locking/mutexes: Introduce cancelable MCS lock for adaptive spinning (fb0527bd) · Commits · e / devices / android_kernel_fairphone_FP3

include/linux/mutex.h

+2 −2

Original line number	Original line	Diff line number	Diff line
	@@ -46,7 +46,7 @@
	* - detects multi-task circular deadlocks and prints out all affected		* - detects multi-task circular deadlocks and prints out all affected
	* locks and tasks (and only those tasks)		* locks and tasks (and only those tasks)
	*/		*/
	struct mcs_spinlock;		struct optimistic_spin_queue;
	struct mutex {		struct mutex {
	/* 1: unlocked, 0: locked, negative: locked, possible waiters */		/* 1: unlocked, 0: locked, negative: locked, possible waiters */
	atomic_t count;		atomic_t count;
	@@ -56,7 +56,7 @@ struct mutex {
	struct task_struct *owner;		struct task_struct *owner;
	#endif		#endif
	#ifdef CONFIG_MUTEX_SPIN_ON_OWNER		#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
	struct mcs_spinlock mcs_lock; / Spinner MCS lock */		struct optimistic_spin_queue osq; / Spinner MCS lock */
	#endif		#endif
	#ifdef CONFIG_DEBUG_MUTEXES		#ifdef CONFIG_DEBUG_MUTEXES
	const char *name;		const char *name;

kernel/locking/Makefile

+1 −1

Original line number	Original line	Diff line number	Diff line

	obj-y += mutex.o semaphore.o rwsem.o lglock.o		obj-y += mutex.o semaphore.o rwsem.o lglock.o mcs_spinlock.o

	ifdef CONFIG_FUNCTION_TRACER		ifdef CONFIG_FUNCTION_TRACER
	CFLAGS_REMOVE_lockdep.o = -pg		CFLAGS_REMOVE_lockdep.o = -pg

kernel/locking/mcs_spinlock.c

0 → 100644

+178 −0

Original line number	Original line	Diff line number	Diff line

			#include <linux/percpu.h>
			#include <linux/mutex.h>
			#include <linux/sched.h>
			#include "mcs_spinlock.h"

			#ifdef CONFIG_SMP

			/*
			* An MCS like lock especially tailored for optimistic spinning for sleeping
			* lock implementations (mutex, rwsem, etc).
			*
			* Using a single mcs node per CPU is safe because sleeping locks should not be
			* called from interrupt context and we have preemption disabled while
			* spinning.
			*/
			static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_queue, osq_node);

			/*
			* Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
			* Can return NULL in case we were the last queued and we updated @lock instead.
			*/
			static inline struct optimistic_spin_queue *
			osq_wait_next(struct optimistic_spin_queue **lock,
			struct optimistic_spin_queue *node,
			struct optimistic_spin_queue *prev)
			{
			struct optimistic_spin_queue *next = NULL;

			for (;;) {
			if (*lock == node && cmpxchg(lock, node, prev) == node) {
			/*
			* We were the last queued, we moved @lock back. @prev
			* will now observe @lock and will complete its
			* unlock()/unqueue().
			*/
			break;
			}

			/*
			* We must xchg() the @node->next value, because if we were to
			* leave it in, a concurrent unlock()/unqueue() from
			* @node->next might complete Step-A and think its @prev is
			* still valid.
			*
			* If the concurrent unlock()/unqueue() wins the race, we'll
			* wait for either @lock to point to us, through its Step-B, or
			* wait for a new @node->next from its Step-C.
			*/
			if (node->next) {
			next = xchg(&node->next, NULL);
			if (next)
			break;
			}

			arch_mutex_cpu_relax();
			}

			return next;
			}

			bool osq_lock(struct optimistic_spin_queue **lock)
			{
			struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
			struct optimistic_spin_queue prev, next;

			node->locked = 0;
			node->next = NULL;

			node->prev = prev = xchg(lock, node);
			if (likely(prev == NULL))
			return true;

			ACCESS_ONCE(prev->next) = node;

			/*
			* Normally @prev is untouchable after the above store; because at that
			* moment unlock can proceed and wipe the node element from stack.
			*
			* However, since our nodes are static per-cpu storage, we're
			* guaranteed their existence -- this allows us to apply
			* cmpxchg in an attempt to undo our queueing.
			*/

			while (!smp_load_acquire(&node->locked)) {
			/*
			* If we need to reschedule bail... so we can block.
			*/
			if (need_resched())
			goto unqueue;

			arch_mutex_cpu_relax();
			}
			return true;

			unqueue:
			/*
			* Step - A -- stabilize @prev
			*
			* Undo our @prev->next assignment; this will make @prev's
			* unlock()/unqueue() wait for a next pointer since @lock points to us
			* (or later).
			*/

			for (;;) {
			if (prev->next == node &&
			cmpxchg(&prev->next, node, NULL) == node)
			break;

			/*
			* We can only fail the cmpxchg() racing against an unlock(),
			* in which case we should observe @node->locked becomming
			* true.
			*/
			if (smp_load_acquire(&node->locked))
			return true;

			arch_mutex_cpu_relax();

			/*
			* Or we race against a concurrent unqueue()'s step-B, in which
			* case its step-C will write us a new @node->prev pointer.
			*/
			prev = ACCESS_ONCE(node->prev);
			}

			/*
			* Step - B -- stabilize @next
			*
			* Similar to unlock(), wait for @node->next or move @lock from @node
			* back to @prev.
			*/

			next = osq_wait_next(lock, node, prev);
			if (!next)
			return false;

			/*
			* Step - C -- unlink
			*
			* @prev is stable because its still waiting for a new @prev->next
			* pointer, @next is stable because our @node->next pointer is NULL and
			* it will wait in Step-A.
			*/

			ACCESS_ONCE(next->prev) = prev;
			ACCESS_ONCE(prev->next) = next;

			return false;
			}

			void osq_unlock(struct optimistic_spin_queue **lock)
			{
			struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
			struct optimistic_spin_queue *next;

			/*
			* Fast path for the uncontended case.
			*/
			if (likely(cmpxchg(lock, node, NULL) == node))
			return;

			/*
			* Second most likely case.
			*/
			next = xchg(&node->next, NULL);
			if (next) {
			ACCESS_ONCE(next->locked) = 1;
			return;
			}

			next = osq_wait_next(lock, node, NULL);
			if (next)
			ACCESS_ONCE(next->locked) = 1;
			}

			#endif

kernel/locking/mcs_spinlock.h

+15 −0

Original line number	Original line	Diff line number	Diff line
	@@ -111,4 +111,19 @@ void mcs_spin_unlock(struct mcs_spinlock *lock, struct mcs_spinlock node)
	arch_mcs_spin_unlock_contended(&next->locked);		arch_mcs_spin_unlock_contended(&next->locked);
	}		}

			/*
			* Cancellable version of the MCS lock above.
			*
			* Intended for adaptive spinning of sleeping locks:
			* mutex_lock()/rwsem_down_{read,write}() etc.
			*/

			struct optimistic_spin_queue {
			struct optimistic_spin_queue next, prev;
			int locked; /* 1 if lock acquired */
			};

			extern bool osq_lock(struct optimistic_spin_queue **lock);
			extern void osq_unlock(struct optimistic_spin_queue **lock);

	#endif /* __LINUX_MCS_SPINLOCK_H */		#endif /* __LINUX_MCS_SPINLOCK_H */

kernel/locking/mutex.c

+6 −4

Original line number	Original line	Diff line number	Diff line
	@@ -53,7 +53,7 @@ __mutex_init(struct mutex lock, const char name, struct lock_class_key *key)
	INIT_LIST_HEAD(&lock->wait_list);		INIT_LIST_HEAD(&lock->wait_list);
	mutex_clear_owner(lock);		mutex_clear_owner(lock);
	#ifdef CONFIG_MUTEX_SPIN_ON_OWNER		#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
	lock->mcs_lock = NULL;		lock->osq = NULL;
	#endif		#endif

	debug_mutex_init(lock, name, key);		debug_mutex_init(lock, name, key);
	@@ -403,7 +403,9 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
	if (!mutex_can_spin_on_owner(lock))		if (!mutex_can_spin_on_owner(lock))
	goto slowpath;		goto slowpath;

	mcs_spin_lock(&lock->mcs_lock, &node);		if (!osq_lock(&lock->osq))
			goto slowpath;

	for (;;) {		for (;;) {
	struct task_struct *owner;		struct task_struct *owner;

	@@ -442,7 +444,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
	}		}

	mutex_set_owner(lock);		mutex_set_owner(lock);
	mcs_spin_unlock(&lock->mcs_lock, &node);		osq_unlock(&lock->osq);
	preempt_enable();		preempt_enable();
	return 0;		return 0;
	}		}
	@@ -464,7 +466,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
	*/		*/
	arch_mutex_cpu_relax();		arch_mutex_cpu_relax();
	}		}
	mcs_spin_unlock(&lock->mcs_lock, &node);		osq_unlock(&lock->osq);
	slowpath:		slowpath:
	#endif		#endif
	spin_lock_mutex(&lock->wait_lock, flags);		spin_lock_mutex(&lock->wait_lock, flags);