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Subject[RFC][PATCH 3/3] locking/mutex: Add lock handoff to avoid starvation
Now that we have an atomic owner field, we can do explicit lock
handoff. Use this to avoid starvation.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/locking/mutex.c | 44 ++++++++++++++++++++++++++++++++++++++++----
1 file changed, 40 insertions(+), 4 deletions(-)

--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -53,6 +53,7 @@ __mutex_init(struct mutex *lock, const c
EXPORT_SYMBOL(__mutex_init);

#define MUTEX_FLAG_WAITERS 0x01
+#define MUTEX_FLAG_HANDOFF 0x02

#define MUTEX_FLAG_ALL 0x03

@@ -84,6 +85,29 @@ static inline void __mutex_clear_flag(st
atomic_long_andnot(flag, &lock->owner);
}

+static inline bool __mutex_waiter_is_first(struct mutex *lock, struct mutex_waiter *waiter)
+{
+ return list_first_entry(&lock->wait_list, struct mutex_waiter, list) == waiter;
+}
+
+static void __mutex_handoff(struct mutex *lock, struct task_struct *task)
+{
+ unsigned long owner = atomic_long_read(&lock->owner);
+
+ for (;;) {
+ unsigned long old, new;
+
+ new = (owner & MUTEX_FLAG_WAITERS);
+ new |= (unsigned long)task;
+
+ old = atomic_long_cmpxchg(&lock->owner, owner, new);
+ if (old == owner)
+ break;
+
+ owner = old;
+ }
+}
+
#ifndef CONFIG_DEBUG_LOCK_ALLOC
/*
* We split the mutex lock/unlock logic into separate fastpath and
@@ -414,7 +438,7 @@ static bool mutex_optimistic_spin(struct
}
#endif

-static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock);
+static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long owner);

/**
* mutex_unlock - release the mutex
@@ -439,6 +463,9 @@ void __sched mutex_unlock(struct mutex *
for (;;) {
unsigned long old;

+ if (owner & MUTEX_FLAG_HANDOFF)
+ break;
+
old = atomic_long_cmpxchg_release(&lock->owner, owner, owner & 0x03);
if (old == owner)
break;
@@ -447,7 +474,7 @@ void __sched mutex_unlock(struct mutex *
}

if (owner & 0x03);
- __mutex_unlock_slowpath(lock);
+ __mutex_unlock_slowpath(lock, owner);
}
EXPORT_SYMBOL(mutex_unlock);

@@ -545,7 +572,7 @@ __mutex_lock_common(struct mutex *lock,
list_add_tail(&waiter.list, &lock->wait_list);
waiter.task = task;

- if (list_first_entry(&lock->wait_list, struct mutex_waiter, list) == &waiter)
+ if (__mutex_waiter_is_first(lock, &waiter))
__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);

lock_contended(&lock->dep_map, ip);
@@ -573,8 +600,14 @@ __mutex_lock_common(struct mutex *lock,
schedule_preempt_disabled();
spin_lock_mutex(&lock->wait_lock, flags);

+ if (__mutex_owner(lock) == current)
+ break;
+
if (__mutex_trylock(lock))
break;
+
+ if (__mutex_waiter_is_first(lock, &waiter))
+ __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
}
__set_task_state(task, TASK_RUNNING);

@@ -707,7 +740,7 @@ EXPORT_SYMBOL_GPL(__ww_mutex_lock_interr
/*
* Release the lock, slowpath:
*/
-static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock)
+static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long owner)
{
unsigned long flags;
WAKE_Q(wake_q);
@@ -722,6 +755,9 @@ static noinline void __sched __mutex_unl
list_entry(lock->wait_list.next,
struct mutex_waiter, list);

+ if (owner & MUTEX_FLAG_HANDOFF)
+ __mutex_handoff(lock, waiter->task);
+
debug_mutex_wake_waiter(lock, waiter);
wake_q_add(&wake_q, waiter->task);
}

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