powerpc/64s/radix: Fix exit lazy tlb mm switch with irqs enabled

Switching mm and tinkering with current->active_mm should be done with
irqs disabled. There is a path where exit_lazy_flush_tlb can be called
with irqs enabled:

    exit_lazy_flush_tlb
    flush_type_needed
    __flush_all_mm
    tlb_finish_mmu
    exit_mmap

Which results in the switching being done with irqs enabled, which is
incorrect.

Fixes: a665eec0 ("powerpc/64s/radix: Fix mm_cpumask trimming race vs kthread_use_mm")
Cc: stable@vger.kernel.org # v5.10+
Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Link: https://lore.kernel.org/linuxppc-dev/A9A5D83D-BA70-47A4-BCB4-30C1AE19BC22@linux.ibm.com/Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230607005601.583293-1-npiggin@gmail.com

arch/powerpc/mm/book3s64/radix_tlb.c

@@ -795,12 +795,20 @@ void exit_lazy_flush_tlb(struct mm_struct *mm, bool always_flush)
 		goto out;
 
 	if (current->active_mm == mm) {
+		unsigned long flags;
+
 		WARN_ON_ONCE(current->mm != NULL);
-		/* Is a kernel thread and is using mm as the lazy tlb */
+		/*
+		 * It is a kernel thread and is using mm as the lazy tlb, so
+		 * switch it to init_mm. This is not always called from IPI
+		 * (e.g., flush_type_needed), so must disable irqs.
+		 */
+		local_irq_save(flags);
 		mmgrab_lazy_tlb(&init_mm);
 		current->active_mm = &init_mm;
 		switch_mm_irqs_off(mm, &init_mm, current);
 		mmdrop_lazy_tlb(mm);
+		local_irq_restore(flags);
 	}
 
 	/*