linux-kernel - Re: Question about kthread_mod_delayed

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Message-ID: <20200213083333.jglatxs5j76z2634@pathway.suse.cz>
Date:   Thu, 13 Feb 2020 09:33:33 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        netdev <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: Question about kthread_mod_delayed_work() allowed context

On Wed 2020-02-12 21:17:53, Grygorii Strashko wrote:
> 
> 
> On 12/02/2020 17:41, Petr Mladek wrote:
> > On Tue 2020-02-11 12:23:59, Grygorii Strashko wrote:
> > > Hi All,
> > > 
> > > I'd like to ask question about allowed calling context for kthread_mod_delayed_work().
> > > 
> > > The comment to kthread_mod_delayed_work() says:
> > > 
> > >   * This function is safe to call from any context including IRQ handler.
> > >   * See __kthread_cancel_work() and kthread_delayed_work_timer_fn()
> > >   * for details.
> > >   */
> > > 
> > > But it has del_timer_sync() inside which seems can't be called from hard_irq context:
> > > kthread_mod_delayed_work()
> > >    |-__kthread_cancel_work()
> > >       |- del_timer_sync()
> > > 	|- WARN_ON(in_irq() && !(timer->flags & TIMER_IRQSAFE));
> > 
> > It is safe because kthread_delayed_work_timer_fn() is IRQ safe.
> > Note that it uses raw_spin_lock_irqsave(). It is the reason why
> > the timer could have set TIMER_IRQSAFE flag, see
> > KTHREAD_DELAYED_WORK_INIT().
> > 
> > In more details. The timer is either canceled before the callback
> > is called. Or it waits for the callback but the callback is safe
> > because it can't sleep.
> 
> I think, my issue (warning) could be related to the fact that kthread_init_delayed_work()
> is used, which seems doesn't set TIMER_IRQSAFE flag.

Great catch!

It is a bug. Would you like to send the fix for
kthread_init_delayed_work() or would you prefer me doing so?

Best Regards,
Petr