stop_machine: Clean up the usage of the preemption counter in cpu_stopper_thread()

1. Change this code to use preempt_count_inc/preempt_count_dec; this way
   it works even if CONFIG_PREEMPT_COUNT=n, and we avoid the unnecessary
   __preempt_schedule() check (stop_sched_class is not preemptible).

   And this makes clear that we only want to make preempt_count() != 0
   for __might_sleep() / schedule_debug().

2. Change WARN_ONCE() to use %pf to print the function name and remove
   kallsyms_lookup/ksym_buf.

3. Move "int ret" into the "if (work)" block, this looks more consistent.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Milos Vyletel <milos@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20151115193332.GA8281@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 7ff7acee2c766021d476aa044f525e3051814db6..61101193967e09005affb9c5ea3f324a82178c3a 100644 (file)
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -435,7 +435,6 @@ static void cpu_stopper_thread(unsigned int cpu)
 {
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
        struct cpu_stop_work *work;
-       int ret;
 
 repeat:
        work = NULL;
@@ -451,23 +450,19 @@ repeat:
                cpu_stop_fn_t fn = work->fn;
                void *arg = work->arg;
                struct cpu_stop_done *done = work->done;
-               char ksym_buf[KSYM_NAME_LEN] __maybe_unused;
+               int ret;
 
-               /* cpu stop callbacks are not allowed to sleep */
-               preempt_disable();
+               /* cpu stop callbacks must not sleep, make in_atomic() == T */
+               preempt_count_inc();
                ret = fn(arg);
                if (done) {
                        if (ret)
                                done->ret = ret;
                        cpu_stop_signal_done(done);
                }
-               /* restore preemption and check it's still balanced */
-               preempt_enable();
+               preempt_count_dec();
                WARN_ONCE(preempt_count(),
-                         "cpu_stop: %s(%p) leaked preempt count\n",
-                         kallsyms_lookup((unsigned long)fn, NULL, NULL, NULL,
-                                         ksym_buf), arg);
-
+                         "cpu_stop: %pf(%p) leaked preempt count\n", fn, arg);
                goto repeat;
        }
 }