/* * Queued spinlock * * 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. * * (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@hpe.com> *          Peter Zijlstra <peterz@infradead.org> */ #ifndef _GEN_PV_LOCK_SLOWPATH #include <linux/smp.h>#include <linux/bug.h>#include <linux/cpumask.h>#include <linux/percpu.h>#include <linux/hardirq.h>#include <linux/mutex.h>#include <linux/prefetch.h>#include <asm/byteorder.h>#include <asm/qspinlock.h> /* * The basic principle of a queue-based spinlock can best be understood * by studying a classic queue-based spinlock implementation called the * MCS lock. The paper below provides a good description for this kind * of lock. * * http://www.cise.ufl.edu/tr/DOC/REP-1992-71.pdf * * This queued spinlock implementation is based on the MCS lock, however to make * it fit the 4 bytes we assume spinlock_t to be, and preserve its existing * API, we must modify it somehow. * * In particular; where the traditional MCS lock consists of a tail pointer * (8 bytes) and needs the next pointer (another 8 bytes) of its own node to * unlock the next pending (next->locked), we compress both these: {tail, * next->locked} into a single u32 value. * * Since a spinlock disables recursion of its own context and there is a limit * to the contexts that can nest; namely: task, softirq, hardirq, nmi. As there * are at most 4 nesting levels, it can be encoded by a 2-bit number. Now * we can encode the tail by combining the 2-bit nesting level with the cpu * number. With one byte for the lock value and 3 bytes for the tail, only a * 32-bit word is now needed. Even though we only need 1 bit for the lock, * we extend it to a full byte to achieve better performance for architectures * that support atomic byte write. * * We also change the first spinner to spin on the lock bit instead of its * node; whereby avoiding the need to carry a node from lock to unlock, and * preserving existing lock API. This also makes the unlock code simpler and * faster. * * N.B. The current implementation only supports architectures that allow *      atomic operations on smaller 8-bit and 16-bit data types. * */ #include "mcs_spinlock.h" #ifdef CONFIG_PARAVIRT_SPINLOCKS#define MAX_NODES	8#else#define MAX_NODES	4#endif /* * Per-CPU queue node structures; we can never have more than 4 nested * contexts: task, softirq, hardirq, nmi. * * Exactly fits one 64-byte cacheline on a 64-bit architecture. * * PV doubles the storage and uses the second cacheline for PV state. */static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]); /* * We must be able to distinguish between no-tail and the tail at 0:0, * therefore increment the cpu number by one. */ static inline __pure u32 encode_tail(int cpu, int idx){	u32 tail; #ifdef CONFIG_DEBUG_SPINLOCK	BUG_ON(idx > 3);#endif	tail  = (cpu + 1) << _Q_TAIL_CPU_OFFSET;	tail |= idx << _Q_TAIL_IDX_OFFSET; /* assume < 4 */ 	return tail;} static inline __pure struct mcs_spinlock *decode_tail(u32 tail){	int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;	int idx = (tail &  _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET; 	return per_cpu_ptr(&mcs_nodes[idx], cpu);} #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK) /* * By using the whole 2nd least significant byte for the pending bit, we * can allow better optimization of the lock acquisition for the pending * bit holder. * * This internal structure is also used by the set_locked function which * is not restricted to _Q_PENDING_BITS == 8. */struct __qspinlock {	union {		atomic_t val;#ifdef __LITTLE_ENDIAN		struct {			u8	locked;			u8	pending;		};		struct {			u16	locked_pending;			u16	tail;		};#else		struct {			u16	tail;			u16	locked_pending;		};		struct {			u8	reserved[2];			u8	pending;			u8	locked;		};#endif	};}; #if _Q_PENDING_BITS == 8/** * clear_pending_set_locked - take ownership and clear the pending bit. * @lock: Pointer to queued spinlock structure * * *,1,0 -> *,0,1 * * Lock stealing is not allowed if this function is used. */static __always_inline void clear_pending_set_locked(struct qspinlock *lock){	struct __qspinlock *l = (void *)lock; 	WRITE_ONCE(l->locked_pending, _Q_LOCKED_VAL);} /* * xchg_tail - Put in the new queue tail code word & retrieve previous one * @lock : Pointer to queued spinlock structure * @tail : The new queue tail code word * Return: The previous queue tail code word * * xchg(lock, tail) * * p,*,* -> n,*,* ; prev = xchg(lock, node) */static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail){	struct __qspinlock *l = (void *)lock; 	/*	 * 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 */ /** * clear_pending_set_locked - take ownership and clear the pending bit. * @lock: Pointer to queued spinlock structure * * *,1,0 -> *,0,1 */static __always_inline void clear_pending_set_locked(struct qspinlock *lock){	atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);} /** * xchg_tail - Put in the new queue tail code word & retrieve previous one * @lock : Pointer to queued spinlock structure * @tail : The new queue tail code word * Return: The previous queue tail code word * * xchg(lock, tail) * * p,*,* -> n,*,* ; prev = xchg(lock, node) */static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail){	u32 old, new, val = atomic_read(&lock->val); 	for (;;) {		new = (val & _Q_LOCKED_PENDING_MASK) | tail;		/*		 * 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; 		val = old;	}	return old;}#endif /* _Q_PENDING_BITS == 8 */ /** * set_locked - Set the lock bit and own the lock * @lock: Pointer to queued spinlock structure * * *,*,0 -> *,0,1 */static __always_inline void set_locked(struct qspinlock *lock){	struct __qspinlock *l = (void *)lock; 	WRITE_ONCE(l->locked, _Q_LOCKED_VAL);}  /* * Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for * all the PV callbacks. */ static __always_inline void __pv_init_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 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_or_lock	__pv_wait_head_or_lock #ifdef CONFIG_PARAVIRT_SPINLOCKS#define queued_spin_lock_slowpath	native_queued_spin_lock_slowpath#endif /* * Various notes on spin_is_locked() and spin_unlock_wait(), which are * 'interesting' functions: * * PROBLEM: some architectures have an interesting issue with atomic ACQUIRE * operations in that the ACQUIRE applies to the LOAD _not_ the STORE (ARM64, * PPC). Also qspinlock has a similar issue per construction, the setting of * the locked byte can be unordered acquiring the lock proper. * * This gets to be 'interesting' in the following cases, where the /should/s * end up false because of this issue. * * * CASE 1: * * So the spin_is_locked() correctness issue comes from something like: * *   CPU0				CPU1 * *   global_lock();			local_lock(i) *     spin_lock(&G)			  spin_lock(&L[i]) *     for (i)				  if (!spin_is_locked(&G)) { *       spin_unlock_wait(&L[i]);	    smp_acquire__after_ctrl_dep(); *					    return; *					  } *					  // deal with fail * * Where it is important CPU1 sees G locked or CPU0 sees L[i] locked such * that there is exclusion between the two critical sections. * * The load from spin_is_locked(&G) /should/ be constrained by the ACQUIRE from * spin_lock(&L[i]), and similarly the load(s) from spin_unlock_wait(&L[i]) * /should/ be constrained by the ACQUIRE from spin_lock(&G). * * Similarly, later stuff is constrained by the ACQUIRE from CTRL+RMB. * * * CASE 2: * * For spin_unlock_wait() there is a second correctness issue, namely: * *   CPU0				CPU1 * *   flag = set; *   smp_mb();				spin_lock(&l) *   spin_unlock_wait(&l);		if (!flag) *					  // add to lockless list *					spin_unlock(&l); *   // iterate lockless list * * Which wants to ensure that CPU1 will stop adding bits to the list and CPU0 * will observe the last entry on the list (if spin_unlock_wait() had ACQUIRE * semantics etc..) * * Where flag /should/ be ordered against the locked store of l. */ /* * queued_spin_lock_slowpath() can (load-)ACQUIRE the lock before * issuing an _unordered_ store to set _Q_LOCKED_VAL. * * This means that the store can be delayed, but no later than the * store-release from the unlock. This means that simply observing * _Q_LOCKED_VAL is not sufficient to determine if the lock is acquired. * * There are two paths that can issue the unordered store: * *  (1) clear_pending_set_locked():	*,1,0 -> *,0,1 * *  (2) set_locked():			t,0,0 -> t,0,1 ; t != 0 *      atomic_cmpxchg_relaxed():	t,0,0 -> 0,0,1 * * However, in both cases we have other !0 state we've set before to queue * ourseves: * * For (1) we have the atomic_cmpxchg_acquire() that set _Q_PENDING_VAL, our * load is constrained by that ACQUIRE to not pass before that, and thus must * observe the store. * * For (2) we have a more intersting scenario. We enqueue ourselves using * xchg_tail(), which ends up being a RELEASE. This in itself is not * sufficient, however that is followed by an smp_cond_acquire() on the same * word, giving a RELEASE->ACQUIRE ordering. This again constrains our load and * guarantees we must observe that store. * * Therefore both cases have other !0 state that is observable before the * unordered locked byte store comes through. This means we can use that to * wait for the lock store, and then wait for an unlock. */#ifndef queued_spin_unlock_waitvoid queued_spin_unlock_wait(struct qspinlock *lock){	u32 val; 	for (;;) {		val = atomic_read(&lock->val); 		if (!val) /* not locked, we're done */			goto done; 		if (val & _Q_LOCKED_MASK) /* locked, go wait for unlock */			break; 		/* not locked, but pending, wait until we observe the lock */		cpu_relax();	} 	/* any unlock is good */	while (atomic_read(&lock->val) & _Q_LOCKED_MASK)		cpu_relax(); done:	smp_acquire__after_ctrl_dep();}EXPORT_SYMBOL(queued_spin_unlock_wait);#endif #endif /* _GEN_PV_LOCK_SLOWPATH */ /** * queued_spin_lock_slowpath - acquire the queued spinlock * @lock: Pointer to queued spinlock structure * @val: Current value of the queued spinlock 32-bit word * * (queue tail, pending bit, lock value) * *              fast     :    slow                                  :    unlock *                       :                                          : * uncontended  (0,0,0) -:--> (0,0,1) ------------------------------:--> (*,*,0) *                       :       | ^--------.------.             /  : *                       :       v           \      \            |  : * pending               :    (0,1,1) +--> (0,1,0)   \           |  : *                       :       | ^--'              |           |  : *                       :       v                   |           |  : * uncontended           :    (n,x,y) +--> (n,0,0) --'           |  : *   queue               :       | ^--'                          |  : *                       :       v                               |  : * contended             :    (*,x,y) +--> (*,0,0) ---> (*,0,1) -'  : *   queue               :         ^--'                             : */void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val){	struct mcs_spinlock *prev, *next, *node;	u32 new, old, tail;	int idx; 	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS)); 	if (pv_enabled())		goto queue; 	if (virt_spin_lock(lock))		return; 	/*	 * wait for in-progress pending->locked hand-overs	 *	 * 0,1,0 -> 0,0,1	 */	if (val == _Q_PENDING_VAL) {		while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)			cpu_relax();	} 	/*	 * trylock || pending	 *	 * 0,0,0 -> 0,0,1 ; trylock	 * 0,0,1 -> 0,1,1 ; pending	 */	for (;;) {		/*		 * If we observe any contention; queue.		 */		if (val & ~_Q_LOCKED_MASK)			goto queue; 		new = _Q_LOCKED_VAL;		if (val == new)			new |= _Q_PENDING_VAL; 		/*		 * 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; 		val = old;	} 	/*	 * we won the trylock	 */	if (new == _Q_LOCKED_VAL)		return; 	/*	 * we're pending, wait for the owner to go away.	 *	 * *,1,1 -> *,1,0	 *	 * this wait loop must be a load-acquire such that we match the	 * store-release that clears the locked bit and create lock	 * sequentiality; this is because not all clear_pending_set_locked()	 * implementations imply full barriers.	 */	smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK)); 	/*	 * take ownership and clear the pending bit.	 *	 * *,1,0 -> *,0,1	 */	clear_pending_set_locked(lock);	return; 	/*	 * End of pending bit optimistic spinning and beginning of MCS	 * queuing.	 */queue:	node = this_cpu_ptr(&mcs_nodes[0]);	idx = node->count++;	tail = encode_tail(smp_processor_id(), idx); 	node += idx;	node->locked = 0;	node->next = NULL;

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