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

[Jürgen Groß <jgross@xxxxxxxx>]

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.


Juergen

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