![Peter Zijlstra](/assets/img/avatar_default.png)
Since we want a task waiting for a mutex_lock() to go to sleep and reschedule on need_resched() we must be able to abort the mcs_spin_lock() around the adaptive spin. Therefore implement a cancelable mcs lock. Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: chegu_vinod@hp.com Cc: paulmck@linux.vnet.ibm.com Cc: Waiman.Long@hp.com Cc: torvalds@linux-foundation.org Cc: tglx@linutronix.de Cc: riel@redhat.com Cc: akpm@linux-foundation.org Cc: davidlohr@hp.com Cc: hpa@zytor.com Cc: andi@firstfloor.org Cc: aswin@hp.com Cc: scott.norton@hp.com Cc: Jason Low <jason.low2@hp.com> Link: http://lkml.kernel.org/n/tip-62hcl5wxydmjzd182zhvk89m@git.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
179 lines
4.1 KiB
C
179 lines
4.1 KiB
C
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#include <linux/percpu.h>
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#include <linux/mutex.h>
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#include <linux/sched.h>
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#include "mcs_spinlock.h"
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#ifdef CONFIG_SMP
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/*
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* An MCS like lock especially tailored for optimistic spinning for sleeping
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* lock implementations (mutex, rwsem, etc).
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*
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* Using a single mcs node per CPU is safe because sleeping locks should not be
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* called from interrupt context and we have preemption disabled while
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* spinning.
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*/
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static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_queue, osq_node);
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/*
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* Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
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* Can return NULL in case we were the last queued and we updated @lock instead.
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*/
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static inline struct optimistic_spin_queue *
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osq_wait_next(struct optimistic_spin_queue **lock,
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struct optimistic_spin_queue *node,
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struct optimistic_spin_queue *prev)
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{
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struct optimistic_spin_queue *next = NULL;
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for (;;) {
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if (*lock == node && cmpxchg(lock, node, prev) == node) {
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/*
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* We were the last queued, we moved @lock back. @prev
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* will now observe @lock and will complete its
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* unlock()/unqueue().
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*/
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break;
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}
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/*
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* We must xchg() the @node->next value, because if we were to
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* leave it in, a concurrent unlock()/unqueue() from
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* @node->next might complete Step-A and think its @prev is
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* still valid.
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*
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* If the concurrent unlock()/unqueue() wins the race, we'll
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* wait for either @lock to point to us, through its Step-B, or
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* wait for a new @node->next from its Step-C.
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*/
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if (node->next) {
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next = xchg(&node->next, NULL);
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if (next)
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break;
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}
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arch_mutex_cpu_relax();
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}
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return next;
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}
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bool osq_lock(struct optimistic_spin_queue **lock)
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{
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struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
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struct optimistic_spin_queue *prev, *next;
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node->locked = 0;
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node->next = NULL;
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node->prev = prev = xchg(lock, node);
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if (likely(prev == NULL))
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return true;
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ACCESS_ONCE(prev->next) = node;
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/*
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* Normally @prev is untouchable after the above store; because at that
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* moment unlock can proceed and wipe the node element from stack.
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*
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* However, since our nodes are static per-cpu storage, we're
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* guaranteed their existence -- this allows us to apply
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* cmpxchg in an attempt to undo our queueing.
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*/
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while (!smp_load_acquire(&node->locked)) {
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/*
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* If we need to reschedule bail... so we can block.
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*/
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if (need_resched())
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goto unqueue;
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arch_mutex_cpu_relax();
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}
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return true;
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unqueue:
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/*
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* Step - A -- stabilize @prev
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*
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* Undo our @prev->next assignment; this will make @prev's
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* unlock()/unqueue() wait for a next pointer since @lock points to us
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* (or later).
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*/
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for (;;) {
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if (prev->next == node &&
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cmpxchg(&prev->next, node, NULL) == node)
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break;
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/*
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* We can only fail the cmpxchg() racing against an unlock(),
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* in which case we should observe @node->locked becomming
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* true.
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*/
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if (smp_load_acquire(&node->locked))
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return true;
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arch_mutex_cpu_relax();
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/*
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* Or we race against a concurrent unqueue()'s step-B, in which
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* case its step-C will write us a new @node->prev pointer.
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*/
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prev = ACCESS_ONCE(node->prev);
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}
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/*
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* Step - B -- stabilize @next
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*
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* Similar to unlock(), wait for @node->next or move @lock from @node
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* back to @prev.
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*/
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next = osq_wait_next(lock, node, prev);
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if (!next)
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return false;
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/*
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* Step - C -- unlink
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*
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* @prev is stable because its still waiting for a new @prev->next
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* pointer, @next is stable because our @node->next pointer is NULL and
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* it will wait in Step-A.
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*/
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ACCESS_ONCE(next->prev) = prev;
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ACCESS_ONCE(prev->next) = next;
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return false;
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}
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void osq_unlock(struct optimistic_spin_queue **lock)
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{
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struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
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struct optimistic_spin_queue *next;
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/*
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* Fast path for the uncontended case.
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*/
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if (likely(cmpxchg(lock, node, NULL) == node))
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return;
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/*
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* Second most likely case.
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*/
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next = xchg(&node->next, NULL);
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if (next) {
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ACCESS_ONCE(next->locked) = 1;
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return;
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}
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next = osq_wait_next(lock, node, NULL);
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if (next)
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ACCESS_ONCE(next->locked) = 1;
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}
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#endif
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