Re: BUG in 2.6.20-rt8

Ingo Molnar
2007-02-25T06:34:10Z

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> I got the following running stock 2.6.20-rt8 on an 4-CPU 1.8GHz
> Opteron box. The machine continued to run a few rounds of kernbench
> and LTP. Looks a bit scary -- a tasklet was "stolen" from
> __tasklet_action().
>
> Thoughts? In the meantime, kicking it off again to see if it repeats.

> BUG: at kernel/softirq.c:559 __tasklet_action()

this seems to happen very sporadically. Seems to happen more likely on
hyperthreading CPUs. It is very likely caused by the
redesign-tasklet-locking-to-be-sane patch below - which is a quick hack
of mine from early -rt days. Can you see any obvious bug in it? The
cmpxchg logic is certainly a bit ... tricky, locking-wise.

Ingo

---------------->
From: Ingo Molnar <mingo@elte.hu>

tasklet redesign: make it saner and make it easier to thread.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

----
include/linux/interrupt.h | 39 ++++++-----
kernel/softirq.c | 155 +++++++++++++++++++++++++++++++---------------
2 files changed, 128 insertions(+), 66 deletions(-)

Index: linux/include/linux/interrupt.h
===================================================================
--- linux.orig/include/linux/interrupt.h
+++ linux/include/linux/interrupt.h
@@ -276,8 +276,9 @@ extern void wait_for_softirq(int softirq
to be executed on some cpu at least once after this.
* If the tasklet is already scheduled, but its excecution is still not
started, it will be executed only once.
- * If this tasklet is already running on another CPU (or schedule is called
- from tasklet itself), it is rescheduled for later.
+ * If this tasklet is already running on another CPU, it is rescheduled
+ for later.
+ * Schedule must not be called from the tasklet itself (a lockup occurs)
* Tasklet is strictly serialized wrt itself, but not
wrt another tasklets. If client needs some intertask synchronization,
he makes it with spinlocks.
@@ -302,15 +303,25 @@ struct tasklet_struct name = { NULL, 0,
enum
{
TASKLET_STATE_SCHED, /* Tasklet is scheduled for execution */
- TASKLET_STATE_RUN /* Tasklet is running (SMP only) */
+ TASKLET_STATE_RUN, /* Tasklet is running (SMP only) */
+ TASKLET_STATE_PENDING /* Tasklet is pending */
};

-#ifdef CONFIG_SMP
+#define TASKLET_STATEF_SCHED (1 << TASKLET_STATE_SCHED)
+#define TASKLET_STATEF_RUN (1 << TASKLET_STATE_RUN)
+#define TASKLET_STATEF_PENDING (1 << TASKLET_STATE_PENDING)
+
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
static inline int tasklet_trylock(struct tasklet_struct *t)
{
return !test_and_set_bit(TASKLET_STATE_RUN, &(t)->state);
}

+static inline int tasklet_tryunlock(struct tasklet_struct *t)
+{
+ return cmpxchg(&t->state, TASKLET_STATEF_RUN, 0) == TASKLET_STATEF_RUN;
+}
+
static inline void tasklet_unlock(struct tasklet_struct *t)
{
smp_mb__before_clear_bit();
@@ -322,9 +333,10 @@ static inline void tasklet_unlock_wait(s
while (test_bit(TASKLET_STATE_RUN, &(t)->state)) { barrier(); }
}
#else
-#define tasklet_trylock(t) 1
-#define tasklet_unlock_wait(t) do { } while (0)
-#define tasklet_unlock(t) do { } while (0)
+# define tasklet_trylock(t) 1
+# define tasklet_tryunlock(t) 1
+# define tasklet_unlock_wait(t) do { } while (0)
+# define tasklet_unlock(t) do { } while (0)
#endif

extern void FASTCALL(__tasklet_schedule(struct tasklet_struct *t));
@@ -357,17 +369,8 @@ static inline void tasklet_disable(struc
smp_mb();
}

-static inline void tasklet_enable(struct tasklet_struct *t)
-{
- smp_mb__before_atomic_dec();
- atomic_dec(&t->count);
-}
-
-static inline void tasklet_hi_enable(struct tasklet_struct *t)
-{
- smp_mb__before_atomic_dec();
- atomic_dec(&t->count);
-}
+extern fastcall void tasklet_enable(struct tasklet_struct *t);
+extern fastcall void tasklet_hi_enable(struct tasklet_struct *t);

extern void tasklet_kill(struct tasklet_struct *t);
extern void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu);
Index: linux/kernel/softirq.c
===================================================================
--- linux.orig/kernel/softirq.c
+++ linux/kernel/softirq.c
@@ -464,14 +464,24 @@ struct tasklet_head
static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec) = { NULL };
static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec) = { NULL };

+static void inline
+__tasklet_common_schedule(struct tasklet_struct *t, struct tasklet_head *head, unsigned int nr)
+{
+ if (tasklet_trylock(t)) {
+ WARN_ON(t->next != NULL);
+ t->next = head->list;
+ head->list = t;
+ raise_softirq_irqoff(nr);
+ tasklet_unlock(t);
+ }
+}
+
void fastcall __tasklet_schedule(struct tasklet_struct *t)
{
unsigned long flags;

local_irq_save(flags);
- t->next = __get_cpu_var(tasklet_vec).list;
- __get_cpu_var(tasklet_vec).list = t;
- raise_softirq_irqoff(TASKLET_SOFTIRQ);
+ __tasklet_common_schedule(t, &__get_cpu_var(tasklet_vec), TASKLET_SOFTIRQ);
local_irq_restore(flags);
}

@@ -482,81 +492,130 @@ void fastcall __tasklet_hi_schedule(stru
unsigned long flags;

local_irq_save(flags);
- t->next = __get_cpu_var(tasklet_hi_vec).list;
- __get_cpu_var(tasklet_hi_vec).list = t;
- raise_softirq_irqoff(HI_SOFTIRQ);
+ __tasklet_common_schedule(t, &__get_cpu_var(tasklet_hi_vec), HI_SOFTIRQ);
local_irq_restore(flags);
}

EXPORT_SYMBOL(__tasklet_hi_schedule);

-static void tasklet_action(struct softirq_action *a)
+void fastcall tasklet_enable(struct tasklet_struct *t)
{
- struct tasklet_struct *list;
+ if (!atomic_dec_and_test(&t->count))
+ return;
+ if (test_and_clear_bit(TASKLET_STATE_PENDING, &t->state))
+ tasklet_schedule(t);
+}

- local_irq_disable();
- list = __get_cpu_var(tasklet_vec).list;
- __get_cpu_var(tasklet_vec).list = NULL;
- local_irq_enable();
+EXPORT_SYMBOL(tasklet_enable);
+
+void fastcall tasklet_hi_enable(struct tasklet_struct *t)
+{
+ if (!atomic_dec_and_test(&t->count))
+ return;
+ if (test_and_clear_bit(TASKLET_STATE_PENDING, &t->state))
+ tasklet_hi_schedule(t);
+}
+
+EXPORT_SYMBOL(tasklet_hi_enable);
+
+static void
+__tasklet_action(struct softirq_action *a, struct tasklet_struct *list)
+{
+ int loops = 1000000;

while (list) {
struct tasklet_struct *t = list;

list = list->next;
+ /*
+ * Should always succeed - after a tasklist got on the
+ * list (after getting the SCHED bit set from 0 to 1),
+ * nothing but the tasklet softirq it got queued to can
+ * lock it:
+ */
+ if (!tasklet_trylock(t)) {
+ WARN_ON(1);
+ continue;
+ }
+
+ t->next = NULL;
+
+ /*
+ * If we cannot handle the tasklet because it's disabled,
+ * mark it as pending. tasklet_enable() will later
+ * re-schedule the tasklet.
+ */
+ if (unlikely(atomic_read(&t->count))) {
+out_disabled:
+ /* implicit unlock: */
+ wmb();
+ t->state = TASKLET_STATEF_PENDING;
+ continue;
+ }

- if (tasklet_trylock(t)) {
- if (!atomic_read(&t->count)) {
- if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
- BUG();
- t->func(t->data);
+ /*
+ * After this point on the tasklet might be rescheduled
+ * on another CPU, but it can only be added to another
+ * CPU's tasklet list if we unlock the tasklet (which we
+ * dont do yet).
+ */
+ if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
+ WARN_ON(1);
+
+again:
+ t->func(t->data);
+
+ /*
+ * Try to unlock the tasklet. We must use cmpxchg, because
+ * another CPU might have scheduled or disabled the tasklet.
+ * We only allow the STATE_RUN -> 0 transition here.
+ */
+ while (!tasklet_tryunlock(t)) {
+ /*
+ * If it got disabled meanwhile, bail out:
+ */
+ if (atomic_read(&t->count))
+ goto out_disabled;
+ /*
+ * If it got scheduled meanwhile, re-execute
+ * the tasklet function:
+ */
+ if (test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
+ goto again;
+ if (!--loops) {
+ printk("hm, tasklet state: %08lx\n", t->state);
+ WARN_ON(1);
tasklet_unlock(t);
- continue;
+ break;
}
- tasklet_unlock(t);
}
-
- local_irq_disable();
- t->next = __get_cpu_var(tasklet_vec).list;
- __get_cpu_var(tasklet_vec).list = t;
- __do_raise_softirq_irqoff(TASKLET_SOFTIRQ);
- local_irq_enable();
}
}

-static void tasklet_hi_action(struct softirq_action *a)
+static void tasklet_action(struct softirq_action *a)
{
struct tasklet_struct *list;

local_irq_disable();
- list = __get_cpu_var(tasklet_hi_vec).list;
- __get_cpu_var(tasklet_hi_vec).list = NULL;
+ list = __get_cpu_var(tasklet_vec).list;
+ __get_cpu_var(tasklet_vec).list = NULL;
local_irq_enable();

- while (list) {
- struct tasklet_struct *t = list;
+ __tasklet_action(a, list);
+}

- list = list->next;
+static void tasklet_hi_action(struct softirq_action *a)
+{
+ struct tasklet_struct *list;

- if (tasklet_trylock(t)) {
- if (!atomic_read(&t->count)) {
- if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
- BUG();
- t->func(t->data);
- tasklet_unlock(t);
- continue;
- }
- tasklet_unlock(t);
- }
+ local_irq_disable();
+ list = __get_cpu_var(tasklet_hi_vec).list;
+ __get_cpu_var(tasklet_hi_vec).list = NULL;
+ local_irq_enable();

- local_irq_disable();
- t->next = __get_cpu_var(tasklet_hi_vec).list;
- __get_cpu_var(tasklet_hi_vec).list = t;
- __do_raise_softirq_irqoff(HI_SOFTIRQ);
- local_irq_enable();
- }
+ __tasklet_action(a, list);
}

-
void tasklet_init(struct tasklet_struct *t,
void (*func)(unsigned long), unsigned long data)
{