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SubjectRe: [2.6.2] Badness in futex_wait revisited
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On Thu, 2004-04-01 at 02:56, Jamie Lokier wrote:
> Was the badness in futex_wait problem ever resolved?

No: Andrew misapplied the patches for ages and ended up ditching it. We
tried a different version but there was a bug which meant real wakups
triggered it...

Here is the current version...

Name: Who's Spuriously Waking Futexes?
Author: Andrew Morton, Rusty Russell
Status: Tested on 2.6.3-bk1

Someone is triggering the WARN_ON() in futex.c. We know that software
suspend could do it, in theory. But noone else should be.

This code adds a PF_FUTEX_DEBUG flag, which is set in the futex code
when we sleep, and also when we wake up. If a task with
PF_FUTEX_DEBUG is woken by a task without PF_FUTEX_DEBUG, we have
found our culprit.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .31103-linux-2.6.3-rc4/include/linux/sched.h .31103-linux-2.6.3-rc4.updated/include/linux/sched.h
--- .31103-linux-2.6.3-rc4/include/linux/sched.h 2004-02-17 17:54:03.000000000 +1100
+++ .31103-linux-2.6.3-rc4.updated/include/linux/sched.h 2004-02-18 14:55:05.000000000 +1100
@@ -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
extern int set_cpus_allowed(task_t *p, cpumask_t new_mask);
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .31103-linux-2.6.3-rc4/kernel/futex.c .31103-linux-2.6.3-rc4.updated/kernel/futex.c
--- .31103-linux-2.6.3-rc4/kernel/futex.c 2004-02-17 17:54:04.000000000 +1100
+++ .31103-linux-2.6.3-rc4.updated/kernel/futex.c 2004-02-18 15:09:15.000000000 +1100
@@ -269,7 +269,11 @@ static void wake_futex(struct futex_q *q
* The lock in wake_up_all() is a crucial memory barrier after the
* list_del_init() and also before assigning to q->lock_ptr.
*/
+
+ current->flags |= PF_FUTEX_DEBUG;
wake_up_all(&q->waiters);
+ current->flags &= ~PF_FUTEX_DEBUG;
+
/*
* The waiting task can free the futex_q as soon as this is written,
* without taking any locks. This must come last.
@@ -490,8 +494,11 @@ 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);

/*
@@ -505,7 +512,11 @@ static int futex_wait(unsigned long uadd
if (time == 0)
return -ETIMEDOUT;
/* A spurious wakeup should never happen. */
- WARN_ON(!signal_pending(current));
+ if (!signal_pending(current)) {
+ printk("futex_wait woken: %lu %i %lu\n",
+ uaddr, val, time);
+ WARN_ON(1);
+ }
return -EINTR;

out_unqueue:
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .31103-linux-2.6.3-rc4/kernel/sched.c .31103-linux-2.6.3-rc4.updated/kernel/sched.c
--- .31103-linux-2.6.3-rc4/kernel/sched.c 2004-02-17 17:54:04.000000000 +1100
+++ .31103-linux-2.6.3-rc4.updated/kernel/sched.c 2004-02-18 14:55:05.000000000 +1100
@@ -658,6 +658,14 @@ static int try_to_wake_up(task_t * p, un
long old_state;
runqueue_t *rq;

+ if ((p->flags & PF_FUTEX_DEBUG)
+ && !(current->flags & PF_FUTEX_DEBUG)) {
+ printk("%s %i waking %s: %i %i\n",
+ current->comm, (int)in_interrupt(),
+ p->comm, p->tgid, p->pid);
+ WARN_ON(1);
+ }
+
repeat_lock_task:
rq = task_rq_lock(p, &flags);
old_state = p->state;
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .31103-linux-2.6.3-rc4/kernel/timer.c .31103-linux-2.6.3-rc4.updated/kernel/timer.c
--- .31103-linux-2.6.3-rc4/kernel/timer.c 2004-02-04 15:39:15.000000000 +1100
+++ .31103-linux-2.6.3-rc4.updated/kernel/timer.c 2004-02-18 14:55:05.000000000 +1100
@@ -971,6 +971,13 @@ static void process_timeout(unsigned lon
wake_up_process((task_t *)__data);
}

+static void futex_timeout(unsigned long __data)
+{
+ current->flags |= PF_FUTEX_DEBUG;
+ wake_up_process((task_t *)__data);
+ current->flags &= ~PF_FUTEX_DEBUG;
+}
+
/**
* schedule_timeout - sleep until timeout
* @timeout: timeout value in jiffies
@@ -1037,7 +1044,10 @@ signed long schedule_timeout(signed long
init_timer(&timer);
timer.expires = expire;
timer.data = (unsigned long) current;
- timer.function = process_timeout;
+ if (current->flags & PF_FUTEX_DEBUG)
+ timer.function = futex_timeout;
+ else
+ timer.function = process_timeout;

add_timer(&timer);
schedule();
--
Anyone who quotes me in their signature is an idiot -- Rusty Russell

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