Re: [PATCH v2 3/3] alpha: do not skip TLB shootdown IPIs for kthread-borrowed mms

[Matt Turner]

On Thu, Oct 08, 2026 at 09:55:18PM +0200, Magnus Lindholm wrote:
> A
> CPU that does have the mm active keeps its slot, and that is the CPU the
> IPI is there for.

That includes a CPU that only holds the mm lazily, idle since the task
migrated away. ipi_flush_tlb_mm() and ipi_flush_icache_page() test
active_mm, so it takes flush_tlb_current() and publishes a new ASN
every time. It stays visible, and each flush_tlb_mm() from the
single-threaded owner sends IPIs again until that CPU runs some other
user task. munmap(), mprotect(), fork() and exit all go through there.

Testing current->mm in those two handlers, as ipi_flush_tlb_page() now
does, would send the lazy CPU to flush_tlb_other() instead. I have not
measured this.

> A CPU that publishes a context is in turn ordered before it goes on to
> access the mm, by two different barriers:

check_mmu_context() is a third publisher. Its __load_new_mm_context()
takes a slot from zero to nonzero after finish_lock_switch() has
dropped the rq lock, and nothing orders that store before the return
to user space. An smp_mb() after the WRITE_ONCE() in
__load_new_mm_context() would cover every caller.

> So a CPU either already holds a
> context and is seen here, or it allocates a fresh one before going on to
> use the mm, and a fresh ASN carries nothing over from the previous
> context.

There is one way for a CPU to run the mm with its slot at zero. When
ev5_switch_mm() allocates a new context it does not set need_new_asn.
With the stale-TLB series, finish_task_switch() enables interrupts
before check_mmu_context() runs. An IPI for the mm taken there sees
asn_locked() and calls flush_tlb_other(), which zeroes the slot.
check_mmu_context() then finds need_new_asn clear and does not reload,
and the task returns to user space on a live ASN that
mm_context_elsewhere() cannot see.

Setting need_new_asn in both branches of ev5_switch_mm() closes it,
since check_mmu_context() only reloads when the slot is zero.