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Subject[PATCH 6/8] locking/mcs: Better differentiate between mcs variants
Date
We have two flavors of the MCS spinlock: standard and cancelable (osq).
While each one is independent of the other, we currently mix and match
them. This patch:

o Moves osq code out of mcs_spinlock.h (which only deals with the traditional
version) into include/linux/osq_lock.h. No unnecessary code is added to the
more global header file, anything locks that make use of osq must include
it anyway.

o Renames mcs_spinlock.c to osq_lock.c. This file only contains osq code.

o Introduces a CONFIG_LOCK_SPIN_ON_OWNER in order to only build osq_lock
if there is support for it.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
include/linux/osq_lock.h | 12 ++-
kernel/Kconfig.locks | 4 +
kernel/locking/Makefile | 3 +-
kernel/locking/mcs_spinlock.c | 208 ------------------------------------------
kernel/locking/mcs_spinlock.h | 16 ----
kernel/locking/osq_lock.c | 203 +++++++++++++++++++++++++++++++++++++++++
6 files changed, 219 insertions(+), 227 deletions(-)
delete mode 100644 kernel/locking/mcs_spinlock.c
create mode 100644 kernel/locking/osq_lock.c

diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
index 90230d5..3a6490e 100644
--- a/include/linux/osq_lock.h
+++ b/include/linux/osq_lock.h
@@ -5,8 +5,11 @@
* An MCS like lock especially tailored for optimistic spinning for sleeping
* lock implementations (mutex, rwsem, etc).
*/
-
-#define OSQ_UNLOCKED_VAL (0)
+struct optimistic_spin_node {
+ struct optimistic_spin_node *next, *prev;
+ int locked; /* 1 if lock acquired */
+ int cpu; /* encoded CPU # + 1 value */
+};

struct optimistic_spin_queue {
/*
@@ -16,6 +19,8 @@ struct optimistic_spin_queue {
atomic_t tail;
};

+#define OSQ_UNLOCKED_VAL (0)
+
/* Init macro and function. */
#define OSQ_LOCK_UNLOCKED { ATOMIC_INIT(OSQ_UNLOCKED_VAL) }

@@ -24,4 +29,7 @@ static inline void osq_lock_init(struct optimistic_spin_queue *lock)
atomic_set(&lock->tail, OSQ_UNLOCKED_VAL);
}

+extern bool osq_lock(struct optimistic_spin_queue *lock);
+extern void osq_unlock(struct optimistic_spin_queue *lock);
+
#endif
diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
index 76768ee..08561f1 100644
--- a/kernel/Kconfig.locks
+++ b/kernel/Kconfig.locks
@@ -231,6 +231,10 @@ config RWSEM_SPIN_ON_OWNER
def_bool y
depends on SMP && RWSEM_XCHGADD_ALGORITHM && ARCH_SUPPORTS_ATOMIC_RMW

+config LOCK_SPIN_ON_OWNER
+ def_bool y
+ depends on MUTEX_SPIN_ON_OWNER || RWSEM_SPIN_ON_OWNER
+
config ARCH_USE_QUEUE_RWLOCK
bool

diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index 8541bfd..4ca8eb1 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -1,5 +1,5 @@

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

ifdef CONFIG_FUNCTION_TRACER
CFLAGS_REMOVE_lockdep.o = -pg
@@ -14,6 +14,7 @@ ifeq ($(CONFIG_PROC_FS),y)
obj-$(CONFIG_LOCKDEP) += lockdep_proc.o
endif
obj-$(CONFIG_SMP) += spinlock.o
+obj-$(CONFIG_LOCK_SPIN_ON_OWNER) += osq_lock.o
obj-$(CONFIG_SMP) += lglock.o
obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
obj-$(CONFIG_RT_MUTEXES) += rtmutex.o
diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
deleted file mode 100644
index 9887a90..0000000
--- a/kernel/locking/mcs_spinlock.c
+++ /dev/null
@@ -1,208 +0,0 @@
-#include <linux/percpu.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_node, osq_node);
-
-/*
- * We use the value 0 to represent "no CPU", thus the encoded value
- * will be the CPU number incremented by 1.
- */
-static inline int encode_cpu(int cpu_nr)
-{
- return cpu_nr + 1;
-}
-
-static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
-{
- int cpu_nr = encoded_cpu_val - 1;
-
- return per_cpu_ptr(&osq_node, cpu_nr);
-}
-
-/*
- * 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_node *
-osq_wait_next(struct optimistic_spin_queue *lock,
- struct optimistic_spin_node *node,
- struct optimistic_spin_node *prev)
-{
- struct optimistic_spin_node *next = NULL;
- int curr = encode_cpu(smp_processor_id());
- int old;
-
- /*
- * If there is a prev node in queue, then the 'old' value will be
- * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
- * we're currently last in queue, then the queue will then become empty.
- */
- old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
-
- for (;;) {
- if (atomic_read(&lock->tail) == curr &&
- atomic_cmpxchg(&lock->tail, curr, old) == curr) {
- /*
- * 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;
- }
-
- cpu_relax_lowlatency();
- }
-
- return next;
-}
-
-bool osq_lock(struct optimistic_spin_queue *lock)
-{
- struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
- struct optimistic_spin_node *prev, *next;
- int curr = encode_cpu(smp_processor_id());
- int old;
-
- node->locked = 0;
- node->next = NULL;
- node->cpu = curr;
-
- old = atomic_xchg(&lock->tail, curr);
- if (old == OSQ_UNLOCKED_VAL)
- return true;
-
- prev = decode_cpu(old);
- node->prev = prev;
- 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;
-
- cpu_relax_lowlatency();
- }
- 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;
-
- cpu_relax_lowlatency();
-
- /*
- * 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_node *node, *next;
- int curr = encode_cpu(smp_processor_id());
-
- /*
- * Fast path for the uncontended case.
- */
- if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr))
- return;
-
- /*
- * Second most likely case.
- */
- node = this_cpu_ptr(&osq_node);
- 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
-
diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index 4d60986..d1fe2ba 100644
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -108,20 +108,4 @@ 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_node {
- struct optimistic_spin_node *next, *prev;
- int locked; /* 1 if lock acquired */
- int cpu; /* encoded CPU # value */
-};
-
-extern bool osq_lock(struct optimistic_spin_queue *lock);
-extern void osq_unlock(struct optimistic_spin_queue *lock);
-
#endif /* __LINUX_MCS_SPINLOCK_H */
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
new file mode 100644
index 0000000..ec83d4d
--- /dev/null
+++ b/kernel/locking/osq_lock.c
@@ -0,0 +1,203 @@
+#include <linux/percpu.h>
+#include <linux/sched.h>
+#include <linux/osq_lock.h>
+
+/*
+ * 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_node, osq_node);
+
+/*
+ * We use the value 0 to represent "no CPU", thus the encoded value
+ * will be the CPU number incremented by 1.
+ */
+static inline int encode_cpu(int cpu_nr)
+{
+ return cpu_nr + 1;
+}
+
+static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
+{
+ int cpu_nr = encoded_cpu_val - 1;
+
+ return per_cpu_ptr(&osq_node, cpu_nr);
+}
+
+/*
+ * 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_node *
+osq_wait_next(struct optimistic_spin_queue *lock,
+ struct optimistic_spin_node *node,
+ struct optimistic_spin_node *prev)
+{
+ struct optimistic_spin_node *next = NULL;
+ int curr = encode_cpu(smp_processor_id());
+ int old;
+
+ /*
+ * If there is a prev node in queue, then the 'old' value will be
+ * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
+ * we're currently last in queue, then the queue will then become empty.
+ */
+ old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
+
+ for (;;) {
+ if (atomic_read(&lock->tail) == curr &&
+ atomic_cmpxchg(&lock->tail, curr, old) == curr) {
+ /*
+ * 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;
+ }
+
+ cpu_relax_lowlatency();
+ }
+
+ return next;
+}
+
+bool osq_lock(struct optimistic_spin_queue *lock)
+{
+ struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
+ struct optimistic_spin_node *prev, *next;
+ int curr = encode_cpu(smp_processor_id());
+ int old;
+
+ node->locked = 0;
+ node->next = NULL;
+ node->cpu = curr;
+
+ old = atomic_xchg(&lock->tail, curr);
+ if (old == OSQ_UNLOCKED_VAL)
+ return true;
+
+ prev = decode_cpu(old);
+ node->prev = prev;
+ 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;
+
+ cpu_relax_lowlatency();
+ }
+ 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;
+
+ cpu_relax_lowlatency();
+
+ /*
+ * 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_node *node, *next;
+ int curr = encode_cpu(smp_processor_id());
+
+ /*
+ * Fast path for the uncontended case.
+ */
+ if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr))
+ return;
+
+ /*
+ * Second most likely case.
+ */
+ node = this_cpu_ptr(&osq_node);
+ 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;
+}
--
2.1.2


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