1 files changed, 44 insertions, 46 deletions

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 7050d864f520..cf87de3fc390 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -46,7 +46,7 @@ static __always_inline bool static_key_false(struct static_key *key);
 
 static inline void __ticket_enter_slowpath(arch_spinlock_t *lock)
 {
-        set_bit(0, (volatile unsigned long *)&lock->tickets.tail);
+        set_bit(0, (volatile unsigned long *)&lock->tickets.head);
 }
 
 #else  /* !CONFIG_PARAVIRT_SPINLOCKS */
@@ -60,10 +60,30 @@ static inline void __ticket_unlock_kick(arch_spinlock_t *lock,
 }
 
 #endif /* CONFIG_PARAVIRT_SPINLOCKS */
+static inline int  __tickets_equal(__ticket_t one, __ticket_t two)
+{
+        return !((one ^ two) & ~TICKET_SLOWPATH_FLAG);
+}
+
+static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock,
+                                                        __ticket_t head)
+{
+        if (head & TICKET_SLOWPATH_FLAG) {
+                arch_spinlock_t old, new;
+
+                old.tickets.head = head;
+                new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
+                old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
+                new.tickets.tail = old.tickets.tail;
+
+                /* try to clear slowpath flag when there are no contenders */
+                cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
+        }
+}
 
 static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
 {
-        return lock.tickets.head == lock.tickets.tail;
+        return __tickets_equal(lock.tickets.head, lock.tickets.tail);
 }
 
 /*
@@ -87,18 +107,21 @@ static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
         if (likely(inc.head == inc.tail))
                 goto out;
 
-        inc.tail &= ~TICKET_SLOWPATH_FLAG;
         for (;;) {
                 unsigned count = SPIN_THRESHOLD;
 
                 do {
-                        if (READ_ONCE(lock->tickets.head) == inc.tail)
-                                goto out;
+                        inc.head = READ_ONCE(lock->tickets.head);
+                        if (__tickets_equal(inc.head, inc.tail))
+                                goto clear_slowpath;
                         cpu_relax();
                 } while (--count);
                 __ticket_lock_spinning(lock, inc.tail);
         }
-out:    barrier();      /* make sure nothing creeps before the lock is taken */
+clear_slowpath:
+        __ticket_check_and_clear_slowpath(lock, inc.head);
+out:
+        barrier();      /* make sure nothing creeps before the lock is taken */
 }
 
 static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
@@ -106,56 +129,30 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
         arch_spinlock_t old, new;
 
         old.tickets = READ_ONCE(lock->tickets);
-        if (old.tickets.head != (old.tickets.tail & ~TICKET_SLOWPATH_FLAG))
+        if (!__tickets_equal(old.tickets.head, old.tickets.tail))
                 return 0;
 
         new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT);
+        new.head_tail &= ~TICKET_SLOWPATH_FLAG;
 
         /* cmpxchg is a full barrier, so nothing can move before it */
         return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == old.head_tail;
 }
 
-static inline void __ticket_unlock_slowpath(arch_spinlock_t *lock,
-                                            arch_spinlock_t old)
-{
-        arch_spinlock_t new;
-
-        BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
-
-        /* Perform the unlock on the "before" copy */
-        old.tickets.head += TICKET_LOCK_INC;
-
-        /* Clear the slowpath flag */
-        new.head_tail = old.head_tail & ~(TICKET_SLOWPATH_FLAG << TICKET_SHIFT);
-
-        /*
-         * If the lock is uncontended, clear the flag - use cmpxchg in
-         * case it changes behind our back though.
-         */
-        if (new.tickets.head != new.tickets.tail ||
-            cmpxchg(&lock->head_tail, old.head_tail,
-                                        new.head_tail) != old.head_tail) {
-                /*
-                 * Lock still has someone queued for it, so wake up an
-                 * appropriate waiter.
-                 */
-                __ticket_unlock_kick(lock, old.tickets.head);
-        }
-}
-
 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
         if (TICKET_SLOWPATH_FLAG &&
-            static_key_false(&paravirt_ticketlocks_enabled)) {
-                arch_spinlock_t prev;
+                static_key_false(&paravirt_ticketlocks_enabled)) {
+                __ticket_t head;
 
-                prev = *lock;
-                add_smp(&lock->tickets.head, TICKET_LOCK_INC);
+                BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
 
-                /* add_smp() is a full mb() */
+                head = xadd(&lock->tickets.head, TICKET_LOCK_INC);
 
-                if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
-                        __ticket_unlock_slowpath(lock, prev);
+                if (unlikely(head & TICKET_SLOWPATH_FLAG)) {
+                        head &= ~TICKET_SLOWPATH_FLAG;
+                        __ticket_unlock_kick(lock, (head + TICKET_LOCK_INC));
+                }
         } else
                 __add(&lock->tickets.head, TICKET_LOCK_INC, UNLOCK_LOCK_PREFIX);
 }
@@ -164,14 +161,15 @@ static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
         struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 
-        return tmp.tail != tmp.head;
+        return !__tickets_equal(tmp.tail, tmp.head);
 }
 
 static inline int arch_spin_is_contended(arch_spinlock_t *lock)
 {
         struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 
-        return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;
+        tmp.head &= ~TICKET_SLOWPATH_FLAG;
+        return (tmp.tail - tmp.head) > TICKET_LOCK_INC;
 }
 #define arch_spin_is_contended  arch_spin_is_contended
 
@@ -191,8 +189,8 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
                  * We need to check "unlocked" in a loop, tmp.head == head
                  * can be false positive because of overflow.
                  */
-                if (tmp.head == (tmp.tail & ~TICKET_SLOWPATH_FLAG) ||
-                    tmp.head != head)
+                if (__tickets_equal(tmp.head, tmp.tail) ||
+                                !__tickets_equal(tmp.head, head))
                         break;
 
                 cpu_relax();