#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
