powerpc/64s/radix: Allow mm_cpumask trimming from external sources

mm_cpumask trimming is currently restricted to be issued by the current
thread of a single-threaded mm. This patch relaxes that and allows the
mask to be trimmed from any context.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201217134731.488135-5-npiggin@gmail.com

diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
index 3d8b494..c27ef2f 100644 (file)
--- a/arch/powerpc/mm/book3s64/radix_tlb.c
+++ b/arch/powerpc/mm/book3s64/radix_tlb.c
@@ -666,20 +666,16 @@ static void do_exit_flush_lazy_tlb(void *arg)
        }
 
        /*
-        * This IPI is only initiated from a CPU which is running mm which
-        * is a single-threaded process, so there will not be another racing
-        * IPI coming in where we would find our cpumask already clear.
-        *
-        * Nothing else clears our bit in the cpumask except CPU offlining,
-        * in which case we should not be taking IPIs here. However check
-        * this just in case the logic is wrong somewhere, and don't underflow
-        * the active_cpus count.
+        * This IPI may be initiated from any source including those not
+        * running the mm, so there may be a racing IPI that comes after
+        * this one which finds the cpumask already clear. Check and avoid
+        * underflowing the active_cpus count in that case. The race should
+        * not otherwise be a problem, but the TLB must be flushed because
+        * that's what the caller expects.
         */
        if (cpumask_test_cpu(cpu, mm_cpumask(mm))) {
                atomic_dec(&mm->context.active_cpus);
                cpumask_clear_cpu(cpu, mm_cpumask(mm));
-       } else {
-               WARN_ON_ONCE(1);
        }
 
 out_flush: