kern/spinlock: fix and optimize

Making the unlock operation block allows tricky deadlocks to occur in
case a thread is interrupted right before announcing itself as the first
waiter in the queue.

Since locking is expected to block, the spinlock implementation is
reworked to move the hand-off performed by the unlock operation into
the lock operation.

As a side effect, the common case of a single waiter is also optimized.

1 files changed, 8 insertions, 30 deletions

diff --git a/kern/spinlock_i.h b/kern/spinlock_i.h
index cd1bd368..da1233b4 100644
--- a/kern/spinlock_i.h
+++ b/kern/spinlock_i.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012-2017 Richard Braun.
+ * Copyright (c) 2012-2018 Richard Braun.
  *
  * 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
@@ -30,12 +30,12 @@
 #include <machine/cpu.h>
 
 /*
- * Non-contended lock values.
+ * Uncontended lock values.
  *
- * Any other lock value implies a contended lock.
+ * Any other value implies a contended lock.
  */
-#define SPINLOCK_UNLOCKED   0
-#define SPINLOCK_LOCKED     1
+#define SPINLOCK_UNLOCKED   0x0
+#define SPINLOCK_LOCKED     0x1
 
 #ifdef SPINLOCK_TRACK_OWNER
 
@@ -63,7 +63,7 @@ spinlock_disown(struct spinlock *lock)
 static inline int
 spinlock_lock_fast(struct spinlock *lock)
 {
-    unsigned int prev;
+    uint32_t prev;
 
     prev = atomic_cas_acquire(&lock->value, SPINLOCK_UNLOCKED, SPINLOCK_LOCKED);
 
@@ -75,25 +75,8 @@ spinlock_lock_fast(struct spinlock *lock)
     return 0;
 }
 
-static inline int
-spinlock_unlock_fast(struct spinlock *lock)
-{
-    unsigned int prev;
-
-    spinlock_disown(lock);
-    prev = atomic_cas_release(&lock->value, SPINLOCK_LOCKED, SPINLOCK_UNLOCKED);
-
-    if (unlikely(prev != SPINLOCK_LOCKED)) {
-        return EBUSY;
-    }
-
-    return 0;
-}
-
 void spinlock_lock_slow(struct spinlock *lock);
 
-void spinlock_unlock_slow(struct spinlock *lock);
-
 static inline void
 spinlock_lock_common(struct spinlock *lock)
 {
@@ -109,13 +92,8 @@ spinlock_lock_common(struct spinlock *lock)
 static inline void
 spinlock_unlock_common(struct spinlock *lock)
 {
-    int error;
-
-    error = spinlock_unlock_fast(lock);
-
-    if (unlikely(error)) {
-        spinlock_unlock_slow(lock);
-    }
+    spinlock_disown(lock);
+    atomic_and(&lock->value, ~SPINLOCK_LOCKED, ATOMIC_RELEASE);
 }
 
 #endif /* KERN_SPINLOCK_I_H */