Re: [Xen-devel] [PATCH v6 3/4] xen/rcu: add assertions to debug build

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.03.2020 07:26, Jürgen Groß wrote:
> On 17.03.20 15:36, Jan Beulich wrote:
> > On 13.03.2020 14:06, Juergen Gross wrote:
> > > Xen's RCU implementation relies on no softirq handling taking place
> > > while being in a RCU critical section. Add ASSERT()s in debug builds
> > > in order to catch any violations.
> > >
> > > For that purpose modify rcu_read_[un]lock() to use a dedicated percpu
> > > counter additional to preempt_[en|dis]able() as this enables to test
> > > that condition in __do_softirq() (ASSERT_NOT_IN_ATOMIC() is not
> > > usable there due to __cpu_up() calling process_pending_softirqs()
> > > while holding the cpu hotplug lock).
> > >
> > > While at it switch the rcu_read_[un]lock() implementation to static
> > > inline functions instead of macros.
> > >
> > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > with one remark:
> >
> > > @@ -91,16 +114,23 @@ typedef struct _rcu_read_lock rcu_read_lock_t;
> > >    * will be deferred until the outermost RCU read-side critical section
> > >    * completes.
> > >    *
> > > - * It is illegal to block while in an RCU read-side critical section.
> > > + * It is illegal to process softirqs while in an RCU read-side critical 
> > > section.
> > The latest with the re-added preempt_disable(), wouldn't this better
> > say "... to process softirqs or block ..."?
> I can add this, but OTOH blocking without processing softirqs is not
> possible, as there is no other (legal) way to enter the scheduler.
Sure, but that's still implicit, but could do with saying explicitly.

Jan

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