[PATCH] lazy tlb: consolidate lazy tlb mm switching fix

[Nicholas Piggin <npiggin@xxxxxxxxx>]

Fix an upstream powerpc bug that was discovered with a WARN_ON added in
"lazy tlb: consolidate lazy tlb mm switching":

   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 incorrectly with irqs enabled.

    Link: https://lore.kernel.org/linuxppc-dev/87a5xcgopc.fsf@mail.lhotse/T/#m105488939d0cd9f980978ed2fdeeb89bf731e673
    Fixes: a665eec0a22e1 ("powerpc/64s/radix: Fix mm_cpumask trimming race vs k>

I will send that fix upstream, which will end up conflicting with these.
The resolution will be to take akpm's side of the merge (with this patch).
Or drop the patch when it conflicts and I'll resend it rebased on the fix.

Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
---
 arch/powerpc/mm/book3s64/radix_tlb.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
index 90953cf9f648..8d71fe8f5cdb 100644
--- a/arch/powerpc/mm/book3s64/radix_tlb.c
+++ b/arch/powerpc/mm/book3s64/radix_tlb.c
@@ -795,8 +795,16 @@ void exit_lazy_flush_tlb(struct mm_struct *mm, bool always_flush)
 		goto out;
 
 	if (current->active_mm == mm) {
-		/* Is a kernel thread and is using mm as the lazy tlb */
+		unsigned long flags;
+
+		/*
+		 * 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);
 		kthread_end_lazy_tlb_mm();
+		local_irq_restore(flags);
 	}
 
 	/*
-- 
2.40.1