[PATCH] sched/core: make "Preemption disabled at" message more useful

[Vegard Nossum]

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=2)

Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Rusty Russel <ru...@rustcorp.com.au>
Signed-off-by: Vegard Nossum <vegard.nos...@oracle.com>
---
 kernel/sched/core.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7171cf9..87689a6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3144,9 +3144,18 @@ static inline void preempt_latency_stop(int val) { }
  */
 static noinline void __schedule_bug(struct task_struct *prev)
 {
+#ifdef CONFIG_DEBUG_PREEMPT
+       unsigned long preempt_disable_ip;
+#endif
+
        if (oops_in_progress)
                return;
 
+#ifdef CONFIG_DEBUG_PREEMPT
+       /* Save this before calling printk(), since that will clobber it */
+       preempt_disable_ip = current->preempt_disable_ip;
+#endif
+
        printk(KERN_ERR "BUG: scheduling while atomic: %s/%d/0x%08x\n",
                prev->comm, prev->pid, preempt_count());
 
@@ -3157,7 +3166,7 @@ static noinline void __schedule_bug(struct task_struct 
*prev)
 #ifdef CONFIG_DEBUG_PREEMPT
        if (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
@@ -7541,6 +7550,9 @@ EXPORT_SYMBOL(__might_sleep);
 void ___might_sleep(const char *file, int line, int preempt_offset)
 {
        static unsigned long prev_jiffy;        /* ratelimiting */
+#ifdef CONFIG_DEBUG_PREEMPT
+       unsigned long preempt_disable_ip;
+#endif
 
        rcu_sleep_check(); /* WARN_ON_ONCE() by default, no rate limit reqd. */
        if ((preempt_count_equals(preempt_offset) && !irqs_disabled() &&
@@ -7551,6 +7563,11 @@ void ___might_sleep(const char *file, int line, int 
preempt_offset)
                return;
        prev_jiffy = jiffies;
 
+#ifdef CONFIG_DEBUG_PREEMPT
+       /* Save this before calling printk(), since that will clobber it */
+       preempt_disable_ip = current->preempt_disable_ip;
+#endif
+
        printk(KERN_ERR
                "BUG: sleeping function called from invalid context at %s:%d\n",
                        file, line);
@@ -7568,7 +7585,7 @@ void ___might_sleep(const char *file, int line, int 
preempt_offset)
 #ifdef CONFIG_DEBUG_PREEMPT
        if (!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
-- 
1.9.1