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locking/rwsem: Remove rwsem-spinlock.c & use rwsem-xadd.c for all archs 

Currently, we have two different implementation of rwsem:

 1) CONFIG_RWSEM_GENERIC_SPINLOCK (rwsem-spinlock.c)
 2) CONFIG_RWSEM_XCHGADD_ALGORITHM (rwsem-xadd.c)

As we are going to use a single generic implementation for rwsem-xadd.c
and no architecture-specific code will be needed, there is no point
in keeping two different implementations of rwsem. In most cases, the
performance of rwsem-spinlock.c will be worse. It also doesn't get all
the performance tuning and optimizations that had been implemented in
rwsem-xadd.c over the years.

For simplication, we are going to remove rwsem-spinlock.c and make all
architectures use a single implementation of rwsem - rwsem-xadd.c.

All references to RWSEM_GENERIC_SPINLOCK and RWSEM_XCHGADD_ALGORITHM
in the code are removed.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-c6x-dev@linux-c6x.org
Cc: linux-m68k@lists.linux-m68k.org
Cc: linux-riscv@lists.infradead.org
Cc: linux-um@lists.infradead.org
Cc: linux-xtensa@linux-xtensa.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: nios2-dev@lists.rocketboards.org
Cc: openrisc@lists.librecores.org
Cc: uclinux-h8-devel@lists.sourceforge.jp
Link: https://lkml.kernel.org/r/20190322143008.21313-3-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Waiman Long 2019-03-22 10:30:07 -04:00 committed by Ingo Molnar
parent 46ad0840b1
commit 390a0c62c2
31 changed files with 2 additions and 520 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
arch/alpha/Kconfig
View File

@ -49,13 +49,6 @@ config MMU
bool
default y
config RWSEM_GENERIC_SPINLOCK
bool
config RWSEM_XCHGADD_ALGORITHM
bool
default y
config ARCH_HAS_ILOG2_U32
bool
default n

3
arch/arc/Kconfig
View File

@ -63,9 +63,6 @@ config SCHED_OMIT_FRAME_POINTER
config GENERIC_CSUM
def_bool y
config RWSEM_GENERIC_SPINLOCK
def_bool y
config ARCH_DISCONTIGMEM_ENABLE
def_bool n

4
arch/arm/Kconfig
View File

@ -178,10 +178,6 @@ config TRACE_IRQFLAGS_SUPPORT
bool
default !CPU_V7M
config RWSEM_XCHGADD_ALGORITHM
bool
default y
config ARCH_HAS_ILOG2_U32
bool

3
arch/arm64/Kconfig
View File

@ -237,9 +237,6 @@ config LOCKDEP_SUPPORT
config TRACE_IRQFLAGS_SUPPORT
def_bool y
config RWSEM_XCHGADD_ALGORITHM
def_bool y
config GENERIC_BUG
def_bool y
depends on BUG

3
arch/c6x/Kconfig
View File

@ -27,9 +27,6 @@ config MMU
config FPU
def_bool n
config RWSEM_GENERIC_SPINLOCK
def_bool y
config GENERIC_CALIBRATE_DELAY
def_bool y

3
arch/csky/Kconfig
View File

@ -92,9 +92,6 @@ config GENERIC_HWEIGHT
config MMU
def_bool y
config RWSEM_GENERIC_SPINLOCK
def_bool y
config STACKTRACE_SUPPORT
def_bool y

3
arch/h8300/Kconfig
View File

@ -27,9 +27,6 @@ config H8300
config CPU_BIG_ENDIAN
def_bool y
config RWSEM_GENERIC_SPINLOCK
def_bool y
config GENERIC_HWEIGHT
def_bool y

6
arch/hexagon/Kconfig
View File

@ -65,12 +65,6 @@ config GENERIC_CSUM
config GENERIC_IRQ_PROBE
def_bool y
config RWSEM_GENERIC_SPINLOCK
def_bool n
config RWSEM_XCHGADD_ALGORITHM
def_bool y
config GENERIC_HWEIGHT
def_bool y

4
arch/ia64/Kconfig
View File

@ -83,10 +83,6 @@ config STACKTRACE_SUPPORT
config GENERIC_LOCKBREAK
def_bool n
config RWSEM_XCHGADD_ALGORITHM
bool
default y
config HUGETLB_PAGE_SIZE_VARIABLE
bool
depends on HUGETLB_PAGE

7
arch/m68k/Kconfig
View File

@ -32,13 +32,6 @@ config M68K
config CPU_BIG_ENDIAN
def_bool y
config RWSEM_GENERIC_SPINLOCK
bool
default y
config RWSEM_XCHGADD_ALGORITHM
bool
config ARCH_HAS_ILOG2_U32
bool

6
arch/microblaze/Kconfig
View File

@ -58,15 +58,9 @@ config CPU_LITTLE_ENDIAN
endchoice
config RWSEM_GENERIC_SPINLOCK
def_bool y
config ZONE_DMA
def_bool y
config RWSEM_XCHGADD_ALGORITHM
bool
config ARCH_HAS_ILOG2_U32
def_bool n

7
arch/mips/Kconfig
View File

@ -1037,13 +1037,6 @@ source "arch/mips/paravirt/Kconfig"
endmenu
config RWSEM_GENERIC_SPINLOCK
bool
default y
config RWSEM_XCHGADD_ALGORITHM
bool
config GENERIC_HWEIGHT
bool
default y

3
arch/nds32/Kconfig
View File

@ -60,9 +60,6 @@ config GENERIC_LOCKBREAK
def_bool y
depends on PREEMPT
config RWSEM_GENERIC_SPINLOCK
def_bool y
config TRACE_IRQFLAGS_SUPPORT
def_bool y

3
arch/nios2/Kconfig
View File

@ -40,9 +40,6 @@ config NO_IOPORT_MAP
config FPU
def_bool n
config RWSEM_GENERIC_SPINLOCK
def_bool y
config TRACE_IRQFLAGS_SUPPORT
def_bool n

6
arch/openrisc/Kconfig
View File

@ -43,12 +43,6 @@ config CPU_BIG_ENDIAN
config MMU
def_bool y
config RWSEM_GENERIC_SPINLOCK
def_bool y
config RWSEM_XCHGADD_ALGORITHM
def_bool n
config GENERIC_HWEIGHT
def_bool y

6
arch/parisc/Kconfig
View File

@ -75,12 +75,6 @@ config GENERIC_LOCKBREAK
default y
depends on SMP && PREEMPT
config RWSEM_GENERIC_SPINLOCK
def_bool y
config RWSEM_XCHGADD_ALGORITHM
bool
config ARCH_HAS_ILOG2_U32
bool
default n

7
arch/powerpc/Kconfig
View File

@ -103,13 +103,6 @@ config LOCKDEP_SUPPORT
bool
default y
config RWSEM_GENERIC_SPINLOCK
bool
config RWSEM_XCHGADD_ALGORITHM
bool
default y
config GENERIC_LOCKBREAK
bool
default y

3
arch/riscv/Kconfig
View File

@ -69,9 +69,6 @@ config STACKTRACE_SUPPORT
config TRACE_IRQFLAGS_SUPPORT
def_bool y
config RWSEM_GENERIC_SPINLOCK
def_bool y
config GENERIC_BUG
def_bool y
depends on BUG

6
arch/s390/Kconfig
View File

@ -14,12 +14,6 @@ config LOCKDEP_SUPPORT
config STACKTRACE_SUPPORT
def_bool y
config RWSEM_GENERIC_SPINLOCK
bool
config RWSEM_XCHGADD_ALGORITHM
def_bool y
config ARCH_HAS_ILOG2_U32
def_bool n

6
arch/sh/Kconfig
View File

@ -90,12 +90,6 @@ config ARCH_DEFCONFIG
default "arch/sh/configs/shx3_defconfig" if SUPERH32
default "arch/sh/configs/cayman_defconfig" if SUPERH64
config RWSEM_GENERIC_SPINLOCK
def_bool y
config RWSEM_XCHGADD_ALGORITHM
bool
config GENERIC_BUG
def_bool y
depends on BUG && SUPERH32

8
arch/sparc/Kconfig
View File

@ -191,14 +191,6 @@ config NR_CPUS
source "kernel/Kconfig.hz"
config RWSEM_GENERIC_SPINLOCK
bool
default y if SPARC32
config RWSEM_XCHGADD_ALGORITHM
bool
default y if SPARC64
config GENERIC_HWEIGHT
bool
default y

6
arch/unicore32/Kconfig
View File

@ -38,12 +38,6 @@ config STACKTRACE_SUPPORT
config LOCKDEP_SUPPORT
def_bool y
config RWSEM_GENERIC_SPINLOCK
def_bool y
config RWSEM_XCHGADD_ALGORITHM
bool
config ARCH_HAS_ILOG2_U32
bool

3
arch/x86/Kconfig
View File

@ -268,9 +268,6 @@ config ARCH_MAY_HAVE_PC_FDC
def_bool y
depends on ISA_DMA_API
config RWSEM_XCHGADD_ALGORITHM
def_bool y
config GENERIC_CALIBRATE_DELAY
def_bool y

6
arch/x86/um/Kconfig
View File

@ -32,12 +32,6 @@ config ARCH_DEFCONFIG
default "arch/um/configs/i386_defconfig" if X86_32
default "arch/um/configs/x86_64_defconfig" if X86_64
config RWSEM_XCHGADD_ALGORITHM
def_bool 64BIT
config RWSEM_GENERIC_SPINLOCK
def_bool !RWSEM_XCHGADD_ALGORITHM
config 3_LEVEL_PGTABLES
bool "Three-level pagetables" if !64BIT
default 64BIT

3
arch/xtensa/Kconfig
View File

@ -46,9 +46,6 @@ config XTENSA
with reasonable minimum requirements. The Xtensa Linux project has
a home page at <http://www.linux-xtensa.org/>.
config RWSEM_XCHGADD_ALGORITHM
def_bool y
config GENERIC_HWEIGHT
def_bool y

47
include/linux/rwsem-spinlock.h
View File

@ -1,47 +0,0 @@
/* SPDX-License-Identifier: GPL-2.0 */
/* rwsem-spinlock.h: fallback C implementation
*
* Copyright (c) 2001 David Howells (dhowells@redhat.com).
* - Derived partially from ideas by Andrea Arcangeli <andrea@suse.de>
* - Derived also from comments by Linus
*/
#ifndef _LINUX_RWSEM_SPINLOCK_H
#define _LINUX_RWSEM_SPINLOCK_H
#ifndef _LINUX_RWSEM_H
#error "please don't include linux/rwsem-spinlock.h directly, use linux/rwsem.h instead"
#endif
#ifdef __KERNEL__
/*
* the rw-semaphore definition
* - if count is 0 then there are no active readers or writers
* - if count is +ve then that is the number of active readers
* - if count is -1 then there is one active writer
* - if wait_list is not empty, then there are processes waiting for the semaphore
*/
struct rw_semaphore {
__s32 count;
raw_spinlock_t wait_lock;
struct list_head wait_list;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
};
#define RWSEM_UNLOCKED_VALUE 0x00000000
extern void __down_read(struct rw_semaphore *sem);
extern int __must_check __down_read_killable(struct rw_semaphore *sem);
extern int __down_read_trylock(struct rw_semaphore *sem);
extern void __down_write(struct rw_semaphore *sem);
extern int __must_check __down_write_killable(struct rw_semaphore *sem);
extern int __down_write_trylock(struct rw_semaphore *sem);
extern void __up_read(struct rw_semaphore *sem);
extern void __up_write(struct rw_semaphore *sem);
extern void __downgrade_write(struct rw_semaphore *sem);
extern int rwsem_is_locked(struct rw_semaphore *sem);
#endif /* __KERNEL__ */
#endif /* _LINUX_RWSEM_SPINLOCK_H */

5
include/linux/rwsem.h
View File

@ -22,10 +22,6 @@
struct rw_semaphore;
#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
#include <linux/rwsem-spinlock.h> /* use a generic implementation */
#define __RWSEM_INIT_COUNT(name) .count = RWSEM_UNLOCKED_VALUE
#else
/* All arch specific implementations share the same struct */
struct rw_semaphore {
atomic_long_t count;
@ -65,7 +61,6 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
#define RWSEM_UNLOCKED_VALUE 0L
#define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
#endif
/* Common initializer macros and functions */

2
kernel/Kconfig.locks
View File

@ -229,7 +229,7 @@ config MUTEX_SPIN_ON_OWNER
config RWSEM_SPIN_ON_OWNER
def_bool y
depends on SMP && RWSEM_XCHGADD_ALGORITHM && ARCH_SUPPORTS_ATOMIC_RMW
depends on SMP && ARCH_SUPPORTS_ATOMIC_RMW
config LOCK_SPIN_ON_OWNER
def_bool y

4
kernel/locking/Makefile
View File

@ -3,7 +3,7 @@
# and is generally not a function of system call inputs.
KCOV_INSTRUMENT := n
obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o
obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o rwsem-xadd.o
ifdef CONFIG_FUNCTION_TRACER
CFLAGS_REMOVE_lockdep.o = $(CC_FLAGS_FTRACE)
@ -25,8 +25,6 @@ obj-$(CONFIG_RT_MUTEXES) += rtmutex.o
obj-$(CONFIG_DEBUG_RT_MUTEXES) += rtmutex-debug.o
obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock_debug.o
obj-$(CONFIG_RWSEM_GENERIC_SPINLOCK) += rwsem-spinlock.o
obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem-xadd.o
obj-$(CONFIG_QUEUED_RWLOCKS) += qrwlock.o
obj-$(CONFIG_LOCK_TORTURE_TEST) += locktorture.o
obj-$(CONFIG_WW_MUTEX_SELFTEST) += test-ww_mutex.o

339
kernel/locking/rwsem-spinlock.c
View File

@ -1,339 +0,0 @@
// SPDX-License-Identifier: GPL-2.0
/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
* generic spinlock implementation
*
* Copyright (c) 2001 David Howells (dhowells@redhat.com).
* - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
* - Derived also from comments by Linus
*/
#include <linux/rwsem.h>
#include <linux/sched/signal.h>
#include <linux/sched/debug.h>
#include <linux/export.h>
enum rwsem_waiter_type {
RWSEM_WAITING_FOR_WRITE,
RWSEM_WAITING_FOR_READ
};
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
};
int rwsem_is_locked(struct rw_semaphore *sem)
{
int ret = 1;
unsigned long flags;
if (raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
ret = (sem->count != 0);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
}
return ret;
}
EXPORT_SYMBOL(rwsem_is_locked);
/*
* initialise the semaphore
*/
void __init_rwsem(struct rw_semaphore *sem, const char *name,
struct lock_class_key *key)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
/*
* Make sure we are not reinitializing a held semaphore:
*/
debug_check_no_locks_freed((void *)sem, sizeof(*sem));
lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
sem->count = 0;
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->wait_list);
}
EXPORT_SYMBOL(__init_rwsem);
/*
* handle the lock release when processes blocked on it that can now run
* - if we come here, then:
* - the 'active count' _reached_ zero
* - the 'waiting count' is non-zero
* - the spinlock must be held by the caller
* - woken process blocks are discarded from the list after having task zeroed
* - writers are only woken if wakewrite is non-zero
*/
static inline struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
{
struct rwsem_waiter *waiter;
struct task_struct *tsk;
int woken;
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
if (wakewrite)
/* Wake up a writer. Note that we do not grant it the
* lock - it will have to acquire it when it runs. */
wake_up_process(waiter->task);
goto out;
}
/* grant an infinite number of read locks to the front of the queue */
woken = 0;
do {
struct list_head *next = waiter->list.next;
list_del(&waiter->list);
tsk = waiter->task;
/*
* Make sure we do not wakeup the next reader before
* setting the nil condition to grant the next reader;
* otherwise we could miss the wakeup on the other
* side and end up sleeping again. See the pairing
* in rwsem_down_read_failed().
*/
smp_mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
woken++;
if (next == &sem->wait_list)
break;
waiter = list_entry(next, struct rwsem_waiter, list);
} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
sem->count += woken;
out:
return sem;
}
/*
* wake a single writer
*/
static inline struct rw_semaphore *
__rwsem_wake_one_writer(struct rw_semaphore *sem)
{
struct rwsem_waiter *waiter;
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
wake_up_process(waiter->task);
return sem;
}
/*
* get a read lock on the semaphore
*/
int __sched __down_read_common(struct rw_semaphore *sem, int state)
{
struct rwsem_waiter waiter;
unsigned long flags;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
if (sem->count >= 0 && list_empty(&sem->wait_list)) {
/* granted */
sem->count++;
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
goto out;
}
/* set up my own style of waitqueue */
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_READ;
get_task_struct(current);
list_add_tail(&waiter.list, &sem->wait_list);
/* wait to be given the lock */
for (;;) {
if (!waiter.task)
break;
if (signal_pending_state(state, current))
goto out_nolock;
set_current_state(state);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
schedule();
raw_spin_lock_irqsave(&sem->wait_lock, flags);
}
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
out:
return 0;
out_nolock:
/*
* We didn't take the lock, so that there is a writer, which
* is owner or the first waiter of the sem. If it's a waiter,
* it will be woken by current owner. Not need to wake anybody.
*/
list_del(&waiter.list);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
return -EINTR;
}
void __sched __down_read(struct rw_semaphore *sem)
{
__down_read_common(sem, TASK_UNINTERRUPTIBLE);
}
int __sched __down_read_killable(struct rw_semaphore *sem)
{
return __down_read_common(sem, TASK_KILLABLE);
}
/*
* trylock for reading -- returns 1 if successful, 0 if contention
*/
int __down_read_trylock(struct rw_semaphore *sem)
{
unsigned long flags;
int ret = 0;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
if (sem->count >= 0 && list_empty(&sem->wait_list)) {
/* granted */
sem->count++;
ret = 1;
}
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
return ret;
}
/*
* get a write lock on the semaphore
*/
int __sched __down_write_common(struct rw_semaphore *sem, int state)
{
struct rwsem_waiter waiter;
unsigned long flags;
int ret = 0;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
/* set up my own style of waitqueue */
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
list_add_tail(&waiter.list, &sem->wait_list);
/* wait for someone to release the lock */
for (;;) {
/*
* That is the key to support write lock stealing: allows the
* task already on CPU to get the lock soon rather than put
* itself into sleep and waiting for system woke it or someone
* else in the head of the wait list up.
*/
if (sem->count == 0)
break;
if (signal_pending_state(state, current))
goto out_nolock;
set_current_state(state);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
schedule();
raw_spin_lock_irqsave(&sem->wait_lock, flags);
}
/* got the lock */
sem->count = -1;
list_del(&waiter.list);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
return ret;
out_nolock:
list_del(&waiter.list);
if (!list_empty(&sem->wait_list) && sem->count >= 0)
__rwsem_do_wake(sem, 0);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
return -EINTR;
}
void __sched __down_write(struct rw_semaphore *sem)
{
__down_write_common(sem, TASK_UNINTERRUPTIBLE);
}
int __sched __down_write_killable(struct rw_semaphore *sem)
{
return __down_write_common(sem, TASK_KILLABLE);
}
/*
* trylock for writing -- returns 1 if successful, 0 if contention
*/
int __down_write_trylock(struct rw_semaphore *sem)
{
unsigned long flags;
int ret = 0;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
if (sem->count == 0) {
/* got the lock */
sem->count = -1;
ret = 1;
}
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
return ret;
}
/*
* release a read lock on the semaphore
*/
void __up_read(struct rw_semaphore *sem)
{
unsigned long flags;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
if (--sem->count == 0 && !list_empty(&sem->wait_list))
sem = __rwsem_wake_one_writer(sem);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
}
/*
* release a write lock on the semaphore
*/
void __up_write(struct rw_semaphore *sem)
{
unsigned long flags;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
sem->count = 0;
if (!list_empty(&sem->wait_list))
sem = __rwsem_do_wake(sem, 1);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
}
/*
* downgrade a write lock into a read lock
* - just wake up any readers at the front of the queue
*/
void __downgrade_write(struct rw_semaphore *sem)
{
unsigned long flags;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
sem->count = 1;
if (!list_empty(&sem->wait_list))
sem = __rwsem_do_wake(sem, 0);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
}

3
kernel/locking/rwsem.h
View File

@ -153,7 +153,6 @@ static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
}
#endif
#ifdef CONFIG_RWSEM_XCHGADD_ALGORITHM
/*
* lock for reading
*/
@ -260,5 +259,3 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
if (tmp < 0)
rwsem_downgrade_wake(sem);
}
#endif /* CONFIG_RWSEM_XCHGADD_ALGORITHM */