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authorLinus Torvalds <torvalds@linux-foundation.org>2014-06-21 07:06:02 -1000
committerLinus Torvalds <torvalds@linux-foundation.org>2014-06-21 07:06:02 -1000
commit7b08d618a232aa3bfc538cf1eccd9ce0c239bf03 (patch)
treec440cd867dc7454202115f77f6a46375b770b88a
parent7a8e9c80889732891df50d44328f330316834eb1 (diff)
parent27e35715df54cbc4f2d044f681802ae30479e7fb (diff)
downloadlinux-stericsson-7b08d618a232aa3bfc538cf1eccd9ce0c239bf03.tar.gz
Merge branch 'locking-urgent-for-linus.patch' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull rtmutex fixes from Thomas Gleixner: "Another three patches to make the rtmutex code more robust. That's the last urgent fallout from the big futex/rtmutex investigation" * 'locking-urgent-for-linus.patch' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: rtmutex: Plug slow unlock race rtmutex: Detect changes in the pi lock chain rtmutex: Handle deadlock detection smarter
-rw-r--r--kernel/locking/rtmutex-debug.h5
-rw-r--r--kernel/locking/rtmutex.c243
-rw-r--r--kernel/locking/rtmutex.h5
3 files changed, 218 insertions, 35 deletions
diff --git a/kernel/locking/rtmutex-debug.h b/kernel/locking/rtmutex-debug.h
index 14193d596d78..ab29b6a22669 100644
--- a/kernel/locking/rtmutex-debug.h
+++ b/kernel/locking/rtmutex-debug.h
@@ -31,3 +31,8 @@ static inline int debug_rt_mutex_detect_deadlock(struct rt_mutex_waiter *waiter,
{
return (waiter != NULL);
}
+
+static inline void rt_mutex_print_deadlock(struct rt_mutex_waiter *w)
+{
+ debug_rt_mutex_print_deadlock(w);
+}
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index a620d4d08ca6..fc605941b9b8 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -83,6 +83,47 @@ static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
owner = *p;
} while (cmpxchg(p, owner, owner | RT_MUTEX_HAS_WAITERS) != owner);
}
+
+/*
+ * Safe fastpath aware unlock:
+ * 1) Clear the waiters bit
+ * 2) Drop lock->wait_lock
+ * 3) Try to unlock the lock with cmpxchg
+ */
+static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
+ __releases(lock->wait_lock)
+{
+ struct task_struct *owner = rt_mutex_owner(lock);
+
+ clear_rt_mutex_waiters(lock);
+ raw_spin_unlock(&lock->wait_lock);
+ /*
+ * If a new waiter comes in between the unlock and the cmpxchg
+ * we have two situations:
+ *
+ * unlock(wait_lock);
+ * lock(wait_lock);
+ * cmpxchg(p, owner, 0) == owner
+ * mark_rt_mutex_waiters(lock);
+ * acquire(lock);
+ * or:
+ *
+ * unlock(wait_lock);
+ * lock(wait_lock);
+ * mark_rt_mutex_waiters(lock);
+ *
+ * cmpxchg(p, owner, 0) != owner
+ * enqueue_waiter();
+ * unlock(wait_lock);
+ * lock(wait_lock);
+ * wake waiter();
+ * unlock(wait_lock);
+ * lock(wait_lock);
+ * acquire(lock);
+ */
+ return rt_mutex_cmpxchg(lock, owner, NULL);
+}
+
#else
# define rt_mutex_cmpxchg(l,c,n) (0)
static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
@@ -90,6 +131,17 @@ static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
lock->owner = (struct task_struct *)
((unsigned long)lock->owner | RT_MUTEX_HAS_WAITERS);
}
+
+/*
+ * Simple slow path only version: lock->owner is protected by lock->wait_lock.
+ */
+static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
+ __releases(lock->wait_lock)
+{
+ lock->owner = NULL;
+ raw_spin_unlock(&lock->wait_lock);
+ return true;
+}
#endif
static inline int
@@ -260,27 +312,36 @@ static void rt_mutex_adjust_prio(struct task_struct *task)
*/
int max_lock_depth = 1024;
+static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
+{
+ return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
+}
+
/*
* Adjust the priority chain. Also used for deadlock detection.
* Decreases task's usage by one - may thus free the task.
*
- * @task: the task owning the mutex (owner) for which a chain walk is probably
- * needed
+ * @task: the task owning the mutex (owner) for which a chain walk is
+ * probably needed
* @deadlock_detect: do we have to carry out deadlock detection?
- * @orig_lock: the mutex (can be NULL if we are walking the chain to recheck
- * things for a task that has just got its priority adjusted, and
- * is waiting on a mutex)
+ * @orig_lock: the mutex (can be NULL if we are walking the chain to recheck
+ * things for a task that has just got its priority adjusted, and
+ * is waiting on a mutex)
+ * @next_lock: the mutex on which the owner of @orig_lock was blocked before
+ * we dropped its pi_lock. Is never dereferenced, only used for
+ * comparison to detect lock chain changes.
* @orig_waiter: rt_mutex_waiter struct for the task that has just donated
- * its priority to the mutex owner (can be NULL in the case
- * depicted above or if the top waiter is gone away and we are
- * actually deboosting the owner)
- * @top_task: the current top waiter
+ * its priority to the mutex owner (can be NULL in the case
+ * depicted above or if the top waiter is gone away and we are
+ * actually deboosting the owner)
+ * @top_task: the current top waiter
*
* Returns 0 or -EDEADLK.
*/
static int rt_mutex_adjust_prio_chain(struct task_struct *task,
int deadlock_detect,
struct rt_mutex *orig_lock,
+ struct rt_mutex *next_lock,
struct rt_mutex_waiter *orig_waiter,
struct task_struct *top_task)
{
@@ -314,7 +375,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
}
put_task_struct(task);
- return deadlock_detect ? -EDEADLK : 0;
+ return -EDEADLK;
}
retry:
/*
@@ -339,6 +400,18 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
goto out_unlock_pi;
/*
+ * We dropped all locks after taking a refcount on @task, so
+ * the task might have moved on in the lock chain or even left
+ * the chain completely and blocks now on an unrelated lock or
+ * on @orig_lock.
+ *
+ * We stored the lock on which @task was blocked in @next_lock,
+ * so we can detect the chain change.
+ */
+ if (next_lock != waiter->lock)
+ goto out_unlock_pi;
+
+ /*
* Drop out, when the task has no waiters. Note,
* top_waiter can be NULL, when we are in the deboosting
* mode!
@@ -377,7 +450,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
debug_rt_mutex_deadlock(deadlock_detect, orig_waiter, lock);
raw_spin_unlock(&lock->wait_lock);
- ret = deadlock_detect ? -EDEADLK : 0;
+ ret = -EDEADLK;
goto out_unlock_pi;
}
@@ -422,11 +495,26 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
__rt_mutex_adjust_prio(task);
}
+ /*
+ * Check whether the task which owns the current lock is pi
+ * blocked itself. If yes we store a pointer to the lock for
+ * the lock chain change detection above. After we dropped
+ * task->pi_lock next_lock cannot be dereferenced anymore.
+ */
+ next_lock = task_blocked_on_lock(task);
+
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
top_waiter = rt_mutex_top_waiter(lock);
raw_spin_unlock(&lock->wait_lock);
+ /*
+ * We reached the end of the lock chain. Stop right here. No
+ * point to go back just to figure that out.
+ */
+ if (!next_lock)
+ goto out_put_task;
+
if (!detect_deadlock && waiter != top_waiter)
goto out_put_task;
@@ -536,8 +624,9 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
{
struct task_struct *owner = rt_mutex_owner(lock);
struct rt_mutex_waiter *top_waiter = waiter;
- unsigned long flags;
+ struct rt_mutex *next_lock;
int chain_walk = 0, res;
+ unsigned long flags;
/*
* Early deadlock detection. We really don't want the task to
@@ -548,7 +637,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
* which is wrong, as the other waiter is not in a deadlock
* situation.
*/
- if (detect_deadlock && owner == task)
+ if (owner == task)
return -EDEADLK;
raw_spin_lock_irqsave(&task->pi_lock, flags);
@@ -569,20 +658,28 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
if (!owner)
return 0;
+ raw_spin_lock_irqsave(&owner->pi_lock, flags);
if (waiter == rt_mutex_top_waiter(lock)) {
- raw_spin_lock_irqsave(&owner->pi_lock, flags);
rt_mutex_dequeue_pi(owner, top_waiter);
rt_mutex_enqueue_pi(owner, waiter);
__rt_mutex_adjust_prio(owner);
if (owner->pi_blocked_on)
chain_walk = 1;
- raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
- }
- else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock))
+ } else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock)) {
chain_walk = 1;
+ }
- if (!chain_walk)
+ /* Store the lock on which owner is blocked or NULL */
+ next_lock = task_blocked_on_lock(owner);
+
+ raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
+ /*
+ * Even if full deadlock detection is on, if the owner is not
+ * blocked itself, we can avoid finding this out in the chain
+ * walk.
+ */
+ if (!chain_walk || !next_lock)
return 0;
/*
@@ -594,8 +691,8 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
raw_spin_unlock(&lock->wait_lock);
- res = rt_mutex_adjust_prio_chain(owner, detect_deadlock, lock, waiter,
- task);
+ res = rt_mutex_adjust_prio_chain(owner, detect_deadlock, lock,
+ next_lock, waiter, task);
raw_spin_lock(&lock->wait_lock);
@@ -605,7 +702,8 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
/*
* Wake up the next waiter on the lock.
*
- * Remove the top waiter from the current tasks waiter list and wake it up.
+ * Remove the top waiter from the current tasks pi waiter list and
+ * wake it up.
*
* Called with lock->wait_lock held.
*/
@@ -626,10 +724,23 @@ static void wakeup_next_waiter(struct rt_mutex *lock)
*/
rt_mutex_dequeue_pi(current, waiter);
- rt_mutex_set_owner(lock, NULL);
+ /*
+ * As we are waking up the top waiter, and the waiter stays
+ * queued on the lock until it gets the lock, this lock
+ * obviously has waiters. Just set the bit here and this has
+ * the added benefit of forcing all new tasks into the
+ * slow path making sure no task of lower priority than
+ * the top waiter can steal this lock.
+ */
+ lock->owner = (void *) RT_MUTEX_HAS_WAITERS;
raw_spin_unlock_irqrestore(&current->pi_lock, flags);
+ /*
+ * It's safe to dereference waiter as it cannot go away as
+ * long as we hold lock->wait_lock. The waiter task needs to
+ * acquire it in order to dequeue the waiter.
+ */
wake_up_process(waiter->task);
}
@@ -644,8 +755,8 @@ static void remove_waiter(struct rt_mutex *lock,
{
int first = (waiter == rt_mutex_top_waiter(lock));
struct task_struct *owner = rt_mutex_owner(lock);
+ struct rt_mutex *next_lock = NULL;
unsigned long flags;
- int chain_walk = 0;
raw_spin_lock_irqsave(&current->pi_lock, flags);
rt_mutex_dequeue(lock, waiter);
@@ -669,13 +780,13 @@ static void remove_waiter(struct rt_mutex *lock,
}
__rt_mutex_adjust_prio(owner);
- if (owner->pi_blocked_on)
- chain_walk = 1;
+ /* Store the lock on which owner is blocked or NULL */
+ next_lock = task_blocked_on_lock(owner);
raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
}
- if (!chain_walk)
+ if (!next_lock)
return;
/* gets dropped in rt_mutex_adjust_prio_chain()! */
@@ -683,7 +794,7 @@ static void remove_waiter(struct rt_mutex *lock,
raw_spin_unlock(&lock->wait_lock);
- rt_mutex_adjust_prio_chain(owner, 0, lock, NULL, current);
+ rt_mutex_adjust_prio_chain(owner, 0, lock, next_lock, NULL, current);
raw_spin_lock(&lock->wait_lock);
}
@@ -696,6 +807,7 @@ static void remove_waiter(struct rt_mutex *lock,
void rt_mutex_adjust_pi(struct task_struct *task)
{
struct rt_mutex_waiter *waiter;
+ struct rt_mutex *next_lock;
unsigned long flags;
raw_spin_lock_irqsave(&task->pi_lock, flags);
@@ -706,12 +818,13 @@ void rt_mutex_adjust_pi(struct task_struct *task)
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
return;
}
-
+ next_lock = waiter->lock;
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
/* gets dropped in rt_mutex_adjust_prio_chain()! */
get_task_struct(task);
- rt_mutex_adjust_prio_chain(task, 0, NULL, NULL, task);
+
+ rt_mutex_adjust_prio_chain(task, 0, NULL, next_lock, NULL, task);
}
/**
@@ -763,6 +876,26 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
return ret;
}
+static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
+ struct rt_mutex_waiter *w)
+{
+ /*
+ * If the result is not -EDEADLOCK or the caller requested
+ * deadlock detection, nothing to do here.
+ */
+ if (res != -EDEADLOCK || detect_deadlock)
+ return;
+
+ /*
+ * Yell lowdly and stop the task right here.
+ */
+ rt_mutex_print_deadlock(w);
+ while (1) {
+ set_current_state(TASK_INTERRUPTIBLE);
+ schedule();
+ }
+}
+
/*
* Slow path lock function:
*/
@@ -802,8 +935,10 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
set_current_state(TASK_RUNNING);
- if (unlikely(ret))
+ if (unlikely(ret)) {
remove_waiter(lock, &waiter);
+ rt_mutex_handle_deadlock(ret, detect_deadlock, &waiter);
+ }
/*
* try_to_take_rt_mutex() sets the waiter bit
@@ -859,12 +994,49 @@ rt_mutex_slowunlock(struct rt_mutex *lock)
rt_mutex_deadlock_account_unlock(current);
- if (!rt_mutex_has_waiters(lock)) {
- lock->owner = NULL;
- raw_spin_unlock(&lock->wait_lock);
- return;
+ /*
+ * We must be careful here if the fast path is enabled. If we
+ * have no waiters queued we cannot set owner to NULL here
+ * because of:
+ *
+ * foo->lock->owner = NULL;
+ * rtmutex_lock(foo->lock); <- fast path
+ * free = atomic_dec_and_test(foo->refcnt);
+ * rtmutex_unlock(foo->lock); <- fast path
+ * if (free)
+ * kfree(foo);
+ * raw_spin_unlock(foo->lock->wait_lock);
+ *
+ * So for the fastpath enabled kernel:
+ *
+ * Nothing can set the waiters bit as long as we hold
+ * lock->wait_lock. So we do the following sequence:
+ *
+ * owner = rt_mutex_owner(lock);
+ * clear_rt_mutex_waiters(lock);
+ * raw_spin_unlock(&lock->wait_lock);
+ * if (cmpxchg(&lock->owner, owner, 0) == owner)
+ * return;
+ * goto retry;
+ *
+ * The fastpath disabled variant is simple as all access to
+ * lock->owner is serialized by lock->wait_lock:
+ *
+ * lock->owner = NULL;
+ * raw_spin_unlock(&lock->wait_lock);
+ */
+ while (!rt_mutex_has_waiters(lock)) {
+ /* Drops lock->wait_lock ! */
+ if (unlock_rt_mutex_safe(lock) == true)
+ return;
+ /* Relock the rtmutex and try again */
+ raw_spin_lock(&lock->wait_lock);
}
+ /*
+ * The wakeup next waiter path does not suffer from the above
+ * race. See the comments there.
+ */
wakeup_next_waiter(lock);
raw_spin_unlock(&lock->wait_lock);
@@ -1112,7 +1284,8 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
return 1;
}
- ret = task_blocks_on_rt_mutex(lock, waiter, task, detect_deadlock);
+ /* We enforce deadlock detection for futexes */
+ ret = task_blocks_on_rt_mutex(lock, waiter, task, 1);
if (ret && !rt_mutex_owner(lock)) {
/*
diff --git a/kernel/locking/rtmutex.h b/kernel/locking/rtmutex.h
index a1a1dd06421d..f6a1f3c133b1 100644
--- a/kernel/locking/rtmutex.h
+++ b/kernel/locking/rtmutex.h
@@ -24,3 +24,8 @@
#define debug_rt_mutex_print_deadlock(w) do { } while (0)
#define debug_rt_mutex_detect_deadlock(w,d) (d)
#define debug_rt_mutex_reset_waiter(w) do { } while (0)
+
+static inline void rt_mutex_print_deadlock(struct rt_mutex_waiter *w)
+{
+ WARN(1, "rtmutex deadlock detected\n");
+}