locking/mutexes: Introduce cancelable MCS lock for adaptive spinning (fb0527bd) · Commits · 戴 / test

include/linux/mutex.h

+2 −2

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

kernel/locking/Makefile

+1 −1

Original line number	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
		CFLAGS_REMOVE_lockdep.o = -pg

kernel/locking/mcs_spinlock.c

0 → 100644

+178 −0

Original line number	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	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);
		}

		/*
		* 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 */

kernel/locking/mutex.c

+6 −4

Original line number	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);
		mutex_clear_owner(lock);
		#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
		lock->mcs_lock = NULL;
		lock->osq = NULL;
		#endif

		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))
		goto slowpath;

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

		for (;;) {
		struct task_struct *owner;

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

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

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