Re: [PATCH 4/5] rcuwait: Introduce rcuwait_active()

[Davidlohr Bueso]

On Mon, 18 May 2020, Paolo Bonzini wrote:

> On 24/04/20 07:48, Davidlohr Bueso wrote:
> > +/*
> > + * Note: this provides no serialization and, just as with waitqueues,
> > + * requires care to estimate as to whether or not the wait is active.
> > + */
> > +static inline int rcuwait_active(struct rcuwait *w)
> > +{
> > +	return !!rcu_dereference(w->task);
> > +}
>
> This needs to be changed to rcu_access_pointer:
>
>
> --------------- 8< -----------------
> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Subject: [PATCH] rcuwait: avoid lockdep splats from rcuwait_active()
>
> rcuwait_active only returns whether w->task is not NULL.  This is
> exactly one of the usecases that are mentioned in the documentation
> for rcu_access_pointer() where it is correct to bypass lockdep checks.
>
> This avoids a splat from kvm_vcpu_on_spin().
>
> Reported-by: Wanpeng Li <kernellwp@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Acked-by: Davidlohr Bueso <dbueso@xxxxxxx>