[Xen-devel] [PATCH 7/8] x86/ticketlock: add slowpath logic

Jeremy Fitzhardinge
Fri, 02 Sep 2011 23:54:14 -0000

Maintain a flag in the LSB of the ticket lock tail which indicates
whether anyone is in the lock slowpath and may need kicking when
the current holder unlocks.  The flags are set when the first locker
enters the slowpath, and cleared when unlocking to an empty queue (ie,
no contention).

In the specific implementation of lock_spinning(), make sure to set
the slowpath flags on the lock just before blocking.  We must do
this before the last-chance pickup test to prevent a deadlock
with the unlocker:

Unlocker			Locker
				test for lock pickup
					-> fail
unlock
test slowpath
	-> false
				set slowpath flags
				block

Whereas this works in any ordering:

Unlocker			Locker
				set slowpath flags
				test for lock pickup
					-> fail
				block
unlock
test slowpath
	-> true, kick

Note: this code relies on gcc making sure that unlikely() code is out of
line of the fastpath, which only happens when OPTIMIZE_SIZE=n.  If it
doesn''t the generated code isn''t too bad, but its definitely
suboptimal.

(Thanks to Srivatsa Vaddagiri for providing a bugfix to the original
version of this change, which has been folded in.)

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
---
 arch/x86/include/asm/paravirt.h       |    2 +-
 arch/x86/include/asm/spinlock.h       |   72 ++++++++++++++++++++++++++-------
 arch/x86/include/asm/spinlock_types.h |    2 +
 arch/x86/kernel/paravirt-spinlocks.c  |    1 +
 arch/x86/xen/spinlock.c               |    4 ++
 5 files changed, 65 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 50281c7..13b3d8b 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -755,7 +755,7 @@ static __always_inline void __ticket_lock_spinning(struct
arch_spinlock *lock, _
 	PVOP_VCALLEE2(pv_lock_ops.lock_spinning, lock, ticket);
 }

-static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t ticket)
+static __always_inline void __ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t ticket)
 {
 	PVOP_VCALL2(pv_lock_ops.unlock_kick, lock, ticket);
 }
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 7a1c0c4..64422f1 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -40,29 +40,46 @@
 /* How long a lock should spin before we consider blocking */
 #define SPIN_THRESHOLD	(1 << 11)

-#ifndef CONFIG_PARAVIRT_SPINLOCKS
+#ifdef CONFIG_PARAVIRT_SPINLOCKS

-static __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock,
__ticket_t ticket)
+/*
+ * Return true if someone is in the slowpath on this lock.  This
+ * should only be used by the current lock-holder.
+ */
+static inline bool __ticket_in_slowpath(struct arch_spinlock *lock)
 {
+	return !!(lock->tickets.tail & TICKET_SLOWPATH_FLAG);
 }

-static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t ticket)
+static inline void __ticket_enter_slowpath(struct arch_spinlock *lock)
 {
+	if (sizeof(lock->tickets.tail) == sizeof(u8))
+		asm (LOCK_PREFIX "orb %1, %0"
+		     : "+m" (lock->tickets.tail)
+		     : "i" (TICKET_SLOWPATH_FLAG) : "memory");
+	else
+		asm (LOCK_PREFIX "orw %1, %0"
+		     : "+m" (lock->tickets.tail)
+		     : "i" (TICKET_SLOWPATH_FLAG) : "memory");
 }

-#endif	/* CONFIG_PARAVIRT_SPINLOCKS */
+#else  /* !CONFIG_PARAVIRT_SPINLOCKS */
+static inline bool __ticket_in_slowpath(struct arch_spinlock *lock)
+{
+	return false;
+}

+static __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock,
__ticket_t ticket)
+{
+}

-/*
- * If a spinlock has someone waiting on it, then kick the appropriate
- * waiting cpu.
- */
-static __always_inline void __ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t next)
+static inline void __ticket_unlock_kick(struct arch_spinlock *lock, __ticket_t
ticket)
 {
-	if (unlikely(lock->tickets.tail != next))
-		____ticket_unlock_kick(lock, next);
 }

+#endif	/* CONFIG_PARAVIRT_SPINLOCKS */
+
+
 /*
  * Ticket locks are conceptually two parts, one indicating the current head of
  * the queue, and the other indicating the current tail. The lock is acquired
@@ -85,15 +102,17 @@ static __always_inline void arch_spin_lock(struct
arch_spinlock *lock)
 	register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };

 	inc = xadd(&lock->tickets, inc);
+	if (likely(inc.head == inc.tail))
+		goto out;

+	inc.tail &= ~TICKET_SLOWPATH_FLAG;
 	for (;;) {
 		unsigned count = SPIN_THRESHOLD;

 		do {
-			if (inc.head == inc.tail)
+			if (ACCESS_ONCE(lock->tickets.head) == inc.tail)
 				goto out;
 			cpu_relax();
-			inc.head = ACCESS_ONCE(lock->tickets.head);
 		} while (--count);
 		__ticket_lock_spinning(lock, inc.tail);
 	}
@@ -132,12 +151,35 @@ static __always_inline void
__ticket_unlock_release(arch_spinlock_t *lock)
 }
 #endif

+static inline void __ticket_unlock_slowpath(struct arch_spinlock *lock)
+{
+	struct arch_spinlock old, new;
+
+	BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
+
+	old = ACCESS_ONCE(*lock);
+	new = old;
+
+	/* Clear the slowpath flag */
+	new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
+
+	/*
+	 * 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);
+
+	/* Wake up an appropriate waiter */
+ 	__ticket_unlock_kick(lock, new.tickets.head);
+}
+
 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
-	__ticket_t next = lock->tickets.head + TICKET_LOCK_INC;

 	__ticket_unlock_release(lock);
-	__ticket_unlock_kick(lock, next);
+	if (unlikely(__ticket_in_slowpath(lock)))
+		__ticket_unlock_slowpath(lock);
 }

 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
diff --git a/arch/x86/include/asm/spinlock_types.h
b/arch/x86/include/asm/spinlock_types.h
index aa9a205..407f7f7 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -5,8 +5,10 @@

 #ifdef CONFIG_PARAVIRT_SPINLOCKS
 #define __TICKET_LOCK_INC	2
+#define TICKET_SLOWPATH_FLAG	((__ticket_t)1)
 #else
 #define __TICKET_LOCK_INC	1
+#define TICKET_SLOWPATH_FLAG	((__ticket_t)0)
 #endif

 #if (CONFIG_NR_CPUS < (256 / __TICKET_LOCK_INC))
diff --git a/arch/x86/kernel/paravirt-spinlocks.c
b/arch/x86/kernel/paravirt-spinlocks.c
index 4251c1d..0883c48 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -15,3 +15,4 @@ struct pv_lock_ops pv_lock_ops = {
 };
 EXPORT_SYMBOL(pv_lock_ops);

+
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 7a04950..c939723 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -124,6 +124,10 @@ static void xen_lock_spinning(struct arch_spinlock *lock,
__ticket_t want)
 	/* Only check lock once pending cleared */
 	barrier();

+	/* Mark entry to slowpath before doing the pickup test to make
+	   sure we don''t deadlock with an unlocker. */
+	__ticket_enter_slowpath(lock);
+
 	/* check again make sure it didn''t become free while
 	   we weren''t looking  */
 	if (ACCESS_ONCE(lock->tickets.head) == want) {