tracing/context-tracking: Add preempt_schedule_context() for tracing

Dave Jones hit the following bug report:

 ===============================
 [ INFO: suspicious RCU usage. ]
 3.10.0-rc2+ #1 Not tainted
 -------------------------------
 include/linux/rcupdate.h:771 rcu_read_lock() used illegally while idle!
 other info that might help us debug this:
 RCU used illegally from idle CPU! rcu_scheduler_active = 1, debug_locks = 0
 RCU used illegally from extended quiescent state!
 2 locks held by cc1/63645:
  #0:  (&rq->lock){-.-.-.}, at: [<ffffffff816b39fd>] __schedule+0xed/0x9b0
  #1:  (rcu_read_lock){.+.+..}, at: [<ffffffff8109d645>] cpuacct_charge+0x5/0x1f0

 CPU: 1 PID: 63645 Comm: cc1 Not tainted 3.10.0-rc2+ #1 [loadavg: 40.57 27.55 13.39 25/277 64369]
 Hardware name: Gigabyte Technology Co., Ltd. GA-MA78GM-S2H/GA-MA78GM-S2H, BIOS F12a 04/23/2010
  0000000000000000 ffff88010f78fcf8 ffffffff816ae383 ffff88010f78fd28
  ffffffff810b698d ffff88011c092548 000000000023d073 ffff88011c092500
  0000000000000001 ffff88010f78fd60 ffffffff8109d7c5 ffffffff8109d645
 Call Trace:
  [<ffffffff816ae383>] dump_stack+0x19/0x1b
  [<ffffffff810b698d>] lockdep_rcu_suspicious+0xfd/0x130
  [<ffffffff8109d7c5>] cpuacct_charge+0x185/0x1f0
  [<ffffffff8109d645>] ? cpuacct_charge+0x5/0x1f0
  [<ffffffff8108dffc>] update_curr+0xec/0x240
  [<ffffffff8108f528>] put_prev_task_fair+0x228/0x480
  [<ffffffff816b3a71>] __schedule+0x161/0x9b0
  [<ffffffff816b4721>] preempt_schedule+0x51/0x80
  [<ffffffff816b4800>] ? __cond_resched_softirq+0x60/0x60
  [<ffffffff816b6824>] ? retint_careful+0x12/0x2e
  [<ffffffff810ff3cc>] ftrace_ops_control_func+0x1dc/0x210
  [<ffffffff816be280>] ftrace_call+0x5/0x2f
  [<ffffffff816b681d>] ? retint_careful+0xb/0x2e
  [<ffffffff816b4805>] ? schedule_user+0x5/0x70
  [<ffffffff816b4805>] ? schedule_user+0x5/0x70
  [<ffffffff816b6824>] ? retint_careful+0x12/0x2e
 ------------[ cut here ]------------

What happened was that the function tracer traced the schedule_user() code
that tells RCU that the system is coming back from userspace, and to
add the CPU back to the RCU monitoring.

Because the function tracer does a preempt_disable/enable_notrace() calls
the preempt_enable_notrace() checks the NEED_RESCHED flag. If it is set,
then preempt_schedule() is called. But this is called before the user_exit()
function can inform the kernel that the CPU is no longer in user mode and
needs to be accounted for by RCU.

The fix is to create a new preempt_schedule_context() that checks if
the kernel is still in user mode and if so to switch it to kernel mode
before calling schedule. It also switches back to user mode coming back
from schedule in need be.

The only user of this currently is the preempt_enable_notrace(), which is
only used by the tracing subsystem.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1369423420.6828.226.camel@gandalf.local.homeSigned-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/preempt.h

@@ -33,9 +33,25 @@ do { \
 		preempt_schedule(); \
 } while (0)
 
+#ifdef CONFIG_CONTEXT_TRACKING
+
+void preempt_schedule_context(void);
+
+#define preempt_check_resched_context() \
+do { \
+	if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
+		preempt_schedule_context(); \
+} while (0)
+#else
+
+#define preempt_check_resched_context() preempt_check_resched()
+
+#endif /* CONFIG_CONTEXT_TRACKING */
+
 #else /* !CONFIG_PREEMPT */
 
 #define preempt_check_resched()		do { } while (0)
+#define preempt_check_resched_context()	do { } while (0)
 
 #endif /* CONFIG_PREEMPT */
 
@@ -88,7 +104,7 @@ do { \
 do { \
 	preempt_enable_no_resched_notrace(); \
 	barrier(); \
-	preempt_check_resched(); \
+	preempt_check_resched_context(); \
 } while (0)
 
 #else /* !CONFIG_PREEMPT_COUNT */

kernel/context_tracking.c

@@ -71,6 +71,46 @@ void user_enter(void)
 	local_irq_restore(flags);
 }
 
+#ifdef CONFIG_PREEMPT
+/**
+ * preempt_schedule_context - preempt_schedule called by tracing
+ *
+ * The tracing infrastructure uses preempt_enable_notrace to prevent
+ * recursion and tracing preempt enabling caused by the tracing
+ * infrastructure itself. But as tracing can happen in areas coming
+ * from userspace or just about to enter userspace, a preempt enable
+ * can occur before user_exit() is called. This will cause the scheduler
+ * to be called when the system is still in usermode.
+ *
+ * To prevent this, the preempt_enable_notrace will use this function
+ * instead of preempt_schedule() to exit user context if needed before
+ * calling the scheduler.
+ */
+void __sched notrace preempt_schedule_context(void)
+{
+	struct thread_info *ti = current_thread_info();
+	enum ctx_state prev_ctx;
+
+	if (likely(ti->preempt_count || irqs_disabled()))
+		return;
+
+	/*
+	 * Need to disable preemption in case user_exit() is traced
+	 * and the tracer calls preempt_enable_notrace() causing
+	 * an infinite recursion.
+	 */
+	preempt_disable_notrace();
+	prev_ctx = exception_enter();
+	preempt_enable_no_resched_notrace();
+
+	preempt_schedule();
+
+	preempt_disable_notrace();
+	exception_exit(prev_ctx);
+	preempt_enable_notrace();
+}
+EXPORT_SYMBOL_GPL(preempt_schedule_context);
+#endif /* CONFIG_PREEMPT */
 
 /**
  * user_exit - Inform the context tracking that the CPU is