM Pingfan Liu <kernelfans@...il.com> wrote:
> >
> > 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@...il.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.
> 
> Well, I'm still not sure that this can ever trigger in practice, but I

Yes, I also think it hardly happen. I had gone through all
accesses to irq_desc in kernel, and just want to make anything
completely obey the rule.
> guess the locking can be added for extra safety.
> 
> Anyway, the above information should go into the changelog IMO.
> 

OK, I will update it in V2.
> That said ->
> 
> > > > Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> > > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > > Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> > > > To: linux-kernel@...r.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);
> 
> -> is it entirely safe to call synchronize_irq() under irq_lock_sparse?

synchronize_irq - wait for pending IRQ handlers (on other CPUs). It
only holds irq_desc->lock and has no connections with irq sparse tree or
bitmap. I can not see any deadlock issue or miss something?

Thanks for your time.

Regards,

	Pingfan
> 
> > > >         }
> > > > +       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
> > > >