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SubjectRe: [patch 2/4] lglock: introduce special lglock and brlock spin locks
On Fri, Jun 04, 2010 at 04:43:09PM +1000, Nick Piggin wrote:
> This patch introduces "local-global" locks (lglocks). These can be used to:
>
> - Provide fast exclusive access to per-CPU data, with exclusive access to
> another CPU's data allowed but possibly subject to contention, and to provide
> very slow exclusive access to all per-CPU data.
> - Or to provide very fast and scalable read serialisation, and to provide
> very slow exclusive serialisation of data (not necessarily per-CPU data).
>
> Brlocks are also implemented as a short-hand notation for the latter use
> case.
>
> Thanks to Paul for local/global naming convention.

;-)

One set of questions about how this relates to real-time below.

(And I agree with Eric's point about for_each_possible_cpu(), FWIW.)

> Cc: linux-kernel@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: Al Viro <viro@ZenIV.linux.org.uk>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Frank Mayhar <fmayhar@google.com>,
> Cc: John Stultz <johnstul@us.ibm.com>
> Cc: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Nick Piggin <npiggin@suse.de>
> ---
> include/linux/lglock.h | 165 +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 165 insertions(+)
>
> Index: linux-2.6/include/linux/lglock.h
> ===================================================================
> --- /dev/null
> +++ linux-2.6/include/linux/lglock.h
> @@ -0,0 +1,165 @@
> +/*
> + * Specialised local-global spinlock. Can only be declared as global variables
> + * to avoid overhead and keep things simple (and we don't want to start using
> + * these inside dynamically allocated structures).
> + *
> + * "local/global locks" (lglocks) can be used to:
> + *
> + * - Provide fast exclusive access to per-CPU data, with exclusive access to
> + * another CPU's data allowed but possibly subject to contention, and to
> + * provide very slow exclusive access to all per-CPU data.
> + * - Or to provide very fast and scalable read serialisation, and to provide
> + * very slow exclusive serialisation of data (not necessarily per-CPU data).
> + *
> + * Brlocks are also implemented as a short-hand notation for the latter use
> + * case.
> + *
> + * Copyright 2009, 2010, Nick Piggin, Novell Inc.
> + */
> +#ifndef __LINUX_LGLOCK_H
> +#define __LINUX_LGLOCK_H
> +
> +#include <linux/spinlock.h>
> +#include <linux/lockdep.h>
> +#include <linux/percpu.h>
> +#include <asm/atomic.h>
> +
> +/* can make br locks by using local lock for read side, global lock for write */
> +#define br_lock_init(name) name##_lock_init()
> +#define br_read_lock(name) name##_local_lock()
> +#define br_read_unlock(name) name##_local_unlock()
> +#define br_write_lock(name) name##_global_lock()
> +#define br_write_unlock(name) name##_global_unlock()
> +#define atomic_dec_and_br_write_lock(atomic, name) name##_atomic_dec_and_global_lock(atomic)
> +
> +#define DECLARE_BRLOCK(name) DECLARE_LGLOCK(name)
> +#define DEFINE_BRLOCK(name) DEFINE_LGLOCK(name)
> +
> +
> +#define lg_lock_init(name) name##_lock_init()
> +#define lg_local_lock(name) name##_local_lock()
> +#define lg_local_unlock(name) name##_local_unlock()
> +#define lg_local_lock_cpu(name, cpu) name##_local_lock_cpu(cpu)
> +#define lg_local_unlock_cpu(name, cpu) name##_local_unlock_cpu(cpu)
> +#define lg_global_lock(name) name##_global_lock()
> +#define lg_global_unlock(name) name##_global_unlock()
> +#define atomic_dec_and_lg_global_lock(atomic, name) name##_atomic_dec_and_global_lock(atomic)
> +
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +#define LOCKDEP_INIT_MAP lockdep_init_map
> +
> +#define DEFINE_LGLOCK_LOCKDEP(name) \
> + struct lock_class_key name##_lock_key; \
> + struct lockdep_map name##_lock_dep_map; \
> + EXPORT_SYMBOL(name##_lock_dep_map)
> +
> +#else
> +#define LOCKDEP_INIT_MAP(a, b, c, d)
> +
> +#define DEFINE_LGLOCK_LOCKDEP(name)
> +#endif
> +
> +
> +#define DECLARE_LGLOCK(name) \
> + extern void name##_lock_init(void); \
> + extern void name##_local_lock(void); \
> + extern void name##_local_unlock(void); \
> + extern void name##_local_lock_cpu(int cpu); \
> + extern void name##_local_unlock_cpu(int cpu); \
> + extern void name##_global_lock(void); \
> + extern void name##_global_unlock(void); \
> + extern int name##_atomic_dec_and_global_lock(atomic_t *a); \
> +
> +#define DEFINE_LGLOCK(name) \
> + \
> + DEFINE_PER_CPU(arch_spinlock_t, name##_lock); \
> + DEFINE_LGLOCK_LOCKDEP(name); \
> + \
> + void name##_lock_init(void) { \
> + int i; \
> + LOCKDEP_INIT_MAP(&name##_lock_dep_map, #name, &name##_lock_key, 0); \
> + for_each_possible_cpu(i) { \
> + arch_spinlock_t *lock; \
> + lock = &per_cpu(name##_lock, i); \
> + *lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED; \
> + } \
> + } \
> + EXPORT_SYMBOL(name##_lock_init); \
> + \
> + void name##_local_lock(void) { \
> + arch_spinlock_t *lock; \
> + preempt_disable(); \

In a -rt kernel, I believe we would not want the above preempt_disable().
Of course, in this case the arch_spin_lock() would need to become
spin_lock() or some such.

The main point of this approach is to avoid cross-CPU holding of these
locks, correct? And then the point of arch_spin_lock() is to avoid the
redundant preempt_disable(), right?

Thanx, Paul

> + rwlock_acquire_read(&name##_lock_dep_map, 0, 0, _THIS_IP_); \
> + lock = &__get_cpu_var(name##_lock); \
> + arch_spin_lock(lock); \
> + } \
> + EXPORT_SYMBOL(name##_local_lock); \
> + \
> + void name##_local_unlock(void) { \
> + arch_spinlock_t *lock; \
> + rwlock_release(&name##_lock_dep_map, 1, _THIS_IP_); \
> + lock = &__get_cpu_var(name##_lock); \
> + arch_spin_unlock(lock); \
> + preempt_enable(); \
> + } \
> + EXPORT_SYMBOL(name##_local_unlock); \
> + \
> + void name##_local_lock_cpu(int cpu) { \
> + arch_spinlock_t *lock; \
> + preempt_disable(); \
> + rwlock_acquire_read(&name##_lock_dep_map, 0, 0, _THIS_IP_); \
> + lock = &per_cpu(name##_lock, cpu); \
> + arch_spin_lock(lock); \
> + } \
> + EXPORT_SYMBOL(name##_local_lock_cpu); \
> + \
> + void name##_local_unlock_cpu(int cpu) { \
> + arch_spinlock_t *lock; \
> + rwlock_release(&name##_lock_dep_map, 1, _THIS_IP_); \
> + lock = &per_cpu(name##_lock, cpu); \
> + arch_spin_unlock(lock); \
> + preempt_enable(); \
> + } \
> + EXPORT_SYMBOL(name##_local_unlock_cpu); \
> + \
> + void name##_global_lock(void) { \
> + int i; \
> + preempt_disable(); \
> + rwlock_acquire(&name##_lock_dep_map, 0, 0, _RET_IP_); \
> + for_each_online_cpu(i) { \
> + arch_spinlock_t *lock; \
> + lock = &per_cpu(name##_lock, i); \
> + arch_spin_lock(lock); \
> + } \
> + } \
> + EXPORT_SYMBOL(name##_global_lock); \
> + \
> + void name##_global_unlock(void) { \
> + int i; \
> + rwlock_release(&name##_lock_dep_map, 1, _RET_IP_); \
> + for_each_online_cpu(i) { \
> + arch_spinlock_t *lock; \
> + lock = &per_cpu(name##_lock, i); \
> + arch_spin_unlock(lock); \
> + } \
> + preempt_enable(); \
> + } \
> + EXPORT_SYMBOL(name##_global_unlock); \
> + \
> + static int name##_atomic_dec_and_global_lock__failed(atomic_t *a) { \
> + name##_global_lock(); \
> + if (!atomic_dec_and_test(a)) { \
> + name##_global_unlock(); \
> + return 0; \
> + } \
> + return 1; \
> + } \
> + \
> + int name##_atomic_dec_and_global_lock(atomic_t *a) { \
> + if (likely(atomic_add_unless(a, -1, 1))) \
> + return 0; \
> + return name##_atomic_dec_and_global_lock__failed(a); \
> + } \
> + EXPORT_SYMBOL(name##_atomic_dec_and_global_lock);
> +
> +#endif
>
>


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