#ifndef _GEN_PV_LOCK_SLOWPATH
#error "do not include this file"
#endif
#include <linux/hash.h>
#include <linux/bootmem.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().
*/
set_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;
/*
* 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(cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_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>
