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Subject[TIP][RFC 5/7] rt_mutex: add proxy lock routines
From: Darren Hart <dvhltc@us.ibm.com>

This patch is required for the first half of requeue_pi to function. It
basically splits rt_mutex_slowlock() right down the middle, just before the
first call to schedule().

This patch uses a new futex_q field, rt_waiter, for now. I think
I should be able to use task->pi_blocked_on in a future versino of this patch.

NOTE: I believe this patch creates a race condition that the final patch hits
when trying to do requeue_pi with nr_wake=1 and nr_requeue=0. See
that patch header (6/7) for a complete discription.

V5: -remove EXPORT_SYMBOL_GPL from the new routines
-minor cleanups
V4: -made detect_deadlock a parameter to rt_mutex_enqueue_task
-refactored rt_mutex_slowlock to share code with new functions
-renamed rt_mutex_enqueue_task and rt_mutex_handle_wakeup to
rt_mutex_start_proxy_lock and rt_mutex_finish_proxy_lock, respectively

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
---
kernel/rtmutex.c | 192 +++++++++++++++++++++++++++++++++++++----------
kernel/rtmutex_common.h | 8 ++
2 files changed, 160 insertions(+), 40 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 69d9cb9..a96842e 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -411,6 +411,7 @@ static int try_to_take_rt_mutex(struct rt_mutex *lock)
*/
static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
struct rt_mutex_waiter *waiter,
+ struct task_struct *task,
int detect_deadlock)
{
struct task_struct *owner = rt_mutex_owner(lock);
@@ -418,21 +419,21 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
unsigned long flags;
int chain_walk = 0, res;

- spin_lock_irqsave(&current->pi_lock, flags);
- __rt_mutex_adjust_prio(current);
- waiter->task = current;
+ spin_lock_irqsave(&task->pi_lock, flags);
+ __rt_mutex_adjust_prio(task);
+ waiter->task = task;
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, task->prio);
+ plist_node_init(&waiter->pi_list_entry, task->prio);

/* Get the top priority waiter on the lock */
if (rt_mutex_has_waiters(lock))
top_waiter = rt_mutex_top_waiter(lock);
plist_add(&waiter->list_entry, &lock->wait_list);

- current->pi_blocked_on = waiter;
+ task->pi_blocked_on = waiter;

- spin_unlock_irqrestore(&current->pi_lock, flags);
+ spin_unlock_irqrestore(&task->pi_lock, flags);

if (waiter == rt_mutex_top_waiter(lock)) {
spin_lock_irqsave(&owner->pi_lock, flags);
@@ -460,7 +461,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
spin_unlock(&lock->wait_lock);

res = rt_mutex_adjust_prio_chain(owner, detect_deadlock, lock, waiter,
- current);
+ task);

spin_lock(&lock->wait_lock);

@@ -605,37 +606,25 @@ void rt_mutex_adjust_pi(struct task_struct *task)
rt_mutex_adjust_prio_chain(task, 0, NULL, NULL, task);
}

-/*
- * Slow path lock function:
+/**
+ * __rt_mutex_slowlock - perform the wait-wake-try-to-take loop
+ * @lock the rt_mutex to take
+ * @state: the state the task should block in (TASK_INTERRUPTIBLE
+ * or TASK_UNINTERRUPTIBLE)
+ * @timeout: the pre-initialized and started timer, or NULL for none
+ * @waiter: the pre-initialized rt_mutex_waiter
+ * @detect_deadlock: passed to task_blocks_on_rt_mutex
+ *
+ * lock->wait_lock must be held by the caller.
*/
static int __sched
-rt_mutex_slowlock(struct rt_mutex *lock, int state,
- struct hrtimer_sleeper *timeout,
- int detect_deadlock)
+__rt_mutex_slowlock(struct rt_mutex *lock, int state,
+ struct hrtimer_sleeper *timeout,
+ struct rt_mutex_waiter *waiter,
+ int detect_deadlock)
{
- struct rt_mutex_waiter waiter;
int ret = 0;

- debug_rt_mutex_init_waiter(&waiter);
- waiter.task = NULL;
-
- spin_lock(&lock->wait_lock);
-
- /* Try to acquire the lock again: */
- if (try_to_take_rt_mutex(lock)) {
- spin_unlock(&lock->wait_lock);
- return 0;
- }
-
- set_current_state(state);
-
- /* Setup the timer, when timeout != NULL */
- if (unlikely(timeout)) {
- hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
- if (!hrtimer_active(&timeout->timer))
- timeout->task = NULL;
- }
-
for (;;) {
/* Try to acquire the lock: */
if (try_to_take_rt_mutex(lock))
@@ -656,19 +645,19 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
}

/*
- * waiter.task is NULL the first time we come here and
+ * waiter->task is NULL the first time we come here and
* when we have been woken up by the previous owner
* but the lock got stolen by a higher prio task.
*/
- if (!waiter.task) {
- ret = task_blocks_on_rt_mutex(lock, &waiter,
+ if (!waiter->task) {
+ ret = task_blocks_on_rt_mutex(lock, waiter, current,
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)) {
+ if (unlikely(!waiter->task)) {
/*
* Reset the return value. We might
* have returned with -EDEADLK and the
@@ -684,15 +673,52 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,

spin_unlock(&lock->wait_lock);

- debug_rt_mutex_print_deadlock(&waiter);
+ debug_rt_mutex_print_deadlock(waiter);

- if (waiter.task)
+ if (waiter->task)
schedule_rt_mutex(lock);

spin_lock(&lock->wait_lock);
set_current_state(state);
}

+ return ret;
+}
+
+/*
+ * Slow path lock function:
+ */
+static int __sched
+rt_mutex_slowlock(struct rt_mutex *lock, int state,
+ struct hrtimer_sleeper *timeout,
+ int detect_deadlock)
+{
+ struct rt_mutex_waiter waiter;
+ int ret = 0;
+
+ debug_rt_mutex_init_waiter(&waiter);
+ waiter.task = NULL;
+
+ spin_lock(&lock->wait_lock);
+
+ /* Try to acquire the lock again: */
+ if (try_to_take_rt_mutex(lock)) {
+ spin_unlock(&lock->wait_lock);
+ return 0;
+ }
+
+ set_current_state(state);
+
+ /* Setup the timer, when timeout != NULL */
+ if (unlikely(timeout)) {
+ hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
+ if (!hrtimer_active(&timeout->timer))
+ timeout->task = NULL;
+ }
+
+ ret = __rt_mutex_slowlock(lock, state, timeout, &waiter,
+ detect_deadlock);
+
set_current_state(TASK_RUNNING);

if (unlikely(waiter.task))
@@ -986,6 +1012,41 @@ void rt_mutex_proxy_unlock(struct rt_mutex *lock,
}

/**
+ * rt_mutex_start_proxy_lock - prepare another task to take the lock
+ *
+ * @lock: the rt_mutex to take
+ * @waiter: the rt_mutex_waiter initialized by the waiter
+ * @task: the task to prepare
+ * @detext_deadlock: passed to task_blocks_on_rt_mutex
+ *
+ * The lock should have an owner, and it should not be task.
+ * Special API call for FUTEX_REQUEUE_PI support.
+ */
+int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
+ struct rt_mutex_waiter *waiter,
+ struct task_struct *task, int detect_deadlock)
+{
+ int ret;
+
+ spin_lock(&lock->wait_lock);
+ ret = task_blocks_on_rt_mutex(lock, waiter, task, detect_deadlock);
+ if (ret && !waiter->task) {
+ /*
+ * Reset the return value. We might have
+ * returned with -EDEADLK and the owner
+ * released the lock while we were walking the
+ * pi chain. Let the waiter sort it out.
+ */
+ ret = 0;
+ }
+ spin_unlock(&lock->wait_lock);
+
+ debug_rt_mutex_print_deadlock(waiter);
+
+ return ret;
+}
+
+/**
* rt_mutex_next_owner - return the next owner of the lock
*
* @lock: the rt lock query
@@ -1004,3 +1065,54 @@ struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)

return rt_mutex_top_waiter(lock)->task;
}
+
+/**
+ * rt_mutex_finish_proxy_lock - Complete the taking of the lock initialized on
+ * our behalf by another thread.
+ * @lock: the rt_mutex we were woken on
+ * @to: the timeout, null if none. hrtimer should already have been started.
+ * @waiter: the pre-initialized rt_mutex_waiter
+ * @detect_deadlock: for use by __rt_mutex_slowlock
+ *
+ * Special API call for PI-futex requeue support
+ */
+int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
+ struct hrtimer_sleeper *to,
+ struct rt_mutex_waiter *waiter,
+ int detect_deadlock)
+{
+ int ret;
+
+ if (waiter->task)
+ schedule_rt_mutex(lock);
+
+ spin_lock(&lock->wait_lock);
+
+ set_current_state(TASK_INTERRUPTIBLE);
+
+ ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter,
+ detect_deadlock);
+
+ set_current_state(TASK_RUNNING);
+
+ if (unlikely(waiter->task))
+ remove_waiter(lock, waiter);
+
+ /*
+ * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
+ * have to fix that up.
+ */
+ fixup_rt_mutex_waiters(lock);
+
+ spin_unlock(&lock->wait_lock);
+
+ /*
+ * 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);
+
+ return ret;
+}
diff --git a/kernel/rtmutex_common.h b/kernel/rtmutex_common.h
index e124bf5..97a2f81 100644
--- a/kernel/rtmutex_common.h
+++ b/kernel/rtmutex_common.h
@@ -120,6 +120,14 @@ extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
struct task_struct *proxy_owner);
extern void rt_mutex_proxy_unlock(struct rt_mutex *lock,
struct task_struct *proxy_owner);
+extern int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
+ struct rt_mutex_waiter *waiter,
+ struct task_struct *task,
+ int detect_deadlock);
+extern int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
+ struct hrtimer_sleeper *to,
+ struct rt_mutex_waiter *waiter,
+ int detect_deadlock);

#ifdef CONFIG_DEBUG_RT_MUTEXES
# include "rtmutex-debug.h"
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team


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