ARM: 7446/1: spinlock: use ticket algorithm for ARMv6+ locking implementation

Ticket spinlocks ensure locking fairness by introducing a FIFO-like
nature to the granting of lock acquisitions and also reducing the
thundering herd effect when spinning on a lock by allowing the cacheline
to remain in a shared state amongst the waiting CPUs. This is especially
important on systems where memory-access times are not necessarily
uniform when accessing the lock structure (for example, on a
multi-cluster platform where the lock is allocated into L1 when a CPU
releases it).

This patch implements the ticket spinlock algorithm for ARM, replacing
the simpler implementation for ARMv6+ processors.

Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 15 insertions, 2 deletions

diff --git a/arch/arm/include/asm/spinlock_types.h b/arch/arm/include/asm/spinlock_types.h
index d14d197ae04a..b262d2f8b478 100644
--- a/arch/arm/include/asm/spinlock_types.h
+++ b/arch/arm/include/asm/spinlock_types.h
@@ -5,11 +5,24 @@
 # error "please don't include this file directly"
 #endif
 
+#define TICKET_SHIFT	16
+
 typedef struct {
-	volatile unsigned int lock;
+	union {
+		u32 slock;
+		struct __raw_tickets {
+#ifdef __ARMEB__
+			u16 next;
+			u16 owner;
+#else
+			u16 owner;
+			u16 next;
+#endif
+		} tickets;
+	};
 } arch_spinlock_t;
 
-#define __ARCH_SPIN_LOCK_UNLOCKED	{ 0 }
+#define __ARCH_SPIN_LOCK_UNLOCKED	{ { 0 } }
 
 typedef struct {
 	volatile unsigned int lock;