powerpc/64s/radix: add warning and comments in mm_cpumask trim

Add a comment explaining part of the logic for mm_cpumask trimming, and
add a (hopefully graceful) check and warning in case something gets it
wrong.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201217134731.488135-2-npiggin@gmail.com

diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
index fb66d154b26cf90f3a6d7093116021f1402f3387..40f496a45cd80df3310671ce214e47bdcd0bfe05 100644 (file)
--- a/arch/powerpc/mm/book3s64/radix_tlb.c
+++ b/arch/powerpc/mm/book3s64/radix_tlb.c
@@ -653,13 +653,14 @@ static void do_exit_flush_lazy_tlb(void *arg)
 {
        struct mm_struct *mm = arg;
        unsigned long pid = mm->context.id;
+       int cpu = smp_processor_id();
 
        /*
         * A kthread could have done a mmget_not_zero() after the flushing CPU
-        * checked mm_is_singlethreaded, and be in the process of
-        * kthread_use_mm when interrupted here. In that case, current->mm will
-        * be set to mm, because kthread_use_mm() setting ->mm and switching to
-        * the mm is done with interrupts off.
+        * checked mm_cpumask, and be in the process of kthread_use_mm when
+        * interrupted here. In that case, current->mm will be set to mm,
+        * because kthread_use_mm() setting ->mm and switching to the mm is
+        * done with interrupts off.
         */
        if (current->mm == mm)
                goto out_flush;
@@ -673,8 +674,22 @@ static void do_exit_flush_lazy_tlb(void *arg)
                mmdrop(mm);
        }
 
-       atomic_dec(&mm->context.active_cpus);
-       cpumask_clear_cpu(smp_processor_id(), mm_cpumask(mm));
+       /*
+        * 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.
+        */
+       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:
        _tlbiel_pid(pid, RIC_FLUSH_ALL);