Re: [RFC 06/10] Reclaim memory from blocked kernel stacks

[Peter Zijlstra]

On Fri, Aug 28, 2026 at 01:50:05PM -0700, David Stevens wrote:

> > It should have. They're taking spinlock inside raw_spinlock and lockdep
> > should very much warn about that by default.
> 
> It does trylocks on spinlocks and local locks, inside of
> alloc_pages_nolock_noprof(). Lockdep considers that safe.

Argh, that alloc_pages_nolock thing is a horror show.

> On PREEMPT_RT, it is necessary to skip the call to
> alloc_pages_nolock_noprof() from under pi_lock, since with that
> configuration spin_trylock can end up needing to take pi_lock. But at
> least on !PREEMPT_RT, there is no risk of deadlock.

Yeah, which puts the lie to that horrific hack Alexei did. We should
probably teach lockdep about spin_trylock() not being safe and see the
house of cards crumble.