Re: [Xen-devel] [PATCH v8 3/5] xen: don't process rcu callbacks when holding a rcu_read_lock()

[Jürgen Groß <jgross@xxxxxxxx>]

On 27.03.20 00:24, Igor Druzhinin wrote:
> On 26/03/2020 09:19, 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().
> >
> > For that purpose modify process_pending_softirqs() to not allow rcu
> > callback processing when a rcu_read_lock() is being held.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > ---
> > V3:
> > - add RCU_SOFTIRQ to ignore in process_pending_softirqs_norcu()
> >    (Roger Pau Monné)
> >
> > V5:
> > - block rcu processing depending on rch_read_lock() being held or not
> >    (Jan Beulich)
> Juergen,
>
> Our BVT revealed a likely problem with this commit in that form.
> Since 12509bbeb9e ("rwlocks: call preempt_disable() when taking a rwlock")
> preemption is disabled after taking cpu_maps which will block RCU
> callback processing inside rcu_barrier itself. This will result in
Why would that block RCU callback processing?

RCU callbacks should be blocked only if a rcu lock is being held.

Did I miss something in my patches?

> all system hang on boot after 540d4d60378 ("cpu: sync any remaining
> RCU callbacks before CPU up/down").

Juergen