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

[Hillf Danton]

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?

> frowned upon in RT unless the sections are short and bounded of course.
> 
Is this patchset going not to work with the preempt_disable() in
srcu_read_lock_atomic() replaced with rcu_read_lock()?