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

[Hillf Danton]

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().