alpha: run check_mmu_context() from finish_arch_post_lock_switch()

check_mmu_context() clears asn_lock and acts on need_new_asn, but it
runs only as the tail of switch_to(), after alpha_switch_to() returns.

A newly forked user task never gets there: its first context switch
resumes at ret_from_fork and reaches schedule_tail() without returning
to the code following alpha_switch_to().

asn_lock is left set on that CPU, so the new user task can run with
interrupts enabled while shootdown IPIs keep taking the deferred path.
The handshake meant to complete those invalidations never runs.

New kernel threads also reach schedule_tail(), but context_switch()
does not call switch_mm() for a task without an mm. They therefore do
not set asn_lock in the first place and are not affected by this bug.

Move check_mmu_context() to finish_arch_post_lock_switch(), which is
called by finish_task_switch() for both existing and newly created tasks.
Drop the now redundant call from switch_to().

The hook runs after finish_lock_switch() releases the rq lock and enables
interrupts. An IPI arriving before asn_lock is cleared retires the slot
through flush_tlb_other(); an IPI arriving afterwards can flush directly,
since PAL_swpctx has already installed the incoming context.

Set need_new_asn whether ev5_switch_mm() reuses or allocates an ASN. An
IPI in that interval can zero either slot. Without this, the allocated
case can return to user space with a live ASN but a zero context slot,
which a later context-based shootdown shortcut could mistake for an
inactive CPU. check_mmu_context() reloads only when the slot is zero.

Call the hook unconditionally. Alpha selects ARCH_NO_PREEMPT, and neither
kthread_use_mm() nor sched_force_init_mm() sleeps between the direct
switch and this hook. RCU_STRICT_GRACE_PERIOD can enable PREEMPT_COUNT,
but cannot make those callers migrate there. Testing preemptible() would
only leave asn_lock set in that configuration.

This requires the preceding direct-switch fix, "alpha: load the MMU
context when switch_mm() switches the current task". It installs the
borrowed context before kthread_use_mm() reaches this hook, so clearing
asn_lock cannot expose a targeted invalidate against the wrong context.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20261010142533.974797-3-linmag7@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

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