Re: [2.6.1-mm2] Badness in futex_wait at kernel/futex.c:509

[Andrew Morton <akpm@osdl.org>]

Rusty Russell <rusty@rustcorp.com.au> wrote:
>
> In message <20040113223612.4fe709d9.akpm@osdl.org> you write:
> > Horacio de Oro <hgdeoro@yahoo.com> wrote:
> > >
> > > Hi!
> > > 
> > > This happen every time I switch from X to console:
> > > 
> > > Badness in futex_wait at kernel/futex.c:509
> > > Call Trace:
> > >  [futex_wait+434/448] futex_wait+0x1b2/0x1c0
> > >  [default_wake_function+0/32] default_wake_function+0x0/0x20
> > >  [default_wake_function+0/32] default_wake_function+0x0/0x20
> > >  [do_futex+112/128] do_futex+0x70/0x80
> > >  [sys_futex+292/320] sys_futex+0x124/0x140
> > >  [syscall_call+7/11] syscall_call+0x7/0xb
> > > 
> > 
> > 	/* A spurious wakeup should never happen. */
> > 	WARN_ON(!signal_pending(current));
> > 
> > (looks at Rusty)
> 
> You were the one who said spurious wakeups shouldn't happen 8)
> 
> This implies that the console code decided to do wake_up_process() on
> us.  We returned -EINTR to userspace, but there was no signal, which
> is odd.

Well this would seem to imply that this task woke up and ran while it was
still parked on q.waiters, yes?

> Anyone have any ideas *why*?

Nope.  This patch should identify the source of the wakeup.  Horacio, could
you please add it, see if you get a nice trace?  The patch is a bit racy
wrt task exit, but it should hold up for long enough to get a trace.

 include/linux/sched.h |    1 +
 kernel/futex.c        |    5 ++++-
 kernel/sched.c        |    3 +++
 3 files changed, 8 insertions(+), 1 deletion(-)

diff -puN kernel/futex.c~futex-wakeup-debug kernel/futex.c
--- 25/kernel/futex.c~futex-wakeup-debug	2004-01-14 18:01:10.000000000 -0800
+++ 25-akpm/kernel/futex.c	2004-01-14 18:01:10.000000000 -0800
@@ -491,8 +491,11 @@ static int futex_wait(unsigned long uadd
 	 * !list_empty() is safe here without any lock.
 	 * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
 	 */
-	if (likely(!list_empty(&q.list)))
+	if (likely(!list_empty(&q.list))) {
+		current->flags |= PF_FUTEX_DEBUG;
 		time = schedule_timeout(time);
+		current->flags &= ~PF_FUTEX_DEBUG;
+	}
 	__set_current_state(TASK_RUNNING);
 
 	/*
diff -puN include/linux/sched.h~futex-wakeup-debug include/linux/sched.h
--- 25/include/linux/sched.h~futex-wakeup-debug	2004-01-14 18:01:10.000000000 -0800
+++ 25-akpm/include/linux/sched.h	2004-01-14 18:01:10.000000000 -0800
@@ -500,6 +500,7 @@ do { if (atomic_dec_and_test(&(tsk)->usa
 #define PF_SWAPOFF	0x00080000	/* I am in swapoff */
 #define PF_LESS_THROTTLE 0x00100000	/* Throttle me less: I clean memory */
 #define PF_SYNCWRITE	0x00200000	/* I am doing a sync write */
+#define PF_FUTEX_DEBUG	0x00400000
 
 #ifdef CONFIG_SMP
 #define SD_FLAG_NEWIDLE		1	/* Balance when about to become idle */
diff -puN kernel/sched.c~futex-wakeup-debug kernel/sched.c
--- 25/kernel/sched.c~futex-wakeup-debug	2004-01-14 18:01:10.000000000 -0800
+++ 25-akpm/kernel/sched.c	2004-01-14 18:01:10.000000000 -0800
@@ -770,6 +770,9 @@ static int try_to_wake_up(task_t * p, un
 	}
 	goto out_activate;
 
+	if (p->flags & PF_FUTEX_DEBUG)
+		WARN_ON(!signal_pending(p));
+
 repeat_lock_task:
 	task_rq_unlock(rq, &flags);
 	rq = task_rq_lock(p, &flags);

_