[RFC][PATCH 13/13] locking: Remove lglock
From: Peter Zijlstra
Date: Mon Jun 22 2015 - 08:27:06 EST
Since there are no users left of this primitive, make it go away.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
Documentation/locking/lglock.txt | 166 ---------------------------------------
fs/file_table.c | 1
include/linux/lglock.h | 81 -------------------
kernel/locking/Makefile | 1
kernel/locking/lglock.c | 111 --------------------------
5 files changed, 360 deletions(-)
--- a/Documentation/locking/lglock.txt
+++ /dev/null
@@ -1,166 +0,0 @@
-lglock - local/global locks for mostly local access patterns
-------------------------------------------------------------
-
-Origin: Nick Piggin's VFS scalability series introduced during
- 2.6.35++ [1] [2]
-Location: kernel/locking/lglock.c
- include/linux/lglock.h
-Users: currently only the VFS and stop_machine related code
-
-Design Goal:
-------------
-
-Improve scalability of globally used large data sets that are
-distributed over all CPUs as per_cpu elements.
-
-To manage global data structures that are partitioned over all CPUs
-as per_cpu elements but can be mostly handled by CPU local actions
-lglock will be used where the majority of accesses are cpu local
-reading and occasional cpu local writing with very infrequent
-global write access.
-
-
-* deal with things locally whenever possible
- - very fast access to the local per_cpu data
- - reasonably fast access to specific per_cpu data on a different
- CPU
-* while making global action possible when needed
- - by expensive access to all CPUs locks - effectively
- resulting in a globally visible critical section.
-
-Design:
--------
-
-Basically it is an array of per_cpu spinlocks with the
-lg_local_lock/unlock accessing the local CPUs lock object and the
-lg_local_lock_cpu/unlock_cpu accessing a remote CPUs lock object
-the lg_local_lock has to disable preemption as migration protection so
-that the reference to the local CPUs lock does not go out of scope.
-Due to the lg_local_lock/unlock only touching cpu-local resources it
-is fast. Taking the local lock on a different CPU will be more
-expensive but still relatively cheap.
-
-One can relax the migration constraints by acquiring the current
-CPUs lock with lg_local_lock_cpu, remember the cpu, and release that
-lock at the end of the critical section even if migrated. This should
-give most of the performance benefits without inhibiting migration
-though needs careful considerations for nesting of lglocks and
-consideration of deadlocks with lg_global_lock.
-
-The lg_global_lock/unlock locks all underlying spinlocks of all
-possible CPUs (including those off-line). The preemption disable/enable
-are needed in the non-RT kernels to prevent deadlocks like:
-
- on cpu 1
-
- task A task B
- lg_global_lock
- got cpu 0 lock
- <<<< preempt <<<<
- lg_local_lock_cpu for cpu 0
- spin on cpu 0 lock
-
-On -RT this deadlock scenario is resolved by the arch_spin_locks in the
-lglocks being replaced by rt_mutexes which resolve the above deadlock
-by boosting the lock-holder.
-
-
-Implementation:
----------------
-
-The initial lglock implementation from Nick Piggin used some complex
-macros to generate the lglock/brlock in lglock.h - they were later
-turned into a set of functions by Andi Kleen [7]. The change to functions
-was motivated by the presence of multiple lock users and also by them
-being easier to maintain than the generating macros. This change to
-functions is also the basis to eliminated the restriction of not
-being initializeable in kernel modules (the remaining problem is that
-locks are not explicitly initialized - see lockdep-design.txt)
-
-Declaration and initialization:
--------------------------------
-
- #include <linux/lglock.h>
-
- DEFINE_LGLOCK(name)
- or:
- DEFINE_STATIC_LGLOCK(name);
-
- lg_lock_init(&name, "lockdep_name_string");
-
- on UP this is mapped to DEFINE_SPINLOCK(name) in both cases, note
- also that as of 3.18-rc6 all declaration in use are of the _STATIC_
- variant (and it seems that the non-static was never in use).
- lg_lock_init is initializing the lockdep map only.
-
-Usage:
-------
-
-From the locking semantics it is a spinlock. It could be called a
-locality aware spinlock. lg_local_* behaves like a per_cpu
-spinlock and lg_global_* like a global spinlock.
-No surprises in the API.
-
- lg_local_lock(*lglock);
- access to protected per_cpu object on this CPU
- lg_local_unlock(*lglock);
-
- lg_local_lock_cpu(*lglock, cpu);
- access to protected per_cpu object on other CPU cpu
- lg_local_unlock_cpu(*lglock, cpu);
-
- lg_global_lock(*lglock);
- access all protected per_cpu objects on all CPUs
- lg_global_unlock(*lglock);
-
- There are no _trylock variants of the lglocks.
-
-Note that the lg_global_lock/unlock has to iterate over all possible
-CPUs rather than the actually present CPUs or a CPU could go off-line
-with a held lock [4] and that makes it very expensive. A discussion on
-these issues can be found at [5]
-
-Constraints:
-------------
-
- * currently the declaration of lglocks in kernel modules is not
- possible, though this should be doable with little change.
- * lglocks are not recursive.
- * suitable for code that can do most operations on the CPU local
- data and will very rarely need the global lock
- * lg_global_lock/unlock is *very* expensive and does not scale
- * on UP systems all lg_* primitives are simply spinlocks
- * in PREEMPT_RT the spinlock becomes an rt-mutex and can sleep but
- does not change the tasks state while sleeping [6].
- * in PREEMPT_RT the preempt_disable/enable in lg_local_lock/unlock
- is downgraded to a migrate_disable/enable, the other
- preempt_disable/enable are downgraded to barriers [6].
- The deadlock noted for non-RT above is resolved due to rt_mutexes
- boosting the lock-holder in this case which arch_spin_locks do
- not do.
-
-lglocks were designed for very specific problems in the VFS and probably
-only are the right answer in these corner cases. Any new user that looks
-at lglocks probably wants to look at the seqlock and RCU alternatives as
-her first choice. There are also efforts to resolve the RCU issues that
-currently prevent using RCU in place of view remaining lglocks.
-
-Note on brlock history:
------------------------
-
-The 'Big Reader' read-write spinlocks were originally introduced by
-Ingo Molnar in 2000 (2.4/2.5 kernel series) and removed in 2003. They
-later were introduced by the VFS scalability patch set in 2.6 series
-again as the "big reader lock" brlock [2] variant of lglock which has
-been replaced by seqlock primitives or by RCU based primitives in the
-3.13 kernel series as was suggested in [3] in 2003. The brlock was
-entirely removed in the 3.13 kernel series.
-
-Link: 1 http://lkml.org/lkml/2010/8/2/81
-Link: 2 http://lwn.net/Articles/401738/
-Link: 3 http://lkml.org/lkml/2003/3/9/205
-Link: 4 https://lkml.org/lkml/2011/8/24/185
-Link: 5 http://lkml.org/lkml/2011/12/18/189
-Link: 6 https://www.kernel.org/pub/linux/kernel/projects/rt/
- patch series - lglocks-rt.patch.patch
-Link: 7 http://lkml.org/lkml/2012/3/5/26
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -20,7 +20,6 @@
#include <linux/cdev.h>
#include <linux/fsnotify.h>
#include <linux/sysctl.h>
-#include <linux/lglock.h>
#include <linux/percpu_counter.h>
#include <linux/percpu.h>
#include <linux/hardirq.h>
--- a/include/linux/lglock.h
+++ /dev/null
@@ -1,81 +0,0 @@
-/*
- * 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 <linux/cpu.h>
-#include <linux/notifier.h>
-
-#ifdef CONFIG_SMP
-
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
-#define LOCKDEP_INIT_MAP lockdep_init_map
-#else
-#define LOCKDEP_INIT_MAP(a, b, c, d)
-#endif
-
-struct lglock {
- arch_spinlock_t __percpu *lock;
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
- struct lock_class_key lock_key;
- struct lockdep_map lock_dep_map;
-#endif
-};
-
-#define DEFINE_LGLOCK(name) \
- static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock) \
- = __ARCH_SPIN_LOCK_UNLOCKED; \
- struct lglock name = { .lock = &name ## _lock }
-
-#define DEFINE_STATIC_LGLOCK(name) \
- static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock) \
- = __ARCH_SPIN_LOCK_UNLOCKED; \
- static struct lglock name = { .lock = &name ## _lock }
-
-void lg_lock_init(struct lglock *lg, char *name);
-
-void lg_local_lock(struct lglock *lg);
-void lg_local_unlock(struct lglock *lg);
-void lg_local_lock_cpu(struct lglock *lg, int cpu);
-void lg_local_unlock_cpu(struct lglock *lg, int cpu);
-
-void lg_double_lock(struct lglock *lg, int cpu1, int cpu2);
-void lg_double_unlock(struct lglock *lg, int cpu1, int cpu2);
-
-void lg_global_lock(struct lglock *lg);
-void lg_global_unlock(struct lglock *lg);
-
-#else
-/* When !CONFIG_SMP, map lglock to spinlock */
-#define lglock spinlock
-#define DEFINE_LGLOCK(name) DEFINE_SPINLOCK(name)
-#define DEFINE_STATIC_LGLOCK(name) static DEFINE_SPINLOCK(name)
-#define lg_lock_init(lg, name) spin_lock_init(lg)
-#define lg_local_lock spin_lock
-#define lg_local_unlock spin_unlock
-#define lg_local_lock_cpu(lg, cpu) spin_lock(lg)
-#define lg_local_unlock_cpu(lg, cpu) spin_unlock(lg)
-#define lg_global_lock spin_lock
-#define lg_global_unlock spin_unlock
-#endif
-
-#endif
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -15,7 +15,6 @@ obj-$(CONFIG_LOCKDEP) += lockdep_proc.o
endif
obj-$(CONFIG_SMP) += spinlock.o
obj-$(CONFIG_LOCK_SPIN_ON_OWNER) += osq_lock.o
-obj-$(CONFIG_SMP) += lglock.o
obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
obj-$(CONFIG_QUEUED_SPINLOCKS) += qspinlock.o
obj-$(CONFIG_RT_MUTEXES) += rtmutex.o
--- a/kernel/locking/lglock.c
+++ /dev/null
@@ -1,111 +0,0 @@
-/* See include/linux/lglock.h for description */
-#include <linux/module.h>
-#include <linux/lglock.h>
-#include <linux/cpu.h>
-#include <linux/string.h>
-
-/*
- * Note there is no uninit, so lglocks cannot be defined in
- * modules (but it's fine to use them from there)
- * Could be added though, just undo lg_lock_init
- */
-
-void lg_lock_init(struct lglock *lg, char *name)
-{
- LOCKDEP_INIT_MAP(&lg->lock_dep_map, name, &lg->lock_key, 0);
-}
-EXPORT_SYMBOL(lg_lock_init);
-
-void lg_local_lock(struct lglock *lg)
-{
- arch_spinlock_t *lock;
-
- preempt_disable();
- lock_acquire_shared(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
- lock = this_cpu_ptr(lg->lock);
- arch_spin_lock(lock);
-}
-EXPORT_SYMBOL(lg_local_lock);
-
-void lg_local_unlock(struct lglock *lg)
-{
- arch_spinlock_t *lock;
-
- lock_release(&lg->lock_dep_map, 1, _RET_IP_);
- lock = this_cpu_ptr(lg->lock);
- arch_spin_unlock(lock);
- preempt_enable();
-}
-EXPORT_SYMBOL(lg_local_unlock);
-
-void lg_local_lock_cpu(struct lglock *lg, int cpu)
-{
- arch_spinlock_t *lock;
-
- preempt_disable();
- lock_acquire_shared(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
- lock = per_cpu_ptr(lg->lock, cpu);
- arch_spin_lock(lock);
-}
-EXPORT_SYMBOL(lg_local_lock_cpu);
-
-void lg_local_unlock_cpu(struct lglock *lg, int cpu)
-{
- arch_spinlock_t *lock;
-
- lock_release(&lg->lock_dep_map, 1, _RET_IP_);
- lock = per_cpu_ptr(lg->lock, cpu);
- arch_spin_unlock(lock);
- preempt_enable();
-}
-EXPORT_SYMBOL(lg_local_unlock_cpu);
-
-void lg_double_lock(struct lglock *lg, int cpu1, int cpu2)
-{
- BUG_ON(cpu1 == cpu2);
-
- /* lock in cpu order, just like lg_global_lock */
- if (cpu2 < cpu1)
- swap(cpu1, cpu2);
-
- preempt_disable();
- lock_acquire_shared(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
- arch_spin_lock(per_cpu_ptr(lg->lock, cpu1));
- arch_spin_lock(per_cpu_ptr(lg->lock, cpu2));
-}
-
-void lg_double_unlock(struct lglock *lg, int cpu1, int cpu2)
-{
- lock_release(&lg->lock_dep_map, 1, _RET_IP_);
- arch_spin_unlock(per_cpu_ptr(lg->lock, cpu1));
- arch_spin_unlock(per_cpu_ptr(lg->lock, cpu2));
- preempt_enable();
-}
-
-void lg_global_lock(struct lglock *lg)
-{
- int i;
-
- preempt_disable();
- lock_acquire_exclusive(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
- for_each_possible_cpu(i) {
- arch_spinlock_t *lock;
- lock = per_cpu_ptr(lg->lock, i);
- arch_spin_lock(lock);
- }
-}
-EXPORT_SYMBOL(lg_global_lock);
-
-void lg_global_unlock(struct lglock *lg)
-{
- int i;
-
- lock_release(&lg->lock_dep_map, 1, _RET_IP_);
- for_each_possible_cpu(i) {
- arch_spinlock_t *lock;
- lock = per_cpu_ptr(lg->lock, i);
- arch_spin_unlock(lock);
- }
- preempt_enable();
-}
-EXPORT_SYMBOL(lg_global_unlock);
--
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