[patch 3/5] Fix timeout bug in rtmutex in 2.6.18-rt

From: Esben Nielsen
Date: Sun Oct 01 2006 - 07:38:32 EST

This patch fixes

1) The timeout bug in rtmutexes and PI futexes: When a task is blocked on a lock with timeout and times out it will not wakeup until the owner of the lock
is done. This is because the owner is boosted to the same priority as the blocked task and therefore has the CPU such the blocked task never gets around to de-boost it!

2) setscheduler() now does the PI walking - but defers the work to the blocked
task.

3) In general it makes sure that a task, which is boosting another has enough
priority to do the de-boosting no matter how complicated the lock structure is, or how many times the priorities have changed.

The idea behind the patch is simple:
If a task is boosting another it is scheduled in LIFO order and it will never
loose it's priority. This property lasts until it has left the lock operation
(successfully or not).

The needed priority to do the unboosting is stored in task->boosting_prio.
In the current patch this is always increasing (numerically decreasing) while
trying to take a lock. In a future it might be found safe to decrease
boosting_prio before finally leaving the lock operation.

include/linux/rtmutex.h | 1
include/linux/sched.h | 6
kernel/fork.c | 1
kernel/rtmutex.c | 142 ++++++++++++------
kernel/rtmutex_common.h | 1
kernel/sched.c | 21 ++
scripts/rt-tester/t5-l4-pi-boost-deboost-setsched.tst | 36 ++--
scripts/rt-tester/t5-l4-pi-boost-deboost.tst | 12 -
8 files changed, 156 insertions(+), 64 deletions(-)

Index: linux-2.6.18-rt/include/linux/rtmutex.h
===================================================================
--- linux-2.6.18-rt.orig/include/linux/rtmutex.h
+++ linux-2.6.18-rt/include/linux/rtmutex.h
@@ -99,6 +99,7 @@ extern void rt_mutex_unlock(struct rt_mu
#ifdef CONFIG_RT_MUTEXES
# define INIT_RT_MUTEXES(tsk) \
.pi_waiters = PLIST_HEAD_INIT(tsk.pi_waiters, tsk.pi_lock), \
+ .boosting_prio = MAX_PRIO, \
INIT_RT_MUTEX_DEBUG(tsk)
#else
# define INIT_RT_MUTEXES(tsk)
Index: linux-2.6.18-rt/include/linux/sched.h
===================================================================
--- linux-2.6.18-rt.orig/include/linux/sched.h
+++ linux-2.6.18-rt/include/linux/sched.h
@@ -1069,6 +1069,12 @@ struct task_struct {
struct plist_head pi_waiters;
/* Deadlock detection and priority inheritance handling */
struct rt_mutex_waiter *pi_blocked_on;
+
+ /*
+ * Priority to which this task is boosting something. Don't lower the
+ * priority below this before the boosting is resolved;
+ */
+ int boosting_prio;
#endif

#ifdef CONFIG_DEBUG_MUTEXES
Index: linux-2.6.18-rt/kernel/fork.c
===================================================================
--- linux-2.6.18-rt.orig/kernel/fork.c
+++ linux-2.6.18-rt/kernel/fork.c
@@ -966,6 +966,7 @@ static inline void rt_mutex_init_task(st
spin_lock_init(&p->pi_lock);
plist_head_init(&p->pi_waiters, &p->pi_lock);
p->pi_blocked_on = NULL;
+ p->boosting_prio = MAX_PRIO;
# ifdef CONFIG_DEBUG_RT_MUTEXES
p->last_kernel_lock = NULL;
# endif
Index: linux-2.6.18-rt/kernel/rtmutex.c
===================================================================
--- linux-2.6.18-rt.orig/kernel/rtmutex.c
+++ linux-2.6.18-rt/kernel/rtmutex.c
@@ -126,7 +126,7 @@ static inline void init_lists(struct rt_
* Return task->normal_prio when the waiter list is empty or when
* the waiter is not allowed to do priority boosting
*/
-int rt_mutex_getprio(struct task_struct *task)
+static int rt_mutex_getprio_real(struct task_struct *task)
{
if (likely(!task_has_pi_waiters(task)))
return task->normal_prio;
@@ -135,6 +135,11 @@ int rt_mutex_getprio(struct task_struct
task->normal_prio);
}

+int rt_mutex_getprio(struct task_struct *task)
+{
+ return min(rt_mutex_getprio_real(task), task->boosting_prio);
+}
+
/*
* Adjust the priority of a task, after its pi_waiters got modified.
*
@@ -149,6 +154,20 @@ static void __rt_mutex_adjust_prio(struc
}

/*
+ * Adjust the priority of a task, after its pi_waiters got modified.
+ *
+ * This can be both boosting and unboosting.
+ */
+static void rt_mutex_adjust_prio(struct task_struct *task)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&task->pi_lock, flags);
+ __rt_mutex_adjust_prio(task);
+ spin_unlock_irqrestore(&task->pi_lock, flags);
+}
+
+/*
* Adjust task priority (undo boosting). Called from the exit path of
* rt_mutex_slowunlock() and rt_mutex_slowlock().
*
@@ -157,11 +176,14 @@ static void __rt_mutex_adjust_prio(struc
* of task. We do not use the spin_xx_mutex() variants here as we are
* outside of the debug path.)
*/
-static void rt_mutex_adjust_prio(struct task_struct *task)
+static void rt_mutex_adjust_prio_final(struct task_struct *task,
+ int old_sched_lifo)
{
unsigned long flags;

spin_lock_irqsave(&task->pi_lock, flags);
+ task->boosting_prio = MAX_PRIO;
+ leave_sched_lifo_other(task, old_sched_lifo);
__rt_mutex_adjust_prio(task);
spin_unlock_irqrestore(&task->pi_lock, flags);
}
@@ -237,7 +259,8 @@ static int rt_mutex_adjust_prio_chain(st
* When deadlock detection is off then we check, if further
* priority adjustment is necessary.
*/
- if (!detect_deadlock && waiter->list_entry.prio == task->prio)
+ if (!detect_deadlock &&
+ waiter->list_entry.prio == rt_mutex_getprio_real(task))
goto out_unlock_pi;

lock = waiter->lock;
@@ -259,7 +282,9 @@ static int rt_mutex_adjust_prio_chain(st

/* Requeue the waiter */
plist_del(&waiter->list_entry, &lock->wait_list);
- waiter->list_entry.prio = task->prio;
+ waiter->list_entry.prio = rt_mutex_getprio_real(task);
+ task->boosting_prio = min(task->boosting_prio,
+ waiter->list_entry.prio);
plist_add(&waiter->list_entry, &lock->wait_list);

/* Release the task */
@@ -306,6 +331,18 @@ static int rt_mutex_adjust_prio_chain(st
}

/*
+ * Calls the rt_mutex_adjust_prio_chain() above
+ * whith unlocking and locking lock->wait_lock.
+ */
+static void rt_mutex_adjust_prio_chain_unlock(struct rt_mutex *lock)
+{
+ spin_unlock(&lock->wait_lock);
+ get_task_struct(current);
+ rt_mutex_adjust_prio_chain(current, 0, NULL, NULL, NULL);
+ spin_lock(&lock->wait_lock);
+}
+
+/*
* Optimization: check if we can steal the lock from the
* assigned pending owner [which might not have taken the
* lock yet]:
@@ -315,6 +352,7 @@ static inline int try_to_steal_lock(stru
struct task_struct *pendowner = rt_mutex_owner(lock);
struct rt_mutex_waiter *next;
unsigned long flags;
+ int current_prio;

if (!rt_mutex_owner_pending(lock))
return 0;
@@ -322,8 +360,12 @@ static inline int try_to_steal_lock(stru
if (pendowner == current)
return 1;

+ spin_lock_irqsave(&current->pi_lock, flags);
+ current_prio = rt_mutex_getprio_real(current);
+ spin_unlock_irqrestore(&current->pi_lock, flags);
+
spin_lock_irqsave(&pendowner->pi_lock, flags);
- if (current->prio >= pendowner->prio) {
+ if (current_prio >= rt_mutex_getprio_real(pendowner)) {
spin_unlock_irqrestore(&pendowner->pi_lock, flags);
return 0;
}
@@ -432,8 +474,8 @@ static int task_blocks_on_rt_mutex(struc
__rt_mutex_adjust_prio(current);
waiter->task = current;
waiter->lock = lock;
- plist_node_init(&waiter->list_entry, current->prio);
- plist_node_init(&waiter->pi_list_entry, current->prio);
+ plist_node_init(&waiter->list_entry, rt_mutex_getprio_real(current));
+ plist_node_init(&waiter->pi_list_entry, waiter->list_entry.prio);

/* Get the top priority waiter on the lock */
if (rt_mutex_has_waiters(lock))
@@ -441,7 +483,9 @@ static int task_blocks_on_rt_mutex(struc
plist_add(&waiter->list_entry, &lock->wait_list);

current->pi_blocked_on = waiter;
-
+ current->boosting_prio = min(current->boosting_prio,
+ waiter->list_entry.prio);
+ enter_sched_lifo();
spin_unlock_irqrestore(&current->pi_lock, flags);

if (waiter == rt_mutex_top_waiter(lock)) {
@@ -507,7 +551,6 @@ static void wakeup_next_waiter(struct rt
plist_del(&waiter->pi_list_entry, &current->pi_waiters);
pendowner = waiter->task;
waiter->task = NULL;
-
rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING);

spin_unlock_irqrestore(&current->pi_lock, flags);
@@ -526,6 +569,8 @@ static void wakeup_next_waiter(struct rt
WARN_ON(pendowner->pi_blocked_on->lock != lock);

pendowner->pi_blocked_on = NULL;
+ pendowner->boosting_prio = MAX_PRIO;
+ leave_sched_lifo_other(pendowner, waiter->old_sched_lifo);

if (rt_mutex_has_waiters(lock)) {
struct rt_mutex_waiter *next;
@@ -533,6 +578,7 @@ static void wakeup_next_waiter(struct rt
next = rt_mutex_top_waiter(lock);
plist_add(&next->pi_list_entry, &pendowner->pi_waiters);
}
+ __rt_mutex_adjust_prio(pendowner);
spin_unlock_irqrestore(&pendowner->pi_lock, flags);

if (savestate)
@@ -660,6 +706,7 @@ rt_spin_lock_slowlock(struct rt_mutex *l
{
struct rt_mutex_waiter waiter;
unsigned long saved_state, state;
+ int adjust_prio_final = 0;

debug_rt_mutex_init_waiter(&waiter);
waiter.task = NULL;
@@ -686,6 +733,7 @@ rt_spin_lock_slowlock(struct rt_mutex *l
* then we return with the saved state.
*/
saved_state = xchg(&current->state, TASK_UNINTERRUPTIBLE);
+ waiter.old_sched_lifo = get_sched_lifo();

for (;;) {
unsigned long saved_flags;
@@ -699,12 +747,16 @@ rt_spin_lock_slowlock(struct rt_mutex *l
* when we have been woken up by the previous owner
* but the lock got stolen by an higher prio task.
*/
- if (!waiter.task) {
+ if (!waiter.task)
task_blocks_on_rt_mutex(lock, &waiter, 0);
- /* Wakeup during boost ? */
- if (unlikely(!waiter.task))
- continue;
- }
+ else
+ rt_mutex_adjust_prio_chain_unlock(lock);
+
+ /*
+ * We got it while lock->wait_lock was unlocked ?
+ */
+ if (unlikely(!waiter.task))
+ continue;

/*
* Prevent schedule() to drop BKL, while waiting for
@@ -737,8 +789,10 @@ rt_spin_lock_slowlock(struct rt_mutex *l
* highest-prio waiter (due to SCHED_OTHER changing prio), then we
* can end up with a non-NULL waiter.task:
*/
- if (unlikely(waiter.task))
+ if (unlikely(waiter.task)) {
remove_waiter(lock, &waiter);
+ adjust_prio_final = 1;
+ }
/*
* try_to_take_rt_mutex() sets the waiter bit
* unconditionally. We might have to fix that up:
@@ -747,6 +801,11 @@ rt_spin_lock_slowlock(struct rt_mutex *l

spin_unlock(&lock->wait_lock);

+ if (adjust_prio_final)
+ rt_mutex_adjust_prio_final(current, waiter.old_sched_lifo);
+
+ BUG_ON(current->boosting_prio != MAX_PRIO);
+ BUG_ON(waiter.old_sched_lifo != get_sched_lifo());
debug_rt_mutex_free_waiter(&waiter);
}

@@ -757,21 +816,16 @@ static void fastcall noinline __sched
rt_spin_lock_slowunlock(struct rt_mutex *lock)
{
spin_lock(&lock->wait_lock);
-
debug_rt_mutex_unlock(lock);
-
rt_mutex_deadlock_account_unlock(current);
-
if (!rt_mutex_has_waiters(lock)) {
lock->owner = NULL;
spin_unlock(&lock->wait_lock);
return;
}
-
wakeup_next_waiter(lock, 1);
-
spin_unlock(&lock->wait_lock);
-
+ BUG_ON(current->boosting_prio != MAX_PRIO);
/* Undo pi boosting.when necessary */
rt_mutex_adjust_prio(current);
}
@@ -918,6 +972,7 @@ rt_mutex_slowlock(struct rt_mutex *lock,
{
int ret = 0, saved_lock_depth = -1;
struct rt_mutex_waiter waiter;
+ int adjust_prio_final = 0;

debug_rt_mutex_init_waiter(&waiter);
waiter.task = NULL;
@@ -939,6 +994,7 @@ rt_mutex_slowlock(struct rt_mutex *lock,
saved_lock_depth = rt_release_bkl(lock);

set_current_state(state);
+ waiter.old_sched_lifo = get_sched_lifo();

/* Setup the timer, when timeout != NULL */
if (unlikely(timeout))
@@ -971,20 +1027,23 @@ rt_mutex_slowlock(struct rt_mutex *lock,
* when we have been woken up by the previous owner
* but the lock got stolen by a higher prio task.
*/
- if (!waiter.task) {
+ if (!waiter.task)
ret = task_blocks_on_rt_mutex(lock, &waiter,
detect_deadlock);
- /*
- * If we got woken up by the owner then start loop
- * all over without going into schedule to try
- * to get the lock now:
- */
- if (unlikely(!waiter.task))
- continue;
+ else
+ rt_mutex_adjust_prio_chain_unlock(lock);
+
+ /*
+ * If we got woken up by the owner then start loop
+ * all over without going into schedule to try
+ * to get the lock now:
+ */
+ if (unlikely(!waiter.task))
+ continue;
+
+ if (unlikely(ret))
+ break;

- if (unlikely(ret))
- break;
- }
saved_flags = current->flags & PF_NOSCHED;
current->flags &= ~PF_NOSCHED;

@@ -1003,8 +1062,12 @@ rt_mutex_slowlock(struct rt_mutex *lock,

set_current_state(TASK_RUNNING);

- if (unlikely(waiter.task))
+ if (unlikely(waiter.task)) {
remove_waiter(lock, &waiter);
+ adjust_prio_final = 1;
+ }
+ else if (unlikely(ret))
+ adjust_prio_final = 1;

/*
* try_to_take_rt_mutex() sets the waiter bit
@@ -1014,17 +1077,13 @@ rt_mutex_slowlock(struct rt_mutex *lock,

spin_unlock(&lock->wait_lock);

+ if (adjust_prio_final)
+ rt_mutex_adjust_prio_final(current, waiter.old_sched_lifo);
+
/* Remove pending timer: */
if (unlikely(timeout))
hrtimer_cancel(&timeout->timer);

- /*
- * Readjust priority, when we did not get the lock. We might
- * have been the pending owner and boosted. Since we did not
- * take the lock, the PI boost has to go.
- */
- if (unlikely(ret))
- rt_mutex_adjust_prio(current);

/* Must we reaquire the BKL? */
if (unlikely(saved_lock_depth >= 0))
@@ -1032,6 +1091,8 @@ rt_mutex_slowlock(struct rt_mutex *lock,

debug_rt_mutex_free_waiter(&waiter);

+ BUG_ON(current->boosting_prio != MAX_PRIO);
+
return ret;
}

@@ -1085,6 +1146,7 @@ rt_mutex_slowunlock(struct rt_mutex *loc
spin_unlock(&lock->wait_lock);

/* Undo pi boosting if necessary: */
+ BUG_ON(current->boosting_prio != MAX_PRIO);
rt_mutex_adjust_prio(current);
}

Index: linux-2.6.18-rt/kernel/rtmutex_common.h
===================================================================
--- linux-2.6.18-rt.orig/kernel/rtmutex_common.h
+++ linux-2.6.18-rt/kernel/rtmutex_common.h
@@ -49,6 +49,7 @@ struct rt_mutex_waiter {
struct plist_node pi_list_entry;
struct task_struct *task;
struct rt_mutex *lock;
+ int old_sched_lifo;
#ifdef CONFIG_DEBUG_RT_MUTEXES
unsigned long ip;
pid_t deadlock_task_pid;
Index: linux-2.6.18-rt/kernel/sched.c
===================================================================
--- linux-2.6.18-rt.orig/kernel/sched.c
+++ linux-2.6.18-rt/kernel/sched.c
@@ -4654,9 +4654,15 @@ void set_thread_priority(struct task_str
resched_task(rq->curr);
}
__task_rq_unlock(rq);
- spin_unlock_irqrestore(&p->pi_lock, flags);

- rt_mutex_adjust_pi(p);
+ /*
+ * If the process is blocked on rt-mutex, it will now wake up and
+ * reinsert itself into the wait list and boost the owner correctly
+ */
+ if (p->pi_blocked_on)
+ wake_up_process_mutex(p);
+
+ spin_unlock_irqrestore(&p->pi_lock, flags);
}

/**
@@ -4759,9 +4765,15 @@ recheck:
resched_task(rq->curr);
}
__task_rq_unlock(rq);
- spin_unlock_irqrestore(&p->pi_lock, flags);

- rt_mutex_adjust_pi(p);
+ /*
+ * If the process is blocked on rt-mutex, it will now wake up and
+ * reinsert itself into the wait list and boost the owner correctly
+ */
+ if (p->pi_blocked_on)
+ wake_up_process_mutex(p);
+
+ spin_unlock_irqrestore(&p->pi_lock, flags);

return 0;
}
@@ -7627,4 +7639,3 @@ void notrace preempt_enable_no_resched(v

EXPORT_SYMBOL(preempt_enable_no_resched);
#endif
-
Index: linux-2.6.18-rt/scripts/rt-tester/t5-l4-pi-boost-deboost-setsched.tst
===================================================================
--- linux-2.6.18-rt.orig/scripts/rt-tester/t5-l4-pi-boost-deboost-setsched.tst
+++ linux-2.6.18-rt/scripts/rt-tester/t5-l4-pi-boost-deboost-setsched.tst
@@ -107,10 +107,10 @@ T: prioeq: 3: 84
# Reduce prio of T4
C: schedfifo: 4: 80
T: prioeq: 0: 83
-T: prioeq: 1: 83
-T: prioeq: 2: 83
-T: prioeq: 3: 83
-T: prioeq: 4: 80
+T: prioeq: 1: 84
+T: prioeq: 2: 84
+T: prioeq: 3: 84
+T: prioeq: 4: 84

# Increase prio of T4
C: schedfifo: 4: 84
@@ -139,36 +139,46 @@ T: prioeq: 4: 84
# Reduce prio of T3
C: schedfifo: 3: 83
T: prioeq: 0: 84
-T: prioeq: 1: 84
-T: prioeq: 2: 84
-T: prioeq: 3: 84
+T: prioeq: 1: 85
+T: prioeq: 2: 85
+T: prioeq: 3: 85
T: prioeq: 4: 84

# Signal T4
C: signal: 4: 0
W: unlocked: 4: 3
T: prioeq: 0: 83
-T: prioeq: 1: 83
-T: prioeq: 2: 83
-T: prioeq: 3: 83
+T: prioeq: 1: 85
+T: prioeq: 2: 85
+T: prioeq: 3: 85
+T: prioeq: 4: 84

# Signal T3
C: signal: 3: 0
W: unlocked: 3: 2
T: prioeq: 0: 82
-T: prioeq: 1: 82
-T: prioeq: 2: 82
+T: prioeq: 1: 85
+T: prioeq: 2: 85
+T: prioeq: 3: 83
+T: prioeq: 4: 84

# Signal T2
C: signal: 2: 0
W: unlocked: 2: 1
T: prioeq: 0: 81
-T: prioeq: 1: 81
+T: prioeq: 1: 85
+T: prioeq: 2: 82
+T: prioeq: 3: 83
+T: prioeq: 4: 84

# Signal T1
C: signal: 1: 0
W: unlocked: 1: 0
T: priolt: 0: 1
+T: prioeq: 1: 81
+T: prioeq: 2: 82
+T: prioeq: 3: 83
+T: prioeq: 4: 84

# Unlock and exit
C: unlock: 3: 3
Index: linux-2.6.18-rt/scripts/rt-tester/t5-l4-pi-boost-deboost.tst
===================================================================
--- linux-2.6.18-rt.orig/scripts/rt-tester/t5-l4-pi-boost-deboost.tst
+++ linux-2.6.18-rt/scripts/rt-tester/t5-l4-pi-boost-deboost.tst
@@ -108,22 +108,22 @@ T: prioeq: 3: 84
C: signal: 4: 0
W: unlocked: 4: 3
T: prioeq: 0: 83
-T: prioeq: 1: 83
-T: prioeq: 2: 83
-T: prioeq: 3: 83
+T: prioeq: 1: 84
+T: prioeq: 2: 84
+T: prioeq: 3: 84

# Signal T3
C: signal: 3: 0
W: unlocked: 3: 2
T: prioeq: 0: 82
-T: prioeq: 1: 82
-T: prioeq: 2: 82
+T: prioeq: 1: 84
+T: prioeq: 2: 84

# Signal T2
C: signal: 2: 0
W: unlocked: 2: 1
T: prioeq: 0: 81
-T: prioeq: 1: 81
+T: prioeq: 1: 84

# Signal T1
C: signal: 1: 0

--
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/