kernel/locking/qspinlock_paravirt.h - linux - Git at Google

 #ifndef _GEN_PV_LOCK_SLOWPATH
 #error "do not include this file"
 #endif

 #include <linux/hash.h>
 #include <linux/bootmem.h>
 #include <linux/debug_locks.h>

 /*
  * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
  * of spinning them.
  *
  * This relies on the architecture to provide two paravirt hypercalls:
  *
  *   pv_wait(u8 *ptr, u8 val) -- suspends the vcpu if *ptr == val
  *   pv_kick(cpu)             -- wakes a suspended vcpu
  *
  * Using these we implement __pv_queued_spin_lock_slowpath() and
  * __pv_queued_spin_unlock() to replace native_queued_spin_lock_slowpath() and
  * native_queued_spin_unlock().
  */

 #define _Q_SLOW_VAL	(3U << _Q_LOCKED_OFFSET)

 enum vcpu_state {
 	vcpu_running = 0,
 	vcpu_halted,
 };

 struct pv_node {
 	struct mcs_spinlock	mcs;
 	struct mcs_spinlock	__res[3];

 	int			cpu;
 	u8			state;
 };

 /*
  * Lock and MCS node addresses hash table for fast lookup
  *
  * Hashing is done on a per-cacheline basis to minimize the need to access
  * more than one cacheline.
  *
  * Dynamically allocate a hash table big enough to hold at least 4X the
  * number of possible cpus in the system. Allocation is done on page
  * granularity. So the minimum number of hash buckets should be at least
  * 256 (64-bit) or 512 (32-bit) to fully utilize a 4k page.
  *
  * Since we should not be holding locks from NMI context (very rare indeed) the
  * max load factor is 0.75, which is around the point where open addressing
  * breaks down.
  *
  */
 struct pv_hash_entry {
 	struct qspinlock *lock;
 	struct pv_node   *node;
 };

 #define PV_HE_PER_LINE	(SMP_CACHE_BYTES / sizeof(struct pv_hash_entry))
 #define PV_HE_MIN	(PAGE_SIZE / sizeof(struct pv_hash_entry))

 static struct pv_hash_entry *pv_lock_hash;
 static unsigned int pv_lock_hash_bits __read_mostly;

 /*
  * Allocate memory for the PV qspinlock hash buckets
  *
  * This function should be called from the paravirt spinlock initialization
  * routine.
  */
 void __init __pv_init_lock_hash(void)
 {
 	int pv_hash_size = ALIGN(4 * num_possible_cpus(), PV_HE_PER_LINE);

 	if (pv_hash_size < PV_HE_MIN)
 		pv_hash_size = PV_HE_MIN;

 	/*
 	 * Allocate space from bootmem which should be page-size aligned
 	 * and hence cacheline aligned.
 	 */
 	pv_lock_hash = alloc_large_system_hash("PV qspinlock",
 					       sizeof(struct pv_hash_entry),
 					       pv_hash_size, 0, HASH_EARLY,
 					       &pv_lock_hash_bits, NULL,
 					       pv_hash_size, pv_hash_size);
 }

 #define for_each_hash_entry(he, offset, hash)						\
 	for (hash &= ~(PV_HE_PER_LINE - 1), he = &pv_lock_hash[hash], offset = 0;	\
 	     offset < (1 << pv_lock_hash_bits);						\
 	     offset++, he = &pv_lock_hash[(hash + offset) & ((1 << pv_lock_hash_bits) - 1)])

 static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
 {
 	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
 	struct pv_hash_entry *he;

 	for_each_hash_entry(he, offset, hash) {
 		if (!cmpxchg(&he->lock, NULL, lock)) {
 			WRITE_ONCE(he->node, node);
 			return &he->lock;
 		}
 	}
 	/*
 	 * Hard assume there is a free entry for us.
 	 *
 	 * This is guaranteed by ensuring every blocked lock only ever consumes
 	 * a single entry, and since we only have 4 nesting levels per CPU
 	 * and allocated 4*nr_possible_cpus(), this must be so.
 	 *
 	 * The single entry is guaranteed by having the lock owner unhash
 	 * before it releases.
 	 */
 	BUG();
 }

 static struct pv_node *pv_unhash(struct qspinlock *lock)
 {
 	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
 	struct pv_hash_entry *he;
 	struct pv_node *node;

 	for_each_hash_entry(he, offset, hash) {
 		if (READ_ONCE(he->lock) == lock) {
 			node = READ_ONCE(he->node);
 			WRITE_ONCE(he->lock, NULL);
 			return node;
 		}
 	}
 	/*
 	 * Hard assume we'll find an entry.
 	 *
 	 * This guarantees a limited lookup time and is itself guaranteed by
 	 * having the lock owner do the unhash -- IFF the unlock sees the
 	 * SLOW flag, there MUST be a hash entry.
 	 */
 	BUG();
 }

 /*
  * Initialize the PV part of the mcs_spinlock node.
  */
 static void pv_init_node(struct mcs_spinlock *node)
 {
 	struct pv_node *pn = (struct pv_node *)node;

 	BUILD_BUG_ON(sizeof(struct pv_node) > 5*sizeof(struct mcs_spinlock));

 	pn->cpu = smp_processor_id();
 	pn->state = vcpu_running;
 }

 /*
  * Wait for node->locked to become true, halt the vcpu after a short spin.
  * pv_kick_node() is used to wake the vcpu again.
  */
 static void pv_wait_node(struct mcs_spinlock *node)
 {
 	struct pv_node *pn = (struct pv_node *)node;
 	int loop;

 	for (;;) {
 		for (loop = SPIN_THRESHOLD; loop; loop--) {
 			if (READ_ONCE(node->locked))
 				return;
 			cpu_relax();
 		}

 		/*
 		 * Order pn->state vs pn->locked thusly:
 		 *
 		 * [S] pn->state = vcpu_halted	  [S] next->locked = 1
 		 *     MB			      MB
 		 * [L] pn->locked		[RmW] pn->state = vcpu_running
 		 *
 		 * Matches the xchg() from pv_kick_node().
 		 */
 		smp_store_mb(pn->state, vcpu_halted);

 		if (!READ_ONCE(node->locked))
 			pv_wait(&pn->state, vcpu_halted);

 		/*
 		 * Reset the vCPU state to avoid unncessary CPU kicking
 		 */
 		WRITE_ONCE(pn->state, vcpu_running);

 		/*
 		 * If the locked flag is still not set after wakeup, it is a
 		 * spurious wakeup and the vCPU should wait again. However,
 		 * there is a pretty high overhead for CPU halting and kicking.
 		 * So it is better to spin for a while in the hope that the
 		 * MCS lock will be released soon.
 		 */
 	}
 	/*
 	 * By now our node->locked should be 1 and our caller will not actually
 	 * spin-wait for it. We do however rely on our caller to do a
 	 * load-acquire for us.
 	 */
 }

 /*
  * Called after setting next->locked = 1, used to wake those stuck in
  * pv_wait_node().
  */
 static void pv_kick_node(struct mcs_spinlock *node)
 {
 	struct pv_node *pn = (struct pv_node *)node;

 	/*
 	 * Note that because node->locked is already set, this actual
 	 * mcs_spinlock entry could be re-used already.
 	 *
 	 * This should be fine however, kicking people for no reason is
 	 * harmless.
 	 *
 	 * See the comment in pv_wait_node().
 	 */
 	if (xchg(&pn->state, vcpu_running) == vcpu_halted)
 		pv_kick(pn->cpu);
 }

 /*
  * Wait for l->locked to become clear; halt the vcpu after a short spin.
  * __pv_queued_spin_unlock() will wake us.
  */
 static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
 {
 	struct pv_node *pn = (struct pv_node *)node;
 	struct __qspinlock *l = (void *)lock;
 	struct qspinlock **lp = NULL;
 	int loop;

 	for (;;) {
 		for (loop = SPIN_THRESHOLD; loop; loop--) {
 			if (!READ_ONCE(l->locked))
 				return;
 			cpu_relax();
 		}

 		WRITE_ONCE(pn->state, vcpu_halted);
 		if (!lp) { /* ONCE */
 			lp = pv_hash(lock, pn);
 			/*
 			 * lp must be set before setting _Q_SLOW_VAL
 			 *
 			 * [S] lp = lock                [RmW] l = l->locked = 0
 			 *     MB                             MB
 			 * [S] l->locked = _Q_SLOW_VAL  [L]   lp
 			 *
 			 * Matches the cmpxchg() in __pv_queued_spin_unlock().
 			 */
 			if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL)) {
 				/*
 				 * The lock is free and _Q_SLOW_VAL has never
 				 * been set. Therefore we need to unhash before
 				 * getting the lock.
 				 */
 				WRITE_ONCE(*lp, NULL);
 				return;
 			}
 		}
 		pv_wait(&l->locked, _Q_SLOW_VAL);

 		/*
 		 * The unlocker should have freed the lock before kicking the
 		 * CPU. So if the lock is still not free, it is a spurious
 		 * wakeup and so the vCPU should wait again after spinning for
 		 * a while.
 		 */
 	}

 	/*
 	 * Lock is unlocked now; the caller will acquire it without waiting.
 	 * As with pv_wait_node() we rely on the caller to do a load-acquire
 	 * for us.
 	 */
 }

 /*
  * PV version of the unlock function to be used in stead of
  * queued_spin_unlock().
  */
 __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
 {
 	struct __qspinlock *l = (void *)lock;
 	struct pv_node *node;
 	u8 lockval = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);

 	/*
 	 * We must not unlock if SLOW, because in that case we must first
 	 * unhash. Otherwise it would be possible to have multiple @lock
 	 * entries, which would be BAD.
 	 */
 	if (likely(lockval == _Q_LOCKED_VAL))
 		return;

 	if (unlikely(lockval != _Q_SLOW_VAL)) {
 		if (debug_locks_silent)
 			return;
 		WARN(1, "pvqspinlock: lock %p has corrupted value 0x%x!\n", lock, atomic_read(&lock->val));
 		return;
 	}

 	/*
 	 * Since the above failed to release, this must be the SLOW path.
 	 * Therefore start by looking up the blocked node and unhashing it.
 	 */
 	node = pv_unhash(lock);

 	/*
 	 * Now that we have a reference to the (likely) blocked pv_node,
 	 * release the lock.
 	 */
 	smp_store_release(&l->locked, 0);

 	/*
 	 * At this point the memory pointed at by lock can be freed/reused,
 	 * however we can still use the pv_node to kick the CPU.
 	 */
 	if (READ_ONCE(node->state) == vcpu_halted)
 		pv_kick(node->cpu);
 }
 /*
  * Include the architecture specific callee-save thunk of the
  * __pv_queued_spin_unlock(). This thunk is put together with
  * __pv_queued_spin_unlock() near the top of the file to make sure
  * that the callee-save thunk and the real unlock function are close
  * to each other sharing consecutive instruction cachelines.
  */
 #include <asm/qspinlock_paravirt.h>
	#ifndef _GEN_PV_LOCK_SLOWPATH
	#error "do not include this file"
	#endif

	#include <linux/hash.h>
	#include <linux/bootmem.h>
	#include <linux/debug_locks.h>

	/*
	* Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
	* of spinning them.
	*
	* This relies on the architecture to provide two paravirt hypercalls:
	*
	* pv_wait(u8 ptr, u8 val) -- suspends the vcpu if ptr == val
	* pv_kick(cpu) -- wakes a suspended vcpu
	*
	* Using these we implement __pv_queued_spin_lock_slowpath() and
	* __pv_queued_spin_unlock() to replace native_queued_spin_lock_slowpath() and
	* native_queued_spin_unlock().
	*/

	#define _Q_SLOW_VAL (3U << _Q_LOCKED_OFFSET)

	enum vcpu_state {
	vcpu_running = 0,
	vcpu_halted,
	};

	struct pv_node {
	struct mcs_spinlock mcs;
	struct mcs_spinlock __res[3];

	int cpu;
	u8 state;
	};

	/*
	* Lock and MCS node addresses hash table for fast lookup
	*
	* Hashing is done on a per-cacheline basis to minimize the need to access
	* more than one cacheline.
	*
	* Dynamically allocate a hash table big enough to hold at least 4X the
	* number of possible cpus in the system. Allocation is done on page
	* granularity. So the minimum number of hash buckets should be at least
	* 256 (64-bit) or 512 (32-bit) to fully utilize a 4k page.
	*
	* Since we should not be holding locks from NMI context (very rare indeed) the
	* max load factor is 0.75, which is around the point where open addressing
	* breaks down.
	*
	*/
	struct pv_hash_entry {
	struct qspinlock *lock;
	struct pv_node *node;
	};

	#define PV_HE_PER_LINE (SMP_CACHE_BYTES / sizeof(struct pv_hash_entry))
	#define PV_HE_MIN (PAGE_SIZE / sizeof(struct pv_hash_entry))

	static struct pv_hash_entry *pv_lock_hash;
	static unsigned int pv_lock_hash_bits __read_mostly;

	/*
	* Allocate memory for the PV qspinlock hash buckets
	*
	* This function should be called from the paravirt spinlock initialization
	* routine.
	*/
	void __init __pv_init_lock_hash(void)
	{
	int pv_hash_size = ALIGN(4 * num_possible_cpus(), PV_HE_PER_LINE);

	if (pv_hash_size < PV_HE_MIN)
	pv_hash_size = PV_HE_MIN;

	/*
	* Allocate space from bootmem which should be page-size aligned
	* and hence cacheline aligned.
	*/
	pv_lock_hash = alloc_large_system_hash("PV qspinlock",
	sizeof(struct pv_hash_entry),
	pv_hash_size, 0, HASH_EARLY,
	&pv_lock_hash_bits, NULL,
	pv_hash_size, pv_hash_size);
	}

	#define for_each_hash_entry(he, offset, hash) \
	for (hash &= ~(PV_HE_PER_LINE - 1), he = &pv_lock_hash[hash], offset = 0; \
	offset < (1 << pv_lock_hash_bits); \
	offset++, he = &pv_lock_hash[(hash + offset) & ((1 << pv_lock_hash_bits) - 1)])

	static struct qspinlock *pv_hash(struct qspinlock lock, struct pv_node *node)
	{
	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
	struct pv_hash_entry *he;

	for_each_hash_entry(he, offset, hash) {
	if (!cmpxchg(&he->lock, NULL, lock)) {
	WRITE_ONCE(he->node, node);
	return &he->lock;
	}
	}
	/*
	* Hard assume there is a free entry for us.
	*
	* This is guaranteed by ensuring every blocked lock only ever consumes
	* a single entry, and since we only have 4 nesting levels per CPU
	* and allocated 4*nr_possible_cpus(), this must be so.
	*
	* The single entry is guaranteed by having the lock owner unhash
	* before it releases.
	*/
	BUG();
	}

	static struct pv_node pv_unhash(struct qspinlock lock)
	{
	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
	struct pv_hash_entry *he;
	struct pv_node *node;

	for_each_hash_entry(he, offset, hash) {
	if (READ_ONCE(he->lock) == lock) {
	node = READ_ONCE(he->node);
	WRITE_ONCE(he->lock, NULL);
	return node;
	}
	}
	/*
	* Hard assume we'll find an entry.
	*
	* This guarantees a limited lookup time and is itself guaranteed by
	* having the lock owner do the unhash -- IFF the unlock sees the
	* SLOW flag, there MUST be a hash entry.
	*/
	BUG();
	}

	/*
	* Initialize the PV part of the mcs_spinlock node.
	*/
	static void pv_init_node(struct mcs_spinlock *node)
	{
	struct pv_node pn = (struct pv_node )node;

	BUILD_BUG_ON(sizeof(struct pv_node) > 5*sizeof(struct mcs_spinlock));

	pn->cpu = smp_processor_id();
	pn->state = vcpu_running;
	}

	/*
	* Wait for node->locked to become true, halt the vcpu after a short spin.
	* pv_kick_node() is used to wake the vcpu again.
	*/
	static void pv_wait_node(struct mcs_spinlock *node)
	{
	struct pv_node pn = (struct pv_node )node;
	int loop;

	for (;;) {
	for (loop = SPIN_THRESHOLD; loop; loop--) {
	if (READ_ONCE(node->locked))
	return;
	cpu_relax();
	}

	/*
	* Order pn->state vs pn->locked thusly:
	*
	* [S] pn->state = vcpu_halted [S] next->locked = 1
	* MB MB
	* [L] pn->locked [RmW] pn->state = vcpu_running
	*
	* Matches the xchg() from pv_kick_node().
	*/
	smp_store_mb(pn->state, vcpu_halted);

	if (!READ_ONCE(node->locked))
	pv_wait(&pn->state, vcpu_halted);

	/*
	* Reset the vCPU state to avoid unncessary CPU kicking
	*/
	WRITE_ONCE(pn->state, vcpu_running);

	/*
	* If the locked flag is still not set after wakeup, it is a
	* spurious wakeup and the vCPU should wait again. However,
	* there is a pretty high overhead for CPU halting and kicking.
	* So it is better to spin for a while in the hope that the
	* MCS lock will be released soon.
	*/
	}
	/*
	* By now our node->locked should be 1 and our caller will not actually
	* spin-wait for it. We do however rely on our caller to do a
	* load-acquire for us.
	*/
	}

	/*
	* Called after setting next->locked = 1, used to wake those stuck in
	* pv_wait_node().
	*/
	static void pv_kick_node(struct mcs_spinlock *node)
	{
	struct pv_node pn = (struct pv_node )node;

	/*
	* Note that because node->locked is already set, this actual
	* mcs_spinlock entry could be re-used already.
	*
	* This should be fine however, kicking people for no reason is
	* harmless.
	*
	* See the comment in pv_wait_node().
	*/
	if (xchg(&pn->state, vcpu_running) == vcpu_halted)
	pv_kick(pn->cpu);
	}

	/*
	* Wait for l->locked to become clear; halt the vcpu after a short spin.
	* __pv_queued_spin_unlock() will wake us.
	*/
	static void pv_wait_head(struct qspinlock lock, struct mcs_spinlock node)
	{
	struct pv_node pn = (struct pv_node )node;
	struct __qspinlock l = (void )lock;
	struct qspinlock **lp = NULL;
	int loop;

	for (;;) {
	for (loop = SPIN_THRESHOLD; loop; loop--) {
	if (!READ_ONCE(l->locked))
	return;
	cpu_relax();
	}

	WRITE_ONCE(pn->state, vcpu_halted);
	if (!lp) { /* ONCE */
	lp = pv_hash(lock, pn);
	/*
	* lp must be set before setting _Q_SLOW_VAL
	*
	* [S] lp = lock [RmW] l = l->locked = 0
	* MB MB
	* [S] l->locked = _Q_SLOW_VAL [L] lp
	*
	* Matches the cmpxchg() in __pv_queued_spin_unlock().
	*/
	if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL)) {
	/*
	* The lock is free and _Q_SLOW_VAL has never
	* been set. Therefore we need to unhash before
	* getting the lock.
	*/
	WRITE_ONCE(*lp, NULL);
	return;
	}
	}
	pv_wait(&l->locked, _Q_SLOW_VAL);

	/*
	* The unlocker should have freed the lock before kicking the
	* CPU. So if the lock is still not free, it is a spurious
	* wakeup and so the vCPU should wait again after spinning for
	* a while.
	*/
	}

	/*
	* Lock is unlocked now; the caller will acquire it without waiting.
	* As with pv_wait_node() we rely on the caller to do a load-acquire
	* for us.
	*/
	}

	/*
	* PV version of the unlock function to be used in stead of
	* queued_spin_unlock().
	*/
	__visible void __pv_queued_spin_unlock(struct qspinlock *lock)
	{
	struct __qspinlock l = (void )lock;
	struct pv_node *node;
	u8 lockval = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);

	/*
	* We must not unlock if SLOW, because in that case we must first
	* unhash. Otherwise it would be possible to have multiple @lock
	* entries, which would be BAD.
	*/
	if (likely(lockval == _Q_LOCKED_VAL))
	return;

	if (unlikely(lockval != _Q_SLOW_VAL)) {
	if (debug_locks_silent)
	return;
	WARN(1, "pvqspinlock: lock %p has corrupted value 0x%x!\n", lock, atomic_read(&lock->val));
	return;
	}

	/*
	* Since the above failed to release, this must be the SLOW path.
	* Therefore start by looking up the blocked node and unhashing it.
	*/
	node = pv_unhash(lock);

	/*
	* Now that we have a reference to the (likely) blocked pv_node,
	* release the lock.
	*/
	smp_store_release(&l->locked, 0);

	/*
	* At this point the memory pointed at by lock can be freed/reused,
	* however we can still use the pv_node to kick the CPU.
	*/
	if (READ_ONCE(node->state) == vcpu_halted)
	pv_kick(node->cpu);
	}
	/*
	* Include the architecture specific callee-save thunk of the
	* __pv_queued_spin_unlock(). This thunk is put together with
	* __pv_queued_spin_unlock() near the top of the file to make sure
	* that the callee-save thunk and the real unlock function are close
	* to each other sharing consecutive instruction cachelines.
	*/
	#include <asm/qspinlock_paravirt.h>