ftrace: preemptoff selftest not working

Impact: fix preemptoff and preemptirqsoff tracer self-tests

I was wondering why the preemptoff and preemptirqsoff tracer selftests
don't work on s390. After all its just that they get called from
non-preemptible context:

kernel_init() will execute all initcalls, however the first line in
kernel_init() is lock_kernel(), which causes the preempt_count to be
increased. Any later calls to add_preempt_count() (especially those
from the selftests) will therefore not result in a call to
trace_preempt_off() since the check below in add_preempt_count()
will be false:

        if (preempt_count() == val)
                trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));

Hence the trace buffer will be empty.

Fix this by releasing the BKL during the self-tests.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 8 insertions, 0 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 396fda034e3f..16892121cb7c 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -532,6 +532,13 @@ int register_tracer(struct tracer *type)
         }
 
 #ifdef CONFIG_FTRACE_STARTUP_TEST
+        /*
+         * When this gets called we hold the BKL which means that preemption
+         * is disabled. Various trace selftests however need to disable
+         * and enable preemption for successful tests. So we drop the BKL here
+         * and grab it after the tests again.
+         */
+        unlock_kernel();
         if (type->selftest) {
                 struct tracer *saved_tracer = current_trace;
                 struct trace_array *tr = &global_trace;
@@ -562,6 +569,7 @@ int register_tracer(struct tracer *type)
                 }
                 printk(KERN_CONT "PASSED\n");
         }
+        lock_kernel();
 #endif
 
         type->next = trace_types;