Refresh the topic.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
--- /dev/null
+/*
+ * MCS lock defines
+ *
+ * This file contains the main data structure and API definitions of MCS lock.
+ *
+ * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
+ * with the desirable properties of being fair, and with each cpu trying
+ * to acquire the lock spinning on a local variable.
+ * It avoids expensive cache bouncings that common test-and-set spin-lock
+ * implementations incur.
+ */
+#ifndef __LINUX_MCS_SPINLOCK_H
+#define __LINUX_MCS_SPINLOCK_H
+
+struct mcs_spinlock {
+ struct mcs_spinlock *next;
+ int locked; /* 1 if lock acquired */
+};
+
+#ifndef arch_mcs_spin_lock_contended
+/*
+ * Using smp_load_acquire() provides a memory barrier that ensures
+ * subsequent operations happen after the lock is acquired.
+ */
+#define arch_mcs_spin_lock_contended(l) \
+do { \
+ while (!(smp_load_acquire(l))) \
+ arch_mutex_cpu_relax(); \
+} while (0)
+#endif
+
+#ifndef arch_mcs_spin_unlock_contended
+/*
+ * smp_store_release() provides a memory barrier to ensure all
+ * operations in the critical section has been completed before
+ * unlocking.
+ */
+#define arch_mcs_spin_unlock_contended(l) \
+ smp_store_release((l), 1)
+#endif
+
+/*
+ * Note: the smp_load_acquire/smp_store_release pair is not
+ * sufficient to form a full memory barrier across
+ * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
+ * For applications that need a full barrier across multiple cpus
+ * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
+ * used after mcs_lock.
+ */
+
+/*
+ * In order to acquire the lock, the caller should declare a local node and
+ * pass a reference of the node to this function in addition to the lock.
+ * If the lock has already been acquired, then this will proceed to spin
+ * on this node->locked until the previous lock holder sets the node->locked
+ * in mcs_spin_unlock().
+ *
+ * We don't inline mcs_spin_lock() so that perf can correctly account for the
+ * time spent in this lock function.
+ */
+static inline
+void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
+{
+ struct mcs_spinlock *prev;
+
+ /* Init node */
+ node->locked = 0;
+ node->next = NULL;
+
+ prev = xchg(lock, node);
+ if (likely(prev == NULL)) {
+ /*
+ * Lock acquired, don't need to set node->locked to 1. Threads
+ * only spin on its own node->locked value for lock acquisition.
+ * However, since this thread can immediately acquire the lock
+ * and does not proceed to spin on its own node->locked, this
+ * value won't be used. If a debug mode is needed to
+ * audit lock status, then set node->locked value here.
+ */
+ return;
+ }
+ ACCESS_ONCE(prev->next) = node;
+
+ /* Wait until the lock holder passes the lock down. */
+ arch_mcs_spin_lock_contended(&node->locked);
+}
+
+/*
+ * Releases the lock. The caller should pass in the corresponding node that
+ * was used to acquire the lock.
+ */
+static inline
+void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
+{
+ struct mcs_spinlock *next = ACCESS_ONCE(node->next);
+
+ if (likely(!next)) {
+ /*
+ * Release the lock by setting it to NULL
+ */
+ if (likely(cmpxchg(lock, node, NULL) == node))
+ return;
+ /* Wait until the next pointer is set */
+ while (!(next = ACCESS_ONCE(node->next)))
+ arch_mutex_cpu_relax();
+ }
+
+ /* Pass lock to next waiter. */
+ arch_mcs_spin_unlock_contended(&next->locked);
+}
+
+#endif /* __LINUX_MCS_SPINLOCK_H */
* - detects multi-task circular deadlocks and prints out all affected
* locks and tasks (and only those tasks)
*/
+struct mcs_spinlock;
struct mutex {
/* 1: unlocked, 0: locked, negative: locked, possible waiters */
atomic_t count;
struct task_struct *owner;
#endif
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
- void *spin_mlock; /* Spinner MCS lock */
+ struct mcs_spinlock *mcs_lock; /* Spinner MCS lock */
#endif
#ifdef CONFIG_DEBUG_MUTEXES
const char *name;
# define arch_mutex_cpu_relax() cpu_relax()
#endif
-#endif
+#endif /* __LINUX_MUTEX_H */
#include <linux/spinlock.h>
#include <linux/interrupt.h>
#include <linux/debug_locks.h>
+#include <linux/mcs_spinlock.h>
/*
* In the DEBUG case we are using the "NULL fastpath" for mutexes,
INIT_LIST_HEAD(&lock->wait_list);
mutex_clear_owner(lock);
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
- lock->spin_mlock = NULL;
+ lock->mcs_lock = NULL;
#endif
debug_mutex_init(lock, name, key);
* more or less simultaneously, the spinners need to acquire a MCS lock
* first before spinning on the owner field.
*
- * We don't inline mspin_lock() so that perf can correctly account for the
- * time spent in this lock function.
*/
-struct mspin_node {
- struct mspin_node *next ;
- int locked; /* 1 if lock acquired */
-};
-#define MLOCK(mutex) ((struct mspin_node **)&((mutex)->spin_mlock))
-
-static noinline
-void mspin_lock(struct mspin_node **lock, struct mspin_node *node)
-{
- struct mspin_node *prev;
-
- /* Init node */
- node->locked = 0;
- node->next = NULL;
-
- prev = xchg(lock, node);
- if (likely(prev == NULL)) {
- /* Lock acquired */
- node->locked = 1;
- return;
- }
- ACCESS_ONCE(prev->next) = node;
- smp_wmb();
- /* Wait until the lock holder passes the lock down */
- while (!ACCESS_ONCE(node->locked))
- arch_mutex_cpu_relax();
-}
-
-static void mspin_unlock(struct mspin_node **lock, struct mspin_node *node)
-{
- struct mspin_node *next = ACCESS_ONCE(node->next);
-
- if (likely(!next)) {
- /*
- * Release the lock by setting it to NULL
- */
- if (cmpxchg(lock, node, NULL) == node)
- return;
- /* Wait until the next pointer is set */
- while (!(next = ACCESS_ONCE(node->next)))
- arch_mutex_cpu_relax();
- }
- ACCESS_ONCE(next->locked) = 1;
- smp_wmb();
-}
/*
* Mutex spinning code migrated from kernel/sched/core.c
for (;;) {
struct task_struct *owner;
- struct mspin_node node;
+ struct mcs_spinlock node;
if (use_ww_ctx && ww_ctx->acquired > 0) {
struct ww_mutex *ww;
* If there's an owner, wait for it to either
* release the lock or go to sleep.
*/
- mspin_lock(MLOCK(lock), &node);
+ mcs_spin_lock(&lock->mcs_lock, &node);
owner = ACCESS_ONCE(lock->owner);
if (owner && !mutex_spin_on_owner(lock, owner)) {
- mspin_unlock(MLOCK(lock), &node);
+ mcs_spin_unlock(&lock->mcs_lock, &node);
goto slowpath;
}
}
mutex_set_owner(lock);
- mspin_unlock(MLOCK(lock), &node);
+ mcs_spin_unlock(&lock->mcs_lock, &node);
preempt_enable();
return 0;
}
- mspin_unlock(MLOCK(lock), &node);
+ mcs_spin_unlock(&lock->mcs_lock, &node);
/*
* When there's no owner, we might have preempted between the
projects / linux-2.6-block.git / commitdiff