Merge branch 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking updates from Ingo Molnar:
 "So we have a laundry list of locking subsystem changes:

   - continuing barrier API and code improvements

   - futex enhancements

   - atomics API improvements

   - pvqspinlock enhancements: in particular lock stealing and adaptive
     spinning

   - qspinlock micro-enhancements"

* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op
  futex: Cleanup the goto confusion in requeue_pi()
  futex: Remove pointless put_pi_state calls in requeue()
  futex: Document pi_state refcounting in requeue code
  futex: Rename free_pi_state() to put_pi_state()
  futex: Drop refcount if requeue_pi() acquired the rtmutex
  locking/barriers, arch: Remove ambiguous statement in the smp_store_mb() documentation
  lcoking/barriers, arch: Use smp barriers in smp_store_release()
  locking/cmpxchg, arch: Remove tas() definitions
  locking/pvqspinlock: Queue node adaptive spinning
  locking/pvqspinlock: Allow limited lock stealing
  locking/pvqspinlock: Collect slowpath lock statistics
  sched/core, locking: Document Program-Order guarantees
  locking, sched: Introduce smp_cond_acquire() and use it
  locking/pvqspinlock, x86: Optimize the PV unlock code path
  locking/qspinlock: Avoid redundant read of next pointer
  locking/qspinlock: Prefetch the next node cacheline
  locking/qspinlock: Use _acquire/_release() versions of cmpxchg() & xchg()
  atomics: Add test for atomic operations with _relaxed variants

Documentation/memory-barriers.txt

@@ -1673,8 +1673,8 @@ There are some more advanced barrier functions:
  (*) smp_store_mb(var, value)
 
      This assigns the value to the variable and then inserts a full memory
-     barrier after it, depending on the function.  It isn't guaranteed to
-     insert anything more than a compiler barrier in a UP compilation.
+     barrier after it.  It isn't guaranteed to insert anything more than a
+     compiler barrier in a UP compilation.
 
 
  (*) smp_mb__before_atomic();

arch/blackfin/include/asm/cmpxchg.h

@@ -128,6 +128,5 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
 #endif /* !CONFIG_SMP */
 
 #define xchg(ptr, x) ((__typeof__(*(ptr)))__xchg((unsigned long)(x), (ptr), sizeof(*(ptr))))
-#define tas(ptr) ((void)xchg((ptr), 1))
 
 #endif /* __ARCH_BLACKFIN_CMPXCHG__ */

arch/c6x/include/asm/cmpxchg.h

@@ -47,8 +47,6 @@ static inline unsigned int __xchg(unsigned int x, volatile void *ptr, int size)
 #define xchg(ptr, x) \
 	((__typeof__(*(ptr)))__xchg((unsigned int)(x), (void *) (ptr), \
 				    sizeof(*(ptr))))
-#define tas(ptr)    xchg((ptr), 1)
-
 
 #include <asm-generic/cmpxchg-local.h>
 

arch/frv/include/asm/cmpxchg.h

@@ -69,8 +69,6 @@ extern uint32_t __xchg_32(uint32_t i, volatile void *v);
 
 #endif
 
-#define tas(ptr) (xchg((ptr), 1))
-
 /*****************************************************************************/
 /*
  * compare and conditionally exchange value with memory

arch/ia64/include/asm/barrier.h

@@ -77,7 +77,7 @@ do {									\
 	___p1;								\
 })
 
-#define smp_store_mb(var, value)	do { WRITE_ONCE(var, value); mb(); } while (0)
+#define smp_store_mb(var, value) do { WRITE_ONCE(var, value); smp_mb(); } while (0)
 
 /*
  * The group barrier in front of the rsm & ssm are necessary to ensure

arch/powerpc/include/asm/barrier.h

@@ -34,7 +34,7 @@
 #define rmb()  __asm__ __volatile__ ("sync" : : : "memory")
 #define wmb()  __asm__ __volatile__ ("sync" : : : "memory")
 
-#define smp_store_mb(var, value)	do { WRITE_ONCE(var, value); mb(); } while (0)
+#define smp_store_mb(var, value) do { WRITE_ONCE(var, value); smp_mb(); } while (0)
 
 #ifdef __SUBARCH_HAS_LWSYNC
 #    define SMPWMB      LWSYNC

arch/s390/include/asm/barrier.h

@@ -36,7 +36,7 @@
 #define smp_mb__before_atomic()		smp_mb()
 #define smp_mb__after_atomic()		smp_mb()
 
-#define smp_store_mb(var, value)		do { WRITE_ONCE(var, value); mb(); } while (0)
+#define smp_store_mb(var, value)	do { WRITE_ONCE(var, value); smp_mb(); } while (0)
 
 #define smp_store_release(p, v)						\
 do {									\

arch/tile/include/asm/cmpxchg.h

@@ -127,8 +127,6 @@ long long _atomic64_cmpxchg(long long *v, long long o, long long n);
 
 #endif
 
-#define tas(ptr) xchg((ptr), 1)
-
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_TILE_CMPXCHG_H */

arch/x86/Kconfig

@@ -687,6 +687,14 @@ config PARAVIRT_SPINLOCKS
 
 	  If you are unsure how to answer this question, answer Y.
 
+config QUEUED_LOCK_STAT
+	bool "Paravirt queued spinlock statistics"
+	depends on PARAVIRT_SPINLOCKS && DEBUG_FS && QUEUED_SPINLOCKS
+	---help---
+	  Enable the collection of statistical data on the slowpath
+	  behavior of paravirtualized queued spinlocks and report
+	  them on debugfs.
+
 source "arch/x86/xen/Kconfig"
 
 config KVM_GUEST

arch/x86/include/asm/qspinlock_paravirt.h

 #ifndef __ASM_QSPINLOCK_PARAVIRT_H
 #define __ASM_QSPINLOCK_PARAVIRT_H
 
+/*
+ * For x86-64, PV_CALLEE_SAVE_REGS_THUNK() saves and restores 8 64-bit
+ * registers. For i386, however, only 1 32-bit register needs to be saved
+ * and restored. So an optimized version of __pv_queued_spin_unlock() is
+ * hand-coded for 64-bit, but it isn't worthwhile to do it for 32-bit.
+ */
+#ifdef CONFIG_64BIT
+
+PV_CALLEE_SAVE_REGS_THUNK(__pv_queued_spin_unlock_slowpath);
+#define __pv_queued_spin_unlock	__pv_queued_spin_unlock
+#define PV_UNLOCK		"__raw_callee_save___pv_queued_spin_unlock"
+#define PV_UNLOCK_SLOWPATH	"__raw_callee_save___pv_queued_spin_unlock_slowpath"
+
+/*
+ * Optimized assembly version of __raw_callee_save___pv_queued_spin_unlock
+ * which combines the registers saving trunk and the body of the following
+ * C code:
+ *
+ * void __pv_queued_spin_unlock(struct qspinlock *lock)
+ * {
+ *	struct __qspinlock *l = (void *)lock;
+ *	u8 lockval = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
+ *
+ *	if (likely(lockval == _Q_LOCKED_VAL))
+ *		return;
+ *	pv_queued_spin_unlock_slowpath(lock, lockval);
+ * }
+ *
+ * For x86-64,
+ *   rdi = lock              (first argument)
+ *   rsi = lockval           (second argument)
+ *   rdx = internal variable (set to 0)
+ */
+asm    (".pushsection .text;"
+	".globl " PV_UNLOCK ";"
+	".align 4,0x90;"
+	PV_UNLOCK ": "
+	"push  %rdx;"
+	"mov   $0x1,%eax;"
+	"xor   %edx,%edx;"
+	"lock cmpxchg %dl,(%rdi);"
+	"cmp   $0x1,%al;"
+	"jne   .slowpath;"
+	"pop   %rdx;"
+	"ret;"
+	".slowpath: "
+	"push   %rsi;"
+	"movzbl %al,%esi;"
+	"call " PV_UNLOCK_SLOWPATH ";"
+	"pop    %rsi;"
+	"pop    %rdx;"
+	"ret;"
+	".size " PV_UNLOCK ", .-" PV_UNLOCK ";"
+	".popsection");
+
+#else /* CONFIG_64BIT */
+
+extern void __pv_queued_spin_unlock(struct qspinlock *lock);
 PV_CALLEE_SAVE_REGS_THUNK(__pv_queued_spin_unlock);
 
+#endif /* CONFIG_64BIT */
 #endif

include/asm-generic/barrier.h

@@ -93,7 +93,7 @@
 #endif	/* CONFIG_SMP */
 
 #ifndef smp_store_mb
-#define smp_store_mb(var, value)  do { WRITE_ONCE(var, value); mb(); } while (0)
+#define smp_store_mb(var, value)  do { WRITE_ONCE(var, value); smp_mb(); } while (0)
 #endif
 
 #ifndef smp_mb__before_atomic

include/asm-generic/qspinlock.h

@@ -12,8 +12,9 @@
  * GNU General Public License for more details.
  *
  * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
+ * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
  *
- * Authors: Waiman Long <waiman.long@hp.com>
+ * Authors: Waiman Long <waiman.long@hpe.com>
  */
 #ifndef __ASM_GENERIC_QSPINLOCK_H
 #define __ASM_GENERIC_QSPINLOCK_H
@@ -62,7 +63,7 @@ static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
 static __always_inline int queued_spin_trylock(struct qspinlock *lock)
 {
 	if (!atomic_read(&lock->val) &&
-	   (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) == 0))
+	   (atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0))
 		return 1;
 	return 0;
 }
@@ -77,7 +78,7 @@ static __always_inline void queued_spin_lock(struct qspinlock *lock)
 {
 	u32 val;
 
-	val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
+	val = atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL);
 	if (likely(val == 0))
 		return;
 	queued_spin_lock_slowpath(lock, val);
@@ -93,7 +94,7 @@ static __always_inline void queued_spin_unlock(struct qspinlock *lock)
 	/*
 	 * smp_mb__before_atomic() in order to guarantee release semantics
 	 */
-	smp_mb__before_atomic_dec();
+	smp_mb__before_atomic();
 	atomic_sub(_Q_LOCKED_VAL, &lock->val);
 }
 #endif

include/linux/compiler.h

@@ -299,6 +299,23 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	__u.__val;					\
 })
 
+/**
+ * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
+ * @cond: boolean expression to wait for
+ *
+ * Equivalent to using smp_load_acquire() on the condition variable but employs
+ * the control dependency of the wait to reduce the barrier on many platforms.
+ *
+ * The control dependency provides a LOAD->STORE order, the additional RMB
+ * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
+ * aka. ACQUIRE.
+ */
+#define smp_cond_acquire(cond)	do {		\
+	while (!(cond))				\
+		cpu_relax();			\
+	smp_rmb(); /* ctrl + rmb := acquire */	\
+} while (0)
+
 #endif /* __KERNEL__ */
 
 #endif /* __ASSEMBLY__ */

kernel/futex.c

@@ -725,9 +725,12 @@ static struct futex_pi_state * alloc_pi_state(void)
 }
 
 /*
+ * Drops a reference to the pi_state object and frees or caches it
+ * when the last reference is gone.
+ *
  * Must be called with the hb lock held.
  */
-static void free_pi_state(struct futex_pi_state *pi_state)
+static void put_pi_state(struct futex_pi_state *pi_state)
 {
 	if (!pi_state)
 		return;
@@ -1706,31 +1709,35 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
 		 * exist yet, look it up one more time to ensure we have a
 		 * reference to it. If the lock was taken, ret contains the
 		 * vpid of the top waiter task.
+		 * If the lock was not taken, we have pi_state and an initial
+		 * refcount on it. In case of an error we have nothing.
 		 */
 		if (ret > 0) {
 			WARN_ON(pi_state);
 			drop_count++;
 			task_count++;
 			/*
-			 * If we acquired the lock, then the user
-			 * space value of uaddr2 should be vpid. It
-			 * cannot be changed by the top waiter as it
-			 * is blocked on hb2 lock if it tries to do
-			 * so. If something fiddled with it behind our
-			 * back the pi state lookup might unearth
-			 * it. So we rather use the known value than
-			 * rereading and handing potential crap to
-			 * lookup_pi_state.
+			 * If we acquired the lock, then the user space value
+			 * of uaddr2 should be vpid. It cannot be changed by
+			 * the top waiter as it is blocked on hb2 lock if it
+			 * tries to do so. If something fiddled with it behind
+			 * our back the pi state lookup might unearth it. So
+			 * we rather use the known value than rereading and
+			 * handing potential crap to lookup_pi_state.
+			 *
+			 * If that call succeeds then we have pi_state and an
+			 * initial refcount on it.
 			 */
 			ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
 		}
 
 		switch (ret) {
 		case 0:
+			/* We hold a reference on the pi state. */
 			break;
+
+			/* If the above failed, then pi_state is NULL */
 		case -EFAULT:
-			free_pi_state(pi_state);
-			pi_state = NULL;
 			double_unlock_hb(hb1, hb2);
 			hb_waiters_dec(hb2);
 			put_futex_key(&key2);
@@ -1746,8 +1753,6 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
 			 *   exit to complete.
 			 * - The user space value changed.
 			 */
-			free_pi_state(pi_state);
-			pi_state = NULL;
 			double_unlock_hb(hb1, hb2);
 			hb_waiters_dec(hb2);
 			put_futex_key(&key2);
@@ -1801,30 +1806,58 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
 		 * of requeue_pi if we couldn't acquire the lock atomically.
 		 */
 		if (requeue_pi) {
-			/* Prepare the waiter to take the rt_mutex. */
+			/*
+			 * Prepare the waiter to take the rt_mutex. Take a
+			 * refcount on the pi_state and store the pointer in
+			 * the futex_q object of the waiter.
+			 */
 			atomic_inc(&pi_state->refcount);
 			this->pi_state = pi_state;
 			ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
 							this->rt_waiter,
 							this->task);
 			if (ret == 1) {
-				/* We got the lock. */
+				/*
+				 * We got the lock. We do neither drop the
+				 * refcount on pi_state nor clear
+				 * this->pi_state because the waiter needs the
+				 * pi_state for cleaning up the user space
+				 * value. It will drop the refcount after
+				 * doing so.
+				 */
 				requeue_pi_wake_futex(this, &key2, hb2);
 				drop_count++;
 				continue;
 			} else if (ret) {
-				/* -EDEADLK */
+				/*
+				 * rt_mutex_start_proxy_lock() detected a
+				 * potential deadlock when we tried to queue
+				 * that waiter. Drop the pi_state reference
+				 * which we took above and remove the pointer
+				 * to the state from the waiters futex_q
+				 * object.
+				 */
 				this->pi_state = NULL;
-				free_pi_state(pi_state);
-				goto out_unlock;
+				put_pi_state(pi_state);
+				/*
+				 * We stop queueing more waiters and let user
+				 * space deal with the mess.
+				 */
+				break;
 			}
 		}
 		requeue_futex(this, hb1, hb2, &key2);
 		drop_count++;
 	}
 
+	/*
+	 * We took an extra initial reference to the pi_state either
+	 * in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
+	 * need to drop it here again.
+	 */
+	put_pi_state(pi_state);
+
 out_unlock:
-	free_pi_state(pi_state);
 	double_unlock_hb(hb1, hb2);
 	wake_up_q(&wake_q);
 	hb_waiters_dec(hb2);
@@ -1973,7 +2006,7 @@ static void unqueue_me_pi(struct futex_q *q)
 	__unqueue_futex(q);
 
 	BUG_ON(!q->pi_state);
-	free_pi_state(q->pi_state);
+	put_pi_state(q->pi_state);
 	q->pi_state = NULL;
 
 	spin_unlock(q->lock_ptr);
@@ -2755,6 +2788,11 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
 		if (q.pi_state && (q.pi_state->owner != current)) {
 			spin_lock(q.lock_ptr);
 			ret = fixup_pi_state_owner(uaddr2, &q, current);
+			/*
+			 * Drop the reference to the pi state which
+			 * the requeue_pi() code acquired for us.
+			 */
+			put_pi_state(q.pi_state);
 			spin_unlock(q.lock_ptr);
 		}
 	} else {
@@ -3046,7 +3084,8 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 
 	if (op & FUTEX_CLOCK_REALTIME) {
 		flags |= FLAGS_CLOCKRT;
-		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
+		if (cmd != FUTEX_WAIT && cmd != FUTEX_WAIT_BITSET && \
+		    cmd != FUTEX_WAIT_REQUEUE_PI)
 			return -ENOSYS;
 	}
 

kernel/locking/qspinlock.c

@@ -14,8 +14,9 @@
  * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
  * (C) Copyright 2013-2014 Red Hat, Inc.
  * (C) Copyright 2015 Intel Corp.
+ * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
  *
- * Authors: Waiman Long <waiman.long@hp.com>
+ * Authors: Waiman Long <waiman.long@hpe.com>
  *          Peter Zijlstra <peterz@infradead.org>
  */
 
@@ -176,7 +177,12 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
 {
 	struct __qspinlock *l = (void *)lock;
 
-	return (u32)xchg(&l->tail, tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
+	/*
+	 * Use release semantics to make sure that the MCS node is properly
+	 * initialized before changing the tail code.
+	 */
+	return (u32)xchg_release(&l->tail,
+				 tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
 }
 
 #else /* _Q_PENDING_BITS == 8 */
@@ -208,7 +214,11 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
 
 	for (;;) {
 		new = (val & _Q_LOCKED_PENDING_MASK) | tail;
-		old = atomic_cmpxchg(&lock->val, val, new);
+		/*
+		 * Use release semantics to make sure that the MCS node is
+		 * properly initialized before changing the tail code.
+		 */
+		old = atomic_cmpxchg_release(&lock->val, val, new);
 		if (old == val)
 			break;
 
@@ -238,18 +248,20 @@ static __always_inline void set_locked(struct qspinlock *lock)
  */
 
 static __always_inline void __pv_init_node(struct mcs_spinlock *node) { }
-static __always_inline void __pv_wait_node(struct mcs_spinlock *node) { }
+static __always_inline void __pv_wait_node(struct mcs_spinlock *node,
+					   struct mcs_spinlock *prev) { }
 static __always_inline void __pv_kick_node(struct qspinlock *lock,
 					   struct mcs_spinlock *node) { }
-static __always_inline void __pv_wait_head(struct qspinlock *lock,
-					   struct mcs_spinlock *node) { }
+static __always_inline u32  __pv_wait_head_or_lock(struct qspinlock *lock,
+						   struct mcs_spinlock *node)
+						   { return 0; }
 
 #define pv_enabled()		false
 
 #define pv_init_node		__pv_init_node
 #define pv_wait_node		__pv_wait_node
 #define pv_kick_node		__pv_kick_node
-#define pv_wait_head		__pv_wait_head
+#define pv_wait_head_or_lock	__pv_wait_head_or_lock
 
 #ifdef CONFIG_PARAVIRT_SPINLOCKS
 #define queued_spin_lock_slowpath	native_queued_spin_lock_slowpath
@@ -319,7 +331,11 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 		if (val == new)
 			new |= _Q_PENDING_VAL;
 
-		old = atomic_cmpxchg(&lock->val, val, new);
+		/*
+		 * Acquire semantic is required here as the function may
+		 * return immediately if the lock was free.
+		 */
+		old = atomic_cmpxchg_acquire(&lock->val, val, new);
 		if (old == val)
 			break;
 
@@ -382,6 +398,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 * p,*,* -> n,*,*
 	 */
 	old = xchg_tail(lock, tail);
+	next = NULL;
 
 	/*
 	 * if there was a previous node; link it and wait until reaching the
@@ -391,8 +408,18 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 		prev = decode_tail(old);
 		WRITE_ONCE(prev->next, node);
 
-		pv_wait_node(node);
+		pv_wait_node(node, prev);
 		arch_mcs_spin_lock_contended(&node->locked);
+
+		/*
+		 * While waiting for the MCS lock, the next pointer may have
+		 * been set by another lock waiter. We optimistically load
+		 * the next pointer & prefetch the cacheline for writing
+		 * to reduce latency in the upcoming MCS unlock operation.
+		 */
+		next = READ_ONCE(node->next);
+		if (next)
+			prefetchw(next);
 	}
 
 	/*
@@ -406,11 +433,22 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 * sequentiality; this is because the set_locked() function below
 	 * does not imply a full barrier.
 	 *
+	 * The PV pv_wait_head_or_lock function, if active, will acquire
+	 * the lock and return a non-zero value. So we have to skip the
+	 * smp_load_acquire() call. As the next PV queue head hasn't been
+	 * designated yet, there is no way for the locked value to become
+	 * _Q_SLOW_VAL. So both the set_locked() and the
+	 * atomic_cmpxchg_relaxed() calls will be safe.
+	 *
+	 * If PV isn't active, 0 will be returned instead.
+	 *
 	 */
-	pv_wait_head(lock, node);
-	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
-		cpu_relax();
+	if ((val = pv_wait_head_or_lock(lock, node)))
+		goto locked;
 
+	smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
+
+locked:
 	/*
 	 * claim the lock:
 	 *
@@ -422,11 +460,17 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 * to grab the lock.
 	 */
 	for (;;) {
-		if (val != tail) {
+		/* In the PV case we might already have _Q_LOCKED_VAL set */
+		if ((val & _Q_TAIL_MASK) != tail) {
 			set_locked(lock);
 			break;
 		}
-		old = atomic_cmpxchg(&lock->val, val, _Q_LOCKED_VAL);
+		/*
+		 * The smp_load_acquire() call above has provided the necessary
+		 * acquire semantics required for locking. At most two
+		 * iterations of this loop may be ran.
+		 */
+		old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
 		if (old == val)
 			goto release;	/* No contention */
 
@@ -434,10 +478,12 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	}
 
 	/*
-	 * contended path; wait for next, release.
+	 * contended path; wait for next if not observed yet, release.
 	 */
-	while (!(next = READ_ONCE(node->next)))
-		cpu_relax();
+	if (!next) {
+		while (!(next = READ_ONCE(node->next)))
+			cpu_relax();
+	}
 
 	arch_mcs_spin_unlock_contended(&next->locked);
 	pv_kick_node(lock, next);
@@ -462,7 +508,7 @@ EXPORT_SYMBOL(queued_spin_lock_slowpath);
 #undef pv_init_node
 #undef pv_wait_node
 #undef pv_kick_node
-#undef pv_wait_head
+#undef pv_wait_head_or_lock
 
 #undef  queued_spin_lock_slowpath
 #define queued_spin_lock_slowpath	__pv_queued_spin_lock_slowpath

kernel/locking/qspinlock_stat.h

+/*
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * Authors: Waiman Long <waiman.long@hpe.com>
+ */
+
+/*
+ * When queued spinlock statistical counters are enabled, the following
+ * debugfs files will be created for reporting the counter values:
+ *
+ * <debugfs>/qlockstat/
+ *   pv_hash_hops	- average # of hops per hashing operation
+ *   pv_kick_unlock	- # of vCPU kicks issued at unlock time
+ *   pv_kick_wake	- # of vCPU kicks used for computing pv_latency_wake
+ *   pv_latency_kick	- average latency (ns) of vCPU kick operation
+ *   pv_latency_wake	- average latency (ns) from vCPU kick to wakeup
+ *   pv_lock_stealing	- # of lock stealing operations
+ *   pv_spurious_wakeup	- # of spurious wakeups
+ *   pv_wait_again	- # of vCPU wait's that happened after a vCPU kick
+ *   pv_wait_early	- # of early vCPU wait's
+ *   pv_wait_head	- # of vCPU wait's at the queue head
+ *   pv_wait_node	- # of vCPU wait's at a non-head queue node
+ *
+ * Writing to the "reset_counters" file will reset all the above counter
+ * values.
+ *
+ * These statistical counters are implemented as per-cpu variables which are
+ * summed and computed whenever the corresponding debugfs files are read. This
+ * minimizes added overhead making the counters usable even in a production
+ * environment.
+ *
+ * There may be slight difference between pv_kick_wake and pv_kick_unlock.
+ */
+enum qlock_stats {
+	qstat_pv_hash_hops,
+	qstat_pv_kick_unlock,
+	qstat_pv_kick_wake,
+	qstat_pv_latency_kick,
+	qstat_pv_latency_wake,
+	qstat_pv_lock_stealing,
+	qstat_pv_spurious_wakeup,
+	qstat_pv_wait_again,
+	qstat_pv_wait_early,
+	qstat_pv_wait_head,
+	qstat_pv_wait_node,
+	qstat_num,	/* Total number of statistical counters */
+	qstat_reset_cnts = qstat_num,
+};
+
+#ifdef CONFIG_QUEUED_LOCK_STAT
+/*
+ * Collect pvqspinlock statistics
+ */
+#include <linux/debugfs.h>
+#include <linux/sched.h>
+#include <linux/fs.h>
+
+static const char * const qstat_names[qstat_num + 1] = {
+	[qstat_pv_hash_hops]	   = "pv_hash_hops",
+	[qstat_pv_kick_unlock]     = "pv_kick_unlock",
+	[qstat_pv_kick_wake]       = "pv_kick_wake",
+	[qstat_pv_spurious_wakeup] = "pv_spurious_wakeup",
+	[qstat_pv_latency_kick]	   = "pv_latency_kick",
+	[qstat_pv_latency_wake]    = "pv_latency_wake",
+	[qstat_pv_lock_stealing]   = "pv_lock_stealing",
+	[qstat_pv_wait_again]      = "pv_wait_again",
+	[qstat_pv_wait_early]      = "pv_wait_early",
+	[qstat_pv_wait_head]       = "pv_wait_head",
+	[qstat_pv_wait_node]       = "pv_wait_node",
+	[qstat_reset_cnts]         = "reset_counters",
+};
+
+/*
+ * Per-cpu counters
+ */
+static DEFINE_PER_CPU(unsigned long, qstats[qstat_num]);
+static DEFINE_PER_CPU(u64, pv_kick_time);
+
+/*
+ * Function to read and return the qlock statistical counter values
+ *
+ * The following counters are handled specially:
+ * 1. qstat_pv_latency_kick
+ *    Average kick latency (ns) = pv_latency_kick/pv_kick_unlock
+ * 2. qstat_pv_latency_wake
+ *    Average wake latency (ns) = pv_latency_wake/pv_kick_wake
+ * 3. qstat_pv_hash_hops
+ *    Average hops/hash = pv_hash_hops/pv_kick_unlock
+ */
+static ssize_t qstat_read(struct file *file, char __user *user_buf,
+			  size_t count, loff_t *ppos)
+{
+	char buf[64];
+	int cpu, counter, len;
+	u64 stat = 0, kicks = 0;
+
+	/*
+	 * Get the counter ID stored in file->f_inode->i_private
+	 */
+	if (!file->f_inode) {
+		WARN_ON_ONCE(1);
+		return -EBADF;
+	}
+	counter = (long)(file->f_inode->i_private);
+
+	if (counter >= qstat_num)
+		return -EBADF;
+
+	for_each_possible_cpu(cpu) {
+		stat += per_cpu(qstats[counter], cpu);
+		/*
+		 * Need to sum additional counter for some of them
+		 */
+		switch (counter) {
+
+		case qstat_pv_latency_kick:
+		case qstat_pv_hash_hops:
+			kicks += per_cpu(qstats[qstat_pv_kick_unlock], cpu);
+			break;
+
+		case qstat_pv_latency_wake:
+			kicks += per_cpu(qstats[qstat_pv_kick_wake], cpu);
+			break;
+		}
+	}
+
+	if (counter == qstat_pv_hash_hops) {
+		u64 frac;
+
+		frac = 100ULL * do_div(stat, kicks);
+		frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
+
+		/*
+		 * Return a X.XX decimal number
+		 */
+		len = snprintf(buf, sizeof(buf) - 1, "%llu.%02llu\n", stat, frac);
+	} else {
+		/*
+		 * Round to the nearest ns
+		 */
+		if ((counter == qstat_pv_latency_kick) ||
+		    (counter == qstat_pv_latency_wake)) {
+			stat = 0;
+			if (kicks)
+				stat = DIV_ROUND_CLOSEST_ULL(stat, kicks);
+		}
+		len = snprintf(buf, sizeof(buf) - 1, "%llu\n", stat);
+	}
+
+	return simple_read_from_buffer(user_buf, count, ppos, buf, len);
+}
+
+/*
+ * Function to handle write request
+ *
+ * When counter = reset_cnts, reset all the counter values.
+ * Since the counter updates aren't atomic, the resetting is done twice
+ * to make sure that the counters are very likely to be all cleared.
+ */
+static ssize_t qstat_write(struct file *file, const char __user *user_buf,
+			   size_t count, loff_t *ppos)
+{
+	int cpu;
+
+	/*
+	 * Get the counter ID stored in file->f_inode->i_private
+	 */
+	if (!file->f_inode) {
+		WARN_ON_ONCE(1);
+		return -EBADF;
+	}
+	if ((long)(file->f_inode->i_private) != qstat_reset_cnts)
+		return count;
+
+	for_each_possible_cpu(cpu) {
+		int i;
+		unsigned long *ptr = per_cpu_ptr(qstats, cpu);
+
+		for (i = 0 ; i < qstat_num; i++)
+			WRITE_ONCE(ptr[i], 0);
+		for (i = 0 ; i < qstat_num; i++)
+			WRITE_ONCE(ptr[i], 0);
+	}
+	return count;
+}
+
+/*
+ * Debugfs data structures
+ */
+static const struct file_operations fops_qstat = {
+	.read = qstat_read,
+	.write = qstat_write,
+	.llseek = default_llseek,
+};
+
+/*
+ * Initialize debugfs for the qspinlock statistical counters
+ */
+static int __init init_qspinlock_stat(void)
+{
+	struct dentry *d_qstat = debugfs_create_dir("qlockstat", NULL);
+	int i;
+
+	if (!d_qstat) {
+		pr_warn("Could not create 'qlockstat' debugfs directory\n");
+		return 0;
+	}
+
+	/*
+	 * Create the debugfs files
+	 *
+	 * As reading from and writing to the stat files can be slow, only
+	 * root is allowed to do the read/write to limit impact to system
+	 * performance.
+	 */
+	for (i = 0; i < qstat_num; i++)
+		debugfs_create_file(qstat_names[i], 0400, d_qstat,
+				   (void *)(long)i, &fops_qstat);
+
+	debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
+			   (void *)(long)qstat_reset_cnts, &fops_qstat);
+	return 0;
+}
+fs_initcall(init_qspinlock_stat);
+
+/*
+ * Increment the PV qspinlock statistical counters
+ */
+static inline void qstat_inc(enum qlock_stats stat, bool cond)
+{
+	if (cond)
+		this_cpu_inc(qstats[stat]);
+}
+
+/*
+ * PV hash hop count
+ */
+static inline void qstat_hop(int hopcnt)
+{
+	this_cpu_add(qstats[qstat_pv_hash_hops], hopcnt);
+}
+
+/*
+ * Replacement function for pv_kick()
+ */
+static inline void __pv_kick(int cpu)
+{
+	u64 start = sched_clock();
+
+	per_cpu(pv_kick_time, cpu) = start;
+	pv_kick(cpu);
+	this_cpu_add(qstats[qstat_pv_latency_kick], sched_clock() - start);
+}
+
+/*
+ * Replacement function for pv_wait()
+ */
+static inline void __pv_wait(u8 *ptr, u8 val)
+{
+	u64 *pkick_time = this_cpu_ptr(&pv_kick_time);
+
+	*pkick_time = 0;
+	pv_wait(ptr, val);
+	if (*pkick_time) {
+		this_cpu_add(qstats[qstat_pv_latency_wake],
+			     sched_clock() - *pkick_time);
+		qstat_inc(qstat_pv_kick_wake, true);
+	}
+}
+
+#define pv_kick(c)	__pv_kick(c)
+#define pv_wait(p, v)	__pv_wait(p, v)
+
+/*
+ * PV unfair trylock count tracking function
+ */
+static inline int qstat_spin_steal_lock(struct qspinlock *lock)
+{
+	int ret = pv_queued_spin_steal_lock(lock);
+
+	qstat_inc(qstat_pv_lock_stealing, ret);
+	return ret;
+}
+#undef  queued_spin_trylock
+#define queued_spin_trylock(l)	qstat_spin_steal_lock(l)
+
+#else /* CONFIG_QUEUED_LOCK_STAT */
+
+static inline void qstat_inc(enum qlock_stats stat, bool cond)	{ }
+static inline void qstat_hop(int hopcnt)			{ }
+
+#endif /* CONFIG_QUEUED_LOCK_STAT */

kernel/sched/core.c

@@ -1905,6 +1905,97 @@ static void ttwu_queue(struct task_struct *p, int cpu)
 	raw_spin_unlock(&rq->lock);
 }
 
+/*
+ * Notes on Program-Order guarantees on SMP systems.
+ *
+ *  MIGRATION
+ *
+ * The basic program-order guarantee on SMP systems is that when a task [t]
+ * migrates, all its activity on its old cpu [c0] happens-before any subsequent
+ * execution on its new cpu [c1].
+ *
+ * For migration (of runnable tasks) this is provided by the following means:
+ *
+ *  A) UNLOCK of the rq(c0)->lock scheduling out task t
+ *  B) migration for t is required to synchronize *both* rq(c0)->lock and
+ *     rq(c1)->lock (if not at the same time, then in that order).
+ *  C) LOCK of the rq(c1)->lock scheduling in task
+ *
+ * Transitivity guarantees that B happens after A and C after B.
+ * Note: we only require RCpc transitivity.
+ * Note: the cpu doing B need not be c0 or c1
+ *
+ * Example:
+ *
+ *   CPU0            CPU1            CPU2
+ *
+ *   LOCK rq(0)->lock
+ *   sched-out X
+ *   sched-in Y
+ *   UNLOCK rq(0)->lock
+ *
+ *                                   LOCK rq(0)->lock // orders against CPU0
+ *                                   dequeue X
+ *                                   UNLOCK rq(0)->lock
+ *
+ *                                   LOCK rq(1)->lock
+ *                                   enqueue X
+ *                                   UNLOCK rq(1)->lock
+ *
+ *                   LOCK rq(1)->lock // orders against CPU2
+ *                   sched-out Z
+ *                   sched-in X
+ *                   UNLOCK rq(1)->lock
+ *
+ *
+ *  BLOCKING -- aka. SLEEP + WAKEUP
+ *
+ * For blocking we (obviously) need to provide the same guarantee as for
+ * migration. However the means are completely different as there is no lock
+ * chain to provide order. Instead we do:
+ *
+ *   1) smp_store_release(X->on_cpu, 0)
+ *   2) smp_cond_acquire(!X->on_cpu)
+ *
+ * Example:
+ *
+ *   CPU0 (schedule)  CPU1 (try_to_wake_up) CPU2 (schedule)
+ *
+ *   LOCK rq(0)->lock LOCK X->pi_lock
+ *   dequeue X
+ *   sched-out X
+ *   smp_store_release(X->on_cpu, 0);
+ *
+ *                    smp_cond_acquire(!X->on_cpu);
+ *                    X->state = WAKING
+ *                    set_task_cpu(X,2)
+ *
+ *                    LOCK rq(2)->lock
+ *                    enqueue X
+ *                    X->state = RUNNING
+ *                    UNLOCK rq(2)->lock
+ *
+ *                                          LOCK rq(2)->lock // orders against CPU1
+ *                                          sched-out Z
+ *                                          sched-in X
+ *                                          UNLOCK rq(2)->lock
+ *
+ *                    UNLOCK X->pi_lock
+ *   UNLOCK rq(0)->lock
+ *
+ *
+ * However; for wakeups there is a second guarantee we must provide, namely we
+ * must observe the state that lead to our wakeup. That is, not only must our
+ * task observe its own prior state, it must also observe the stores prior to
+ * its wakeup.
+ *
+ * This means that any means of doing remote wakeups must order the CPU doing
+ * the wakeup against the CPU the task is going to end up running on. This,
+ * however, is already required for the regular Program-Order guarantee above,
+ * since the waking CPU is the one issueing the ACQUIRE (smp_cond_acquire).
+ *
+ */
+
 /**
  * try_to_wake_up - wake up a thread
  * @p: the thread to be awakened
@@ -1968,19 +2059,13 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	/*
 	 * If the owning (remote) cpu is still in the middle of schedule() with
 	 * this task as prev, wait until its done referencing the task.
-	 */
-	while (p->on_cpu)
-		cpu_relax();
-	/*
-	 * Combined with the control dependency above, we have an effective
-	 * smp_load_acquire() without the need for full barriers.
 	 *
 	 * Pairs with the smp_store_release() in finish_lock_switch().
 	 *
 	 * This ensures that tasks getting woken will be fully ordered against
 	 * their previous state and preserve Program Order.
 	 */
-	smp_rmb();
+	smp_cond_acquire(!p->on_cpu);
 
 	p->sched_contributes_to_load = !!task_contributes_to_load(p);
 	p->state = TASK_WAKING;

kernel/sched/sched.h

@@ -1076,7 +1076,7 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
 	 * In particular, the load of prev->state in finish_task_switch() must
 	 * happen before this.
 	 *
-	 * Pairs with the control dependency and rmb in try_to_wake_up().
+	 * Pairs with the smp_cond_acquire() in try_to_wake_up().
 	 */
 	smp_store_release(&prev->on_cpu, 0);
 #endif

lib/atomic64_test.c

@@ -27,6 +27,65 @@ do {								\
 		(unsigned long long)r);				\
 } while (0)
 
+/*
+ * Test for a atomic operation family,
+ * @test should be a macro accepting parameters (bit, op, ...)
+ */
+
+#define FAMILY_TEST(test, bit, op, args...)	\
+do {						\
+	test(bit, op, ##args);		\
+	test(bit, op##_acquire, ##args);	\
+	test(bit, op##_release, ##args);	\
+	test(bit, op##_relaxed, ##args);	\
+} while (0)
+
+#define TEST_RETURN(bit, op, c_op, val)				\
+do {								\
+	atomic##bit##_set(&v, v0);				\
+	r = v0;							\
+	r c_op val;						\
+	BUG_ON(atomic##bit##_##op(val, &v) != r);		\
+	BUG_ON(atomic##bit##_read(&v) != r);			\
+} while (0)
+
+#define RETURN_FAMILY_TEST(bit, op, c_op, val)			\
+do {								\
+	FAMILY_TEST(TEST_RETURN, bit, op, c_op, val);		\
+} while (0)
+
+#define TEST_ARGS(bit, op, init, ret, expect, args...)		\
+do {								\
+	atomic##bit##_set(&v, init);				\
+	BUG_ON(atomic##bit##_##op(&v, ##args) != ret);		\
+	BUG_ON(atomic##bit##_read(&v) != expect);		\
+} while (0)
+
+#define XCHG_FAMILY_TEST(bit, init, new)				\
+do {									\
+	FAMILY_TEST(TEST_ARGS, bit, xchg, init, init, new, new);	\
+} while (0)
+
+#define CMPXCHG_FAMILY_TEST(bit, init, new, wrong)			\
+do {									\
+	FAMILY_TEST(TEST_ARGS, bit, cmpxchg, 				\
+			init, init, new, init, new);			\
+	FAMILY_TEST(TEST_ARGS, bit, cmpxchg,				\
+			init, init, init, wrong, new);			\
+} while (0)
+
+#define INC_RETURN_FAMILY_TEST(bit, i)			\
+do {							\
+	FAMILY_TEST(TEST_ARGS, bit, inc_return,		\
+			i, (i) + one, (i) + one);	\
+} while (0)
+
+#define DEC_RETURN_FAMILY_TEST(bit, i)			\
+do {							\
+	FAMILY_TEST(TEST_ARGS, bit, dec_return,		\
+			i, (i) - one, (i) - one);	\
+} while (0)
+
 static __init void test_atomic(void)
 {
 	int v0 = 0xaaa31337;
@@ -45,6 +104,18 @@ static __init void test_atomic(void)
 	TEST(, and, &=, v1);
 	TEST(, xor, ^=, v1);
 	TEST(, andnot, &= ~, v1);
+
+	RETURN_FAMILY_TEST(, add_return, +=, onestwos);
+	RETURN_FAMILY_TEST(, add_return, +=, -one);
+	RETURN_FAMILY_TEST(, sub_return, -=, onestwos);
+	RETURN_FAMILY_TEST(, sub_return, -=, -one);
+
+	INC_RETURN_FAMILY_TEST(, v0);
+	DEC_RETURN_FAMILY_TEST(, v0);
+
+	XCHG_FAMILY_TEST(, v0, v1);
+	CMPXCHG_FAMILY_TEST(, v0, v1, onestwos);
+
 }
 
 #define INIT(c) do { atomic64_set(&v, c); r = c; } while (0)
@@ -74,59 +145,26 @@ static __init void test_atomic64(void)
 	TEST(64, xor, ^=, v1);
 	TEST(64, andnot, &= ~, v1);
 
-	INIT(v0);
-	r += onestwos;
-	BUG_ON(atomic64_add_return(onestwos, &v) != r);
-	BUG_ON(v.counter != r);
-
-	INIT(v0);
-	r += -one;
-	BUG_ON(atomic64_add_return(-one, &v) != r);
-	BUG_ON(v.counter != r);
-
-	INIT(v0);
-	r -= onestwos;
-	BUG_ON(atomic64_sub_return(onestwos, &v) != r);
-	BUG_ON(v.counter != r);
-
-	INIT(v0);
-	r -= -one;
-	BUG_ON(atomic64_sub_return(-one, &v) != r);
-	BUG_ON(v.counter != r);
+	RETURN_FAMILY_TEST(64, add_return, +=, onestwos);
+	RETURN_FAMILY_TEST(64, add_return, +=, -one);
+	RETURN_FAMILY_TEST(64, sub_return, -=, onestwos);
+	RETURN_FAMILY_TEST(64, sub_return, -=, -one);
 
 	INIT(v0);
 	atomic64_inc(&v);
 	r += one;
 	BUG_ON(v.counter != r);
 
-	INIT(v0);
-	r += one;
-	BUG_ON(atomic64_inc_return(&v) != r);
-	BUG_ON(v.counter != r);
-
 	INIT(v0);
 	atomic64_dec(&v);
 	r -= one;
 	BUG_ON(v.counter != r);
 
-	INIT(v0);
-	r -= one;
-	BUG_ON(atomic64_dec_return(&v) != r);
-	BUG_ON(v.counter != r);
-
-	INIT(v0);
-	BUG_ON(atomic64_xchg(&v, v1) != v0);
-	r = v1;
-	BUG_ON(v.counter != r);
-
-	INIT(v0);
-	BUG_ON(atomic64_cmpxchg(&v, v0, v1) != v0);
-	r = v1;
-	BUG_ON(v.counter != r);
+	INC_RETURN_FAMILY_TEST(64, v0);
+	DEC_RETURN_FAMILY_TEST(64, v0);
 
-	INIT(v0);
-	BUG_ON(atomic64_cmpxchg(&v, v2, v1) != v0);
-	BUG_ON(v.counter != r);
+	XCHG_FAMILY_TEST(64, v0, v1);
+	CMPXCHG_FAMILY_TEST(64, v0, v1, v2);
 
 	INIT(v0);
 	BUG_ON(atomic64_add_unless(&v, one, v0));