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

[David Woodhouse]

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
frowned upon in RT unless the sections are short and bounded of course.

We did talk¹ about the atomic SRCU being silently downgraded to the
sleeping version under PREEMPT_RT, as other spinlocks are silently
converted to sleeping locks. But I don't think it's mandatory, is it?

¹ https://lore.kernel.org/all/333c4cdb-8f34-4e3f-a47f-961f47e089a0@paulmck-laptop/

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