trace, lockdep: manual preempt count adding for local_bh_disable

Impact: fix to preempt trace triggering lockdep check_flag failure

In local_bh_disable, the use of add_preempt_count causes the
preempt tracer to start recording the time preemption is off.
But because it already modified the preempt_count to show
softirqs disabled, and before it called the lockdep code to
handle this, it causes a state that lockdep can not handle.

The preempt tracer will reset the ring buffer on start of a trace,
and the ring buffer reset code does a spin_lock_irqsave. This
calls into lockdep and lockdep will fail when it detects the
invalid state of having softirqs disabled but the internal
current->softirqs_enabled is still set.

The fix is to manually add the SOFTIRQ_OFFSET to preempt count
and call the preempt tracer code outside the lockdep critical
area.

Thanks to Peter Zijlstra for suggesting this solution.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 18 insertions, 5 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4cae9b81a1f8..33085b88f87b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -137,6 +137,8 @@ extern unsigned long nr_uninterruptible(void);
 extern unsigned long nr_active(void);
 extern unsigned long nr_iowait(void);
 
+extern unsigned long get_parent_ip(unsigned long addr);
+
 struct seq_file;
 struct cfs_rq;
 struct task_group;
diff --git a/kernel/sched.c b/kernel/sched.c
index 52bbf1c842a8..c154825ae753 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4399,10 +4399,7 @@ void scheduler_tick(void)
 #endif
 }
 
-#if defined(CONFIG_PREEMPT) && (defined(CONFIG_DEBUG_PREEMPT) || \
-                                defined(CONFIG_PREEMPT_TRACER))
-
-static inline unsigned long get_parent_ip(unsigned long addr)
+unsigned long get_parent_ip(unsigned long addr)
 {
         if (in_lock_functions(addr)) {
                 addr = CALLER_ADDR2;
@@ -4412,6 +4409,9 @@ static inline unsigned long get_parent_ip(unsigned long addr)
         return addr;
 }
 
+#if defined(CONFIG_PREEMPT) && (defined(CONFIG_DEBUG_PREEMPT) || \
+                                defined(CONFIG_PREEMPT_TRACER))
+
 void __kprobes add_preempt_count(int val)
 {
 #ifdef CONFIG_DEBUG_PREEMPT
diff --git a/kernel/softirq.c b/kernel/softirq.c
index bdbe9de9cd8d..6edfc2c11d99 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -21,6 +21,7 @@
 #include <linux/freezer.h>
 #include <linux/kthread.h>
 #include <linux/rcupdate.h>
+#include <linux/ftrace.h>
 #include <linux/smp.h>
 #include <linux/tick.h>
 
@@ -79,13 +80,23 @@ static void __local_bh_disable(unsigned long ip)
         WARN_ON_ONCE(in_irq());
 
         raw_local_irq_save(flags);
-        add_preempt_count(SOFTIRQ_OFFSET);
+        /*
+         * The preempt tracer hooks into add_preempt_count and will break
+         * lockdep because it calls back into lockdep after SOFTIRQ_OFFSET
+         * is set and before current->softirq_enabled is cleared.
+         * We must manually increment preempt_count here and manually
+         * call the trace_preempt_off later.
+         */
+        preempt_count() += SOFTIRQ_OFFSET;
         /*
          * Were softirqs turned off above:
          */
         if (softirq_count() == SOFTIRQ_OFFSET)
                 trace_softirqs_off(ip);
         raw_local_irq_restore(flags);
+
+        if (preempt_count() == SOFTIRQ_OFFSET)
+                trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
 }
 #else /* !CONFIG_TRACE_IRQFLAGS */
 static inline void __local_bh_disable(unsigned long ip)