sched/debug: Make the "Preemption disabled at ..." message more useful

This message is currently really useless since it always prints a value
that comes from the printk() we just did, e.g.:

    BUG: sleeping function called from invalid context at mm/slab.h:388
    in_atomic(): 0, irqs_disabled(): 0, pid: 31996, name: trinity-c1
    Preemption disabled at:[<ffffffff8119db33>] down_trylock+0x13/0x80

    BUG: sleeping function called from invalid context at include/linux/freezer.h:56
    in_atomic(): 0, irqs_disabled(): 0, pid: 31996, name: trinity-c1
    Preemption disabled at:[<ffffffff811aaa37>] console_unlock+0x2f7/0x930

Here, both down_trylock() and console_unlock() is somewhere in the
printk() path.

We should save the value before calling printk() and use the saved value
instead. That immediately reveals the offending callsite:

    BUG: sleeping function called from invalid context at mm/slab.h:388
    in_atomic(): 0, irqs_disabled(): 0, pid: 14971, name: trinity-c2
    Preemption disabled at:[<ffffffff819bcd46>] rhashtable_walk_start+0x46/0x150

Bug report:

  http://marc.info/?l=linux-netdev&m=146925979821849&w=2Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russel <rusty@rustcorp.com.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched.h

@@ -3236,6 +3236,15 @@ static inline void cond_resched_rcu(void)
 #endif
 }
 
+static inline unsigned long get_preempt_disable_ip(struct task_struct *p)
+{
+#ifdef CONFIG_DEBUG_PREEMPT
+	return p->preempt_disable_ip;
+#else
+	return 0;
+#endif
+}
+
 /*
  * Does a critical section need to be broken due to another
  * task waiting?: (technically does not depend on CONFIG_PREEMPT,

kernel/sched/core.c

@@ -3171,6 +3171,9 @@ static inline void preempt_latency_stop(int val) { }
  */
 static noinline void __schedule_bug(struct task_struct *prev)
 {
+	/* Save this before calling printk(), since that will clobber it */
+	unsigned long preempt_disable_ip = get_preempt_disable_ip(current);
+
 	if (oops_in_progress)
 		return;
 
@@ -3181,13 +3184,12 @@ static noinline void __schedule_bug(struct task_struct *prev)
 	print_modules();
 	if (irqs_disabled())
 		print_irqtrace_events(prev);
-#ifdef CONFIG_DEBUG_PREEMPT
-	if (in_atomic_preempt_off()) {
+	if (IS_ENABLED(CONFIG_DEBUG_PREEMPT)
+	    && in_atomic_preempt_off()) {
 		pr_err("Preemption disabled at:");
-		print_ip_sym(current->preempt_disable_ip);
+		print_ip_sym(preempt_disable_ip);
 		pr_cont("\n");
 	}
-#endif
 	if (panic_on_warn)
 		panic("scheduling while atomic\n");
 
@@ -7571,6 +7573,7 @@ EXPORT_SYMBOL(__might_sleep);
 void ___might_sleep(const char *file, int line, int preempt_offset)
 {
 	static unsigned long prev_jiffy;	/* ratelimiting */
+	unsigned long preempt_disable_ip;
 
 	rcu_sleep_check(); /* WARN_ON_ONCE() by default, no rate limit reqd. */
 	if ((preempt_count_equals(preempt_offset) && !irqs_disabled() &&
@@ -7581,6 +7584,9 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
 		return;
 	prev_jiffy = jiffies;
 
+	/* Save this before calling printk(), since that will clobber it */
+	preempt_disable_ip = get_preempt_disable_ip(current);
+
 	printk(KERN_ERR
 		"BUG: sleeping function called from invalid context at %s:%d\n",
 			file, line);
@@ -7595,13 +7601,12 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
 	debug_show_held_locks(current);
 	if (irqs_disabled())
 		print_irqtrace_events(current);
-#ifdef CONFIG_DEBUG_PREEMPT
-	if (!preempt_count_equals(preempt_offset)) {
+	if (IS_ENABLED(CONFIG_DEBUG_PREEMPT)
+	    && !preempt_count_equals(preempt_offset)) {
 		pr_err("Preemption disabled at:");
-		print_ip_sym(current->preempt_disable_ip);
+		print_ip_sym(preempt_disable_ip);
 		pr_cont("\n");
 	}
-#endif
 	dump_stack();
 }
 EXPORT_SYMBOL(___might_sleep);