From: Davidlohr Bueso Date: Tue, 6 Jan 2015 19:45:07 +0000 (-0800) Subject: locking/mcs: Better differentiate between MCS variants X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=d84b6728c54dcf73bcef3e3f7cf6767e2d224e39;p=GitHub%2FLineageOS%2FG12%2Fandroid_kernel_amlogic_linux-4.9.git locking/mcs: Better differentiate between MCS variants 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: - Moves the 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. - Renames mcs_spinlock.c to osq_lock.c. This file only contains osq code. - 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 Signed-off-by: Peter Zijlstra (Intel) Cc: Thomas Gleixner Cc: "Paul E. McKenney" Cc: Jason Low Cc: Linus Torvalds Cc: Mikulas Patocka Cc: Waiman Long Link: http://lkml.kernel.org/r/1420573509-24774-5-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar --- diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h index 90230d5811c5..3a6490e81b28 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 76768ee812b2..08561f1acd13 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 8541bfdfd232..4ca8eb151975 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 9887a905a762..000000000000 --- a/kernel/locking/mcs_spinlock.c +++ /dev/null @@ -1,208 +0,0 @@ -#include -#include -#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 4d60986fcbee..d1fe2ba5bac9 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 000000000000..ec83d4db8ec6 --- /dev/null +++ b/kernel/locking/osq_lock.c @@ -0,0 +1,203 @@ +#include +#include +#include + +/* + * 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; +}