timer: Try to survive timer callback preempt_count leak

If a timer callback leaks preempt_count we currently assert a
BUG(). That makes it unnecessarily hard to retrieve information about
the problem especially on laptops and headless stations.

There is a decent chance to survive the preempt_count leak by
restoring the preempt_count to the value before the callback. That
allows in many cases to get valuable information about the root cause
of the problem.

We carried that fixup in preempt-rt for years and were able to decode
such wreckage quite a few times.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linux Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Veen <arjan@infradead.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/kernel/timer.c b/kernel/timer.c
index 45229694dc6a..7e12e7bc7ce6 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -982,9 +982,15 @@ static void call_timer_fn(struct timer_list *timer, void (*fn)(unsigned long),
         lock_map_release(&lockdep_map);
 
         if (preempt_count != preempt_count()) {
-                printk(KERN_ERR "timer: %pF preempt leak: %08x -> %08x\n",
-                       fn, preempt_count, preempt_count());
-                BUG();
+                WARN_ONCE(1, "timer: %pF preempt leak: %08x -> %08x\n",
+                          fn, preempt_count, preempt_count());
+                /*
+                 * Restore the preempt count. That gives us a decent
+                 * chance to survive and extract information. If the
+                 * callback kept a lock held, bad luck, but not worse
+                 * than the BUG() we had.
+                 */
+                preempt_count() = preempt_count;
         }
 }