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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