On Fri, May 18, 2018 at 11:52:04AM +0200, Peter Zijlstra wrote:
> On Fri, May 18, 2018 at 03:49:06AM -0400, Kent Overstreet wrote:
>
> No.. and most certainly not without a _very_ good reason.
Ok, can I ask why?
Here's what it's for:
commit 61782bf71eef83919af100a9747d8d86dfdf3605
Author: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Date: Fri May 18 06:14:56 2018 -0400
bcachefs: SIX locks (shared/intent/exclusive)
New lock for bcachefs, like read/write locks but with a third state,
intent.
Intent locks conflict with each other, but not with read locks; taking a
write lock requires first holding an intent lock.
diff --git a/fs/bcachefs/six.c b/fs/bcachefs/six.c
new file mode 100644
index 0000000000..afa59a476a
--- /dev/null
+++ b/fs/bcachefs/six.c
@@ -0,0 +1,516 @@
+
+#include <linux/log2.h>
+#include <linux/preempt.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/sched/rt.h>
+
+#include "six.h"
+
+#define six_acquire(l, t) lock_acquire(l, 0, t, 0, 0, NULL, _RET_IP_)
+#define six_release(l) lock_release(l, 0, _RET_IP_)
+
+struct six_lock_vals {
+ /* Value we add to the lock in order to take the lock: */
+ u64 lock_val;
+
+ /* If the lock has this value (used as a mask), taking the lock fails: */
+ u64 lock_fail;
+
+ /* Value we add to the lock in order to release the lock: */
+ u64 unlock_val;
+
+ /* Mask that indicates lock is held for this type: */
+ u64 held_mask;
+
+ /* Waitlist we wakeup when releasing the lock: */
+ enum six_lock_type unlock_wakeup;
+};
+
+#define __SIX_LOCK_HELD_read __SIX_VAL(read_lock, ~0)
+#define __SIX_LOCK_HELD_intent __SIX_VAL(intent_lock, ~0)
+#define __SIX_LOCK_HELD_write __SIX_VAL(seq, 1)
+
+#define LOCK_VALS { \
+ [SIX_LOCK_read] = { \
+ .lock_val = __SIX_VAL(read_lock, 1), \
+ .lock_fail = __SIX_LOCK_HELD_write, \
+ .unlock_val = -__SIX_VAL(read_lock, 1), \
+ .held_mask = __SIX_LOCK_HELD_read, \
+ .unlock_wakeup = SIX_LOCK_write, \
+ }, \
+ [SIX_LOCK_intent] = { \
+ .lock_val = __SIX_VAL(intent_lock, 1), \
+ .lock_fail = __SIX_LOCK_HELD_intent, \
+ .unlock_val = -__SIX_VAL(intent_lock, 1), \
+ .held_mask = __SIX_LOCK_HELD_intent, \
+ .unlock_wakeup = SIX_LOCK_intent, \
+ }, \
+ [SIX_LOCK_write] = { \
+ .lock_val = __SIX_VAL(seq, 1), \
+ .lock_fail = __SIX_LOCK_HELD_read, \
+ .unlock_val = __SIX_VAL(seq, 1), \
+ .held_mask = __SIX_LOCK_HELD_write, \
+ .unlock_wakeup = SIX_LOCK_read, \
+ }, \
+}
+
+static inline void six_set_owner(struct six_lock *lock, enum six_lock_type type,
+ union six_lock_state old)
+{
+ if (type != SIX_LOCK_intent)
+ return;
+
+ if (!old.intent_lock) {
+ EBUG_ON(lock->owner);
+ lock->owner = current;
+ } else {
+ EBUG_ON(lock->owner != current);
+ }
+}
+
+static inline void six_clear_owner(struct six_lock *lock, enum six_lock_type type)
+{
+ if (type != SIX_LOCK_intent)
+ return;
+
+ EBUG_ON(lock->owner != current);
+
+ if (lock->state.intent_lock == 1)
+ lock->owner = NULL;
+}
+
+static __always_inline bool do_six_trylock_type(struct six_lock *lock,
+ enum six_lock_type type)
+{
+ const struct six_lock_vals l[] = LOCK_VALS;
+ union six_lock_state old;
+ u64 v = READ_ONCE(lock->state.v);
+
+ EBUG_ON(type == SIX_LOCK_write && lock->owner != current);
+
+ do {
+ old.v = v;
+
+ EBUG_ON(type == SIX_LOCK_write &&
+ ((old.v & __SIX_LOCK_HELD_write) ||
+ !(old.v & __SIX_LOCK_HELD_intent)));
+
+ if (old.v & l[type].lock_fail)
+ return false;
+ } while ((v = atomic64_cmpxchg_acquire(&lock->state.counter,
+ old.v,
+ old.v + l[type].lock_val)) != old.v);
+
+ six_set_owner(lock, type, old);
+ return true;
+}
+
+__always_inline __flatten
+static bool __six_trylock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ if (!do_six_trylock_type(lock, type))
+ return false;
+
+ six_acquire(&lock->dep_map, 1);
+ return true;
+}
+
+__always_inline __flatten
+static bool __six_relock_type(struct six_lock *lock, enum six_lock_type type,
+ unsigned seq)
+{
+ const struct six_lock_vals l[] = LOCK_VALS;
+ union six_lock_state old;
+ u64 v = READ_ONCE(lock->state.v);
+
+ do {
+ old.v = v;
+
+ if (old.seq != seq || old.v & l[type].lock_fail)
+ return false;
+ } while ((v = atomic64_cmpxchg_acquire(&lock->state.counter,
+ old.v,
+ old.v + l[type].lock_val)) != old.v);
+
+ six_set_owner(lock, type, old);
+ six_acquire(&lock->dep_map, 1);
+ return true;
+}
+
+struct six_lock_waiter {
+ struct list_head list;
+ struct task_struct *task;
+};
+
+/* This is probably up there with the more evil things I've done */
+#define waitlist_bitnr(id) ilog2((((union six_lock_state) { .waiters = 1 << (id) }).l))
+
+#ifdef CONFIG_LOCK_SPIN_ON_OWNER
+
+static inline int six_can_spin_on_owner(struct six_lock *lock)
+{
+ struct task_struct *owner;
+ int retval = 1;
+
+ if (need_resched())
+ return 0;
+
+ rcu_read_lock();
+ owner = READ_ONCE(lock->owner);
+ if (owner)
+ retval = owner->on_cpu;
+ rcu_read_unlock();
+ /*
+ * if lock->owner is not set, the mutex owner may have just acquired
+ * it and not set the owner yet or the mutex has been released.
+ */
+ return retval;
+}
+
+static inline bool six_spin_on_owner(struct six_lock *lock,
+ struct task_struct *owner)
+{
+ bool ret = true;
+
+ rcu_read_lock();
+ while (lock->owner == owner) {
+ /*
+ * Ensure we emit the owner->on_cpu, dereference _after_
+ * checking lock->owner still matches owner. If that fails,
+ * owner might point to freed memory. If it still matches,
+ * the rcu_read_lock() ensures the memory stays valid.
+ */
+ barrier();
+
+ if (!owner->on_cpu || need_resched()) {
+ ret = false;
+ break;
+ }
+
+ cpu_relax();
+ }
+ rcu_read_unlock();
+
+ return ret;
+}
+
+static inline bool six_optimistic_spin(struct six_lock *lock, enum six_lock_type type)
+{
+ struct task_struct *task = current;
+
+ if (type == SIX_LOCK_write)
+ return false;
+
+ preempt_disable();
+ if (!six_can_spin_on_owner(lock))
+ goto fail;
+
+ if (!osq_lock(&lock->osq))
+ goto fail;
+
+ while (1) {
+ struct task_struct *owner;
+
+ /*
+ * If there's an owner, wait for it to either
+ * release the lock or go to sleep.
+ */
+ owner = READ_ONCE(lock->owner);
+ if (owner && !six_spin_on_owner(lock, owner))
+ break;
+
+ if (do_six_trylock_type(lock, type)) {
+ osq_unlock(&lock->osq);
+ preempt_enable();
+ return true;
+ }
+
+ /*
+ * When there's no owner, we might have preempted between the
+ * owner acquiring the lock and setting the owner field. If
+ * we're an RT task that will live-lock because we won't let
+ * the owner complete.
+ */
+ if (!owner && (need_resched() || rt_task(task)))
+ break;
+
+ /*
+ * The cpu_relax() call is a compiler barrier which forces
+ * everything in this loop to be re-loaded. We don't need
+ * memory barriers as we'll eventually observe the right
+ * values at the cost of a few extra spins.
+ */
+ cpu_relax();
+ }
+
+ osq_unlock(&lock->osq);
+fail:
+ preempt_enable();
+
+ /*
+ * If we fell out of the spin path because of need_resched(),
+ * reschedule now, before we try-lock again. This avoids getting
+ * scheduled out right after we obtained the lock.
+ */
+ if (need_resched())
+ schedule();
+
+ return false;
+}
+
+#else /* CONFIG_LOCK_SPIN_ON_OWNER */
+
+static inline bool six_optimistic_spin(struct six_lock *lock, enum six_lock_type type)
+{
+ return false;
+}
+
+#endif
+
+noinline
+static void __six_lock_type_slowpath(struct six_lock *lock, enum six_lock_type type)
+{
+ const struct six_lock_vals l[] = LOCK_VALS;
+ union six_lock_state old, new;
+ struct six_lock_waiter wait;
+ u64 v;
+
+ if (six_optimistic_spin(lock, type))
+ return;
+
+ lock_contended(&lock->dep_map, _RET_IP_);
+
+ INIT_LIST_HEAD(&wait.list);
+ wait.task = current;
+
+ while (1) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ if (type == SIX_LOCK_write)
+ EBUG_ON(lock->owner != current);
+ else if (list_empty_careful(&wait.list)) {
+ raw_spin_lock(&lock->wait_lock);
+ list_add_tail(&wait.list, &lock->wait_list[type]);
+ raw_spin_unlock(&lock->wait_lock);
+ }
+
+ v = READ_ONCE(lock->state.v);
+ do {
+ new.v = old.v = v;
+
+ if (!(old.v & l[type].lock_fail))
+ new.v += l[type].lock_val;
+ else if (!(new.waiters & (1 << type)))
+ new.waiters |= 1 << type;
+ else
+ break; /* waiting bit already set */
+ } while ((v = atomic64_cmpxchg_acquire(&lock->state.counter,
+ old.v, new.v)) != old.v);
+
+ if (!(old.v & l[type].lock_fail))
+ break;
+
+ schedule();
+ }
+
+ six_set_owner(lock, type, old);
+
+ __set_current_state(TASK_RUNNING);
+
+ if (!list_empty_careful(&wait.list)) {
+ raw_spin_lock(&lock->wait_lock);
+ list_del_init(&wait.list);
+ raw_spin_unlock(&lock->wait_lock);
+ }
+}
+
+__always_inline
+static void __six_lock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ six_acquire(&lock->dep_map, 0);
+
+ if (!do_six_trylock_type(lock, type))
+ __six_lock_type_slowpath(lock, type);
+
+ lock_acquired(&lock->dep_map, _RET_IP_);
+}
+
+static inline void six_lock_wakeup(struct six_lock *lock,
+ union six_lock_state state,
+ unsigned waitlist_id)
+{
+ struct list_head *wait_list = &lock->wait_list[waitlist_id];
+ struct six_lock_waiter *w, *next;
+
+ if (waitlist_id == SIX_LOCK_write && state.read_lock)
+ return;
+
+ if (!(state.waiters & (1 << waitlist_id)))
+ return;
+
+ clear_bit(waitlist_bitnr(waitlist_id),
+ (unsigned long *) &lock->state.v);
+
+ if (waitlist_id == SIX_LOCK_write) {
+ struct task_struct *p = READ_ONCE(lock->owner);
+
+ if (p)
+ wake_up_process(p);
+ return;
+ }
+
+ raw_spin_lock(&lock->wait_lock);
+
+ list_for_each_entry_safe(w, next, wait_list, list) {
+ list_del_init(&w->list);
+
+ if (wake_up_process(w->task) &&
+ waitlist_id != SIX_LOCK_read) {
+ if (!list_empty(wait_list))
+ set_bit(waitlist_bitnr(waitlist_id),
+ (unsigned long *) &lock->state.v);
+ break;
+ }
+ }
+
+ raw_spin_unlock(&lock->wait_lock);
+}
+
+__always_inline __flatten
+static void __six_unlock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ const struct six_lock_vals l[] = LOCK_VALS;
+ union six_lock_state state;
+
+ EBUG_ON(!(lock->state.v & l[type].held_mask));
+ EBUG_ON(type == SIX_LOCK_write &&
+ !(lock->state.v & __SIX_LOCK_HELD_intent));
+
+ six_clear_owner(lock, type);
+
+ state.v = atomic64_add_return_release(l[type].unlock_val,
+ &lock->state.counter);
+ six_release(&lock->dep_map);
+ six_lock_wakeup(lock, state, l[type].unlock_wakeup);
+}
+
+#ifdef SIX_LOCK_SEPARATE_LOCKFNS
+
+#define __SIX_LOCK(type) \
+bool six_trylock_##type(struct six_lock *lock) \
+{ \
+ return __six_trylock_type(lock, SIX_LOCK_##type); \
+} \
+ \
+bool six_relock_##type(struct six_lock *lock, u32 seq) \
+{ \
+ return __six_relock_type(lock, SIX_LOCK_##type, seq); \
+} \
+ \
+void six_lock_##type(struct six_lock *lock) \
+{ \
+ __six_lock_type(lock, SIX_LOCK_##type); \
+} \
+ \
+void six_unlock_##type(struct six_lock *lock) \
+{ \
+ __six_unlock_type(lock, SIX_LOCK_##type); \
+}
+
+__SIX_LOCK(read)
+__SIX_LOCK(intent)
+__SIX_LOCK(write)
+
+#undef __SIX_LOCK
+
+#else
+
+bool six_trylock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ return __six_trylock_type(lock, type);
+}
+
+bool six_relock_type(struct six_lock *lock, enum six_lock_type type,
+ unsigned seq)
+{
+ return __six_relock_type(lock, type, seq);
+
+}
+
+void six_lock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ __six_lock_type(lock, type);
+}
+
+void six_unlock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ __six_unlock_type(lock, type);
+}
+
+#endif
+
+/* Convert from intent to read: */
+void six_lock_downgrade(struct six_lock *lock)
+{
+ six_lock_increment(lock, SIX_LOCK_read);
+ six_unlock_intent(lock);
+}
+
+bool six_lock_tryupgrade(struct six_lock *lock)
+{
+ const struct six_lock_vals l[] = LOCK_VALS;
+ union six_lock_state old, new;
+ u64 v = READ_ONCE(lock->state.v);
+
+ do {
+ new.v = old.v = v;
+
+ EBUG_ON(!(old.v & l[SIX_LOCK_read].held_mask));
+
+ new.v += l[SIX_LOCK_read].unlock_val;
+
+ if (new.v & l[SIX_LOCK_intent].lock_fail)
+ return false;
+
+ new.v += l[SIX_LOCK_intent].lock_val;
+ } while ((v = atomic64_cmpxchg_acquire(&lock->state.counter,
+ old.v, new.v)) != old.v);
+
+ six_set_owner(lock, SIX_LOCK_intent, old);
+ six_lock_wakeup(lock, new, l[SIX_LOCK_read].unlock_wakeup);
+
+ return true;
+}
+
+bool six_trylock_convert(struct six_lock *lock,
+ enum six_lock_type from,
+ enum six_lock_type to)
+{
+ EBUG_ON(to == SIX_LOCK_write || from == SIX_LOCK_write);
+
+ if (to == from)
+ return true;
+
+ if (to == SIX_LOCK_read) {
+ six_lock_downgrade(lock);
+ return true;
+ } else {
+ return six_lock_tryupgrade(lock);
+ }
+}
+
+/*
+ * Increment read/intent lock count, assuming we already have it read or intent
+ * locked:
+ */
+void six_lock_increment(struct six_lock *lock, enum six_lock_type type)
+{
+ const struct six_lock_vals l[] = LOCK_VALS;
+
+ EBUG_ON(type == SIX_LOCK_write);
+ six_acquire(&lock->dep_map, 0);
+
+ /* XXX: assert already locked, and that we don't overflow: */
+
+ atomic64_add(l[type].lock_val, &lock->state.counter);
+}
diff --git a/fs/bcachefs/six.h b/fs/bcachefs/six.h
new file mode 100644
index 0000000000..f518c64c40
--- /dev/null
+++ b/fs/bcachefs/six.h
@@ -0,0 +1,190 @@
+#ifndef _BCACHEFS_SIX_H
+#define _BCACHEFS_SIX_H
+
+#include <linux/lockdep.h>
+#include <linux/osq_lock.h>
+#include <linux/sched.h>
+#include <linux/types.h>
+
+#include "util.h"
+
+#define SIX_LOCK_SEPARATE_LOCKFNS
+
+/*
+ * LOCK STATES:
+ *
+ * read, intent, write (i.e. shared/intent/exclusive, hence the name)
+ *
+ * read and write work as with normal read/write locks - a lock can have
+ * multiple readers, but write excludes reads and other write locks.
+ *
+ * Intent does not block read, but it does block other intent locks. The idea is
+ * by taking an intent lock, you can then later upgrade to a write lock without
+ * dropping your read lock and without deadlocking - because no other thread has
+ * the intent lock and thus no other thread could be trying to take the write
+ * lock.
+ */
+
+union six_lock_state {
+ struct {
+ atomic64_t counter;
+ };
+
+ struct {
+ u64 v;
+ };
+
+ struct {
+ /* for waitlist_bitnr() */
+ unsigned long l;
+ };
+
+ struct {
+ unsigned read_lock:26;
+ unsigned intent_lock:3;
+ unsigned waiters:3;
+ /*
+ * seq works much like in seqlocks: it's incremented every time
+ * we lock and unlock for write.
+ *
+ * If it's odd write lock is held, even unlocked.
+ *
+ * Thus readers can unlock, and then lock again later iff it
+ * hasn't been modified in the meantime.
+ */
+ u32 seq;
+ };
+};
+
+#define SIX_LOCK_MAX_RECURSE ((1 << 3) - 1)
+
+enum six_lock_type {
+ SIX_LOCK_read,
+ SIX_LOCK_intent,
+ SIX_LOCK_write,
+};
+
+struct six_lock {
+ union six_lock_state state;
+ struct task_struct *owner;
+ struct optimistic_spin_queue osq;
+
+ raw_spinlock_t wait_lock;
+ struct list_head wait_list[2];
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map dep_map;
+#endif
+};
+
+static __always_inline void __six_lock_init(struct six_lock *lock,
+ const char *name,
+ struct lock_class_key *key)
+{
+ atomic64_set(&lock->state.counter, 0);
+ raw_spin_lock_init(&lock->wait_lock);
+ INIT_LIST_HEAD(&lock->wait_list[SIX_LOCK_read]);
+ INIT_LIST_HEAD(&lock->wait_list[SIX_LOCK_intent]);
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ debug_check_no_locks_freed((void *) lock, sizeof(*lock));
+ lockdep_init_map(&lock->dep_map, name, key, 0);
+#endif
+}
+
+#define six_lock_init(lock) \
+do { \
+ static struct lock_class_key __key; \
+ \
+ __six_lock_init((lock), #lock, &__key); \
+} while (0)
+
+#define __SIX_VAL(field, _v) (((union six_lock_state) { .field = _v }).v)
+
+#ifdef SIX_LOCK_SEPARATE_LOCKFNS
+
+#define __SIX_LOCK(type) \
+bool six_trylock_##type(struct six_lock *); \
+bool six_relock_##type(struct six_lock *, u32); \
+void six_lock_##type(struct six_lock *); \
+void six_unlock_##type(struct six_lock *);
+
+__SIX_LOCK(read)
+__SIX_LOCK(intent)
+__SIX_LOCK(write)
+#undef __SIX_LOCK
+
+#define SIX_LOCK_DISPATCH(type, fn, ...) \
+ switch (type) { \
+ case SIX_LOCK_read: \
+ return fn##_read(__VA_ARGS__); \
+ case SIX_LOCK_intent: \
+ return fn##_intent(__VA_ARGS__); \
+ case SIX_LOCK_write: \
+ return fn##_write(__VA_ARGS__); \
+ default: \
+ BUG(); \
+ }
+
+static inline bool six_trylock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ SIX_LOCK_DISPATCH(type, six_trylock, lock);
+}
+
+static inline bool six_relock_type(struct six_lock *lock, enum six_lock_type type,
+ unsigned seq)
+{
+ SIX_LOCK_DISPATCH(type, six_relock, lock, seq);
+}
+
+static inline void six_lock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ SIX_LOCK_DISPATCH(type, six_lock, lock);
+}
+
+static inline void six_unlock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ SIX_LOCK_DISPATCH(type, six_unlock, lock);
+}
+
+#else
+
+bool six_trylock_type(struct six_lock *, enum six_lock_type);
+bool six_relock_type(struct six_lock *, enum six_lock_type, unsigned);
+void six_lock_type(struct six_lock *, enum six_lock_type);
+void six_unlock_type(struct six_lock *, enum six_lock_type);
+
+#define __SIX_LOCK(type) \
+static __always_inline bool six_trylock_##type(struct six_lock *lock) \
+{ \
+ return six_trylock_type(lock, SIX_LOCK_##type); \
+} \
+ \
+static __always_inline bool six_relock_##type(struct six_lock *lock, u32 seq)\
+{ \
+ return six_relock_type(lock, SIX_LOCK_##type, seq); \
+} \
+ \
+static __always_inline void six_lock_##type(struct six_lock *lock) \
+{ \
+ six_lock_type(lock, SIX_LOCK_##type); \
+} \
+ \
+static __always_inline void six_unlock_##type(struct six_lock *lock) \
+{ \
+ six_unlock_type(lock, SIX_LOCK_##type); \
+}
+
+__SIX_LOCK(read)
+__SIX_LOCK(intent)
+__SIX_LOCK(write)
+#undef __SIX_LOCK
+
+#endif
+
+void six_lock_downgrade(struct six_lock *);
+bool six_lock_tryupgrade(struct six_lock *);
+bool six_trylock_convert(struct six_lock *, enum six_lock_type,
+ enum six_lock_type);
+
+void six_lock_increment(struct six_lock *, enum six_lock_type);
+
+#endif /* _BCACHEFS_SIX_H */
