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Commit 546c2896 authored by Will Deacon's avatar Will Deacon Committed by Russell King
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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: default avatarNicolas Pitre <nico@linaro.org>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent 575320d6

arch/arm/include/asm/spinlock.h

+48 −24
Original line number Diff line number Diff line
@@ -59,18 +59,13 @@ static inline void dsb_sev(void) 
}



/*

 * ARMv6 Spin-locking.

 * ARMv6 ticket-based spin-locking.

 *

 * We exclusively read the old value.  If it is zero, we may have

 * won the lock, so we try exclusively storing it.  A memory barrier

 * is required after we get a lock, and before we release it, because

 * V6 CPUs are assumed to have weakly ordered memory.

 *

 * Unlocked value: 0

 * Locked value: 1

 * A memory barrier is required after we get a lock, and before we

 * release it, because V6 CPUs are assumed to have weakly ordered

 * memory.

 */



#define arch_spin_is_locked(x)		((x)->lock != 0)

#define arch_spin_unlock_wait(lock) \

	do { while (arch_spin_is_locked(lock)) cpu_relax(); } while (0)
@@ -79,31 +74,39 @@ static inline void dsb_sev(void) 
static inline void arch_spin_lock(arch_spinlock_t *lock)

{

	unsigned long tmp;

	u32 newval;

	arch_spinlock_t lockval;



	__asm__ __volatile__(

"1:	ldrex	%0, [%1]\n"

"	teq	%0, #0\n"

	WFE("ne")

"	strexeq	%0, %2, [%1]\n"

"	teqeq	%0, #0\n"

"1:	ldrex	%0, [%3]\n"

"	add	%1, %0, %4\n"

"	strex	%2, %1, [%3]\n"

"	teq	%2, #0\n"

"	bne	1b"

	: "=&r" (tmp)

	: "r" (&lock->lock), "r" (1)

	: "=&r" (lockval), "=&r" (newval), "=&r" (tmp)

	: "r" (&lock->slock), "I" (1 << TICKET_SHIFT)

	: "cc");



	while (lockval.tickets.next != lockval.tickets.owner) {

		wfe();

		lockval.tickets.owner = ACCESS_ONCE(lock->tickets.owner);

	}



	smp_mb();

}



static inline int arch_spin_trylock(arch_spinlock_t *lock)

{

	unsigned long tmp;

	u32 slock;



	__asm__ __volatile__(

"	ldrex	%0, [%1]\n"

"	teq	%0, #0\n"

"	strexeq	%0, %2, [%1]"

	: "=&r" (tmp)

	: "r" (&lock->lock), "r" (1)

"	ldrex	%0, [%2]\n"

"	subs	%1, %0, %0, ror #16\n"

"	addeq	%0, %0, %3\n"

"	strexeq	%1, %0, [%2]"

	: "=&r" (slock), "=&r" (tmp)

	: "r" (&lock->slock), "I" (1 << TICKET_SHIFT)

	: "cc");



	if (tmp == 0) {
@@ -116,17 +119,38 @@ static inline int arch_spin_trylock(arch_spinlock_t *lock) 


static inline void arch_spin_unlock(arch_spinlock_t *lock)

{

	unsigned long tmp;

	u32 slock;



	smp_mb();



	__asm__ __volatile__(

"	str	%1, [%0]\n"

	:

	: "r" (&lock->lock), "r" (0)

"	mov	%1, #1\n"

"1:	ldrex	%0, [%2]\n"

"	uadd16	%0, %0, %1\n"

"	strex	%1, %0, [%2]\n"

"	teq	%1, #0\n"

"	bne	1b"

	: "=&r" (slock), "=&r" (tmp)

	: "r" (&lock->slock)

	: "cc");



	dsb_sev();

}



static inline int arch_spin_is_locked(arch_spinlock_t *lock)

{

	struct __raw_tickets tickets = ACCESS_ONCE(lock->tickets);

	return tickets.owner != tickets.next;

}



static inline int arch_spin_is_contended(arch_spinlock_t *lock)

{

	struct __raw_tickets tickets = ACCESS_ONCE(lock->tickets);

	return (tickets.next - tickets.owner) > 1;

}

#define arch_spin_is_contended	arch_spin_is_contended



/*

 * RWLOCKS

 *

arch/arm/include/asm/spinlock_types.h

+15 −2
Original line number Diff line number Diff line
@@ -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;