Re: [PATCH 1/6] alpha: run check_mmu_context() from finish_arch_post_lock_switch()

[Magnus Lindholm]

On Mon, Aug 10, 2026 at 4:40 AM Matt Turner <mattst88@xxxxxxxxx> wrote:

> > arch/alpha/include/asm/switch_to.h:12 calls it, then finish_task_switch() calls
> > the new hook a few lines later. Between them the rq lock is held and IRQs are
> > off (finish_lock_switch() → raw_spin_rq_unlock_irq), so no shootdown IPI can
> > land in that window.
> >
> > I think we should drop it from switch_to.h?
>

Thanks Matt, agreed. I’ll drop the now-redundant check_mmu_context() from
switch_to() and let finish_arch_post_lock_switch() handle it.

> Another thing. Pre-existing issue, I believe:
>
> preemptible() is true in kthread_use_mm(), so the hook does nothing,
> and ev5_switch_mm()'s asn_lock = 1 stays set on that CPU until the
> next real switch_to.
>
> Benign, but patch 2 depends on this. patch 2's own commit message says
> current->mm can be true with the mm's context not loaded
> (kthread_use_mm), and the only thing saving that case is the leaked
> asn_lock forcing asn_locked() → flush_tlb_other(). Should probably
> document that in one of these patches.

Good point, thanks. I’ll document in the commit message that kthread_use_mm()
can leave asn_lock set until the next real switch, and that the later
TLB handling
relies on this.

Magnus