ftrace: printk and trace irqsoff and wakeups

printk called from wakeup critical timings and irqs off can
cause deadlocks since printk might do a wakeup itself. If the
call to printk happens with the runqueue lock held, it can
deadlock.

This patch protects the printk from being called in trace irqs off
with a test to see if the runqueue for the current CPU is locked.
If it is locked, the printk is skipped.

The wakeup always holds the runqueue lock, so the printk is
simply removed.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 14 insertions, 12 deletions

diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index 7a4dc014b8ab..d0c1748b1e2c 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -165,18 +165,20 @@ check_critical_timing(struct trace_array *tr,
 
         update_max_tr_single(tr, current, cpu);
 
-        if (tracing_thresh) {
-                printk(KERN_INFO "(%16s-%-5d|#%d):"
-                        " %lu us critical section violates %lu us threshold.\n",
-                                current->comm, current->pid,
-                                raw_smp_processor_id(),
-                                latency, nsecs_to_usecs(tracing_thresh));
-        } else {
-                printk(KERN_INFO "(%16s-%-5d|#%d):"
-                       " new %lu us maximum-latency critical section.\n",
-                                current->comm, current->pid,
-                                raw_smp_processor_id(),
-                                latency);
+        if (!runqueue_is_locked()) {
+                if (tracing_thresh) {
+                        printk(KERN_INFO "(%16s-%-5d|#%d): %lu us critical"
+                               " section violates %lu us threshold.\n",
+                               current->comm, current->pid,
+                               raw_smp_processor_id(),
+                               latency, nsecs_to_usecs(tracing_thresh));
+                } else {
+                        printk(KERN_INFO "(%16s-%-5d|#%d): new %lu us"
+                               " maximum-latency critical section.\n",
+                               current->comm, current->pid,
+                               raw_smp_processor_id(),
+                               latency);
+                }
         }
 
         max_sequence++;