[RFC PATCH 1/5] x86,smp: move waiting on contended ticket lock out of line

[Rik van Riel <riel@redhat.com>]

Moving the wait loop for congested loops to its own function allows
us to add things to that wait loop, without growing the size of the
kernel text appreciably.

Signed-off-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Steven Rostedt <rostedt@goodmiss.org>
Reviewed-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Rafael Aquini <aquini@redhat.com>
---
v2: clean up the code a little, after Michel's suggestion 

 arch/x86/include/asm/spinlock.h |   11 +++++------
 arch/x86/kernel/smp.c           |   14 ++++++++++++++
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 33692ea..dc492f6 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -34,6 +34,8 @@
 # define UNLOCK_LOCK_PREFIX
 #endif
 
+extern void ticket_spin_lock_wait(arch_spinlock_t *, struct __raw_tickets);
+
 /*
  * 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
@@ -53,12 +55,9 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
 
 	inc = xadd(&lock->tickets, inc);
 
-	for (;;) {
-		if (inc.head == inc.tail)
-			break;
-		cpu_relax();
-		inc.head = ACCESS_ONCE(lock->tickets.head);
-	}
+	if (inc.head != inc.tail)
+		ticket_spin_lock_wait(lock, inc);
+
 	barrier();		/* make sure nothing creeps before the lock is taken */
 }
 
diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 48d2b7d..20da354 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -113,6 +113,20 @@ static atomic_t stopping_cpu = ATOMIC_INIT(-1);
 static bool smp_no_nmi_ipi = false;
 
 /*
+ * Wait on a congested ticket spinlock.
+ */
+void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
+{
+	for (;;) {
+		cpu_relax();
+		inc.head = ACCESS_ONCE(lock->tickets.head);
+
+		if (inc.head == inc.tail)
+			break;
+	}
+}
+
+/*
  * this function sends a 'reschedule' IPI to another CPU.
  * it goes straight through and wastes no time serializing
  * anything. Worst case is that we lose a reschedule ...