Re: [Xen-devel] [PATCH v2 3/4] xen: add process_pending_softirqs_norcu() for keyhandlers

[Jürgen Groß <jgross@xxxxxxxx>]

On 24.02.20 12:25, Roger Pau Monné wrote:
> On Tue, Feb 18, 2020 at 01:21:13PM +0100, Juergen Gross wrote:
> > Some keyhandlers are calling process_pending_softirqs() while holding
> > a rcu_read_lock(). This is wrong, as process_pending_softirqs() might
> > activate rcu calls which should not happen inside a rcu_read_lock().
> It might be helpful to turn the ASSERT in process_pending_softirqs
> into ASSERT_NOT_IN_ATOMIC also, as it would catch such missuses
> AFAICT.
No, this would be triggering in __cpu_up() at system boot.

> > For that purpose add process_pending_softirqs_norcu() which will not
> > do any rcu activity and use this for keyhandlers.
> I wonder if for keyhandlers it might be easier to just disable the
> watchdog in handle_keypress and remove the softirq processing from the
> handlers.
>
> At the end of day we want the keyhanders to run as fast as possible in
> order to get the data out, and we only care about the watchdog not
> triggering? (maybe I'm missing something here)
It is not that simple, I believe.

You'd need to be very careful that other functionality wouldn't suffer.
I'm e.g. not sure time_calibration won't lead to a hanging system then.

> > +void process_pending_softirqs_norcu(void)
> > +{
> > +    ASSERT(!in_irq() && local_irq_is_enabled());
> > +    /* Do not enter scheduler as it can preempt the calling context. */
> > +    __do_softirq((1ul << SCHEDULE_SOFTIRQ) | (1ul << SCHED_SLAVE_SOFTIRQ),
> Don't you also need to pass RCU_SOFTIRQ to the ignore mask in order to
> avoid any RCU work happening?
Yes, that's probably a good idea.


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel