Re: [PATCH 01/17] KVM: pfncache: Use atomic SRCU for readers instead of a rwlock

[Paolo Bonzini]

On Mon, Sep 21, 2026 at 1:21 PM Hillf Danton <hdanton@xxxxxxxx> wrote:
>
> On Mon, 21 Sep 2026 11:54:36 +0200 David Woodhouse wrote:
> > On Mon, 2026-09-21 at 09:42 +0800, Hillf Danton wrote:
> > > On Sun, 20 Sep 2026 21:49:29 +0100 David Woodhouse wrote:
> > > >
> > > > Replace the per-cache rwlock with a dedicated per-VM atomic SRCU
> > > > domain (kvm->gpc_srcu) for the read side. Readers now run under
> > > > srcu_read_lock_atomic() alone — preemption disabled and non-blocking
> > > > enforced, which every GPC read-side section already satisfied as a
> > > > former rwlock critical section. It works in any context
> > > > (including hardirq and sched-out paths) and never fails or spins,
> > > > eliminating the
> > > > read_trylock() contortions in the atomic paths, the double-lock dance
> > > > with the lockdep subclass hack in the runstate update, and the
> > > > PREEMPT_RT problems inherent to taking an rwlock in those contexts.
> > > > In particular, kvm_xen_set_evtchn_fast() is called from hardirq
> > > > context (timer callback, kvm_arch_set_irq_inatomic()), where taking
> > > > gpc->lock is a sleeping-lock-in-atomic-context bug on PREEMPT_RT.
> > >
> > > Given the preempt_disable() in srcu_read_lock_atomic(), as PREEMPT_RT
> > > has no way of handling preempt_disable(), you need to add another
> > > version of srcu_read_lock_atomic() with preempt_disable() replaced with
> > > something like rcu_read_lock().
> >
> > Hm? What's wrong with preempt_disable()? It works OK, but is generally
>
> How is preempt_disable() handled by RT?

It disables preemption. :)

The difference is that *spinlocks* (by virtue of becoming sleeping
locks) do not disable it, but the low-level primitives preempt_disable
and get_cpu do not change their behavior.

Older PREEMPT_RT patches had get_cpu_light() which did
migrate_disable() instead of preempt_disable(), but then
migrate_disable() was deprecated.

Paolo