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 09:35, Jan Beulich wrote:
> On 27.03.2020 09:10, Jürgen Groß wrote:
> > 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?
> Igor, are you perhaps running without "rcu: add assertions to debug
> build"? I think this actually fixes what you describe. Without it
> rcu_barrier(), in its second loop, calling process_pending_softirqs(),
> would cause the RCU softirq to not be invoked anymore with preemption
> disabled. Of course the title of this change doesn't reflect this at
> all.
Right. This explains why I don't see the hang on my test system.

> Jürgen, as an aside, while looking at the code again, I think the
> comment near the end of process_pending_softirqs() would now rather
> belong at its very beginning; should have spotted this while
> reviewing.
Oh, indeed. Will send a patch.


Juergen