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![Waiman Long](/assets/img/avatar_default.png)
Currently, when a reader acquires a lock, it only sets the RWSEM_READER_OWNED bit in the owner field. The other bits are simply not used. When debugging hanging cases involving rwsems and readers, the owner value does not provide much useful information at all. This patch modifies the current behavior to always store the task_struct pointer of the last rwsem-acquiring reader in a reader-owned rwsem. This may be useful in debugging rwsem hanging cases especially if only one reader is involved. However, the task in the owner field may not the real owner or one of the real owners at all when the owner value is examined, for example, in a crash dump. So it is just an additional hint about the past history. If CONFIG_DEBUG_RWSEMS=y is enabled, the owner field will be checked at unlock time too to make sure the task pointer value is valid. That does have a slight performance cost and so is only enabled as part of that debug option. From the performance point of view, it is expected that the changes shouldn't have any noticeable performance impact. A rwsem microbenchmark (with 48 worker threads and 1:1 reader/writer ratio) was ran on a 2-socket 24-core 48-thread Haswell system. The locking rates on a 4.19-rc1 based kernel were as follows: 1) Unpatched kernel: 543.3 kops/s 2) Patched kernel: 549.2 kops/s 3) Patched kernel (CONFIG_DEBUG_RWSEMS on): 546.6 kops/s There was actually a slight increase in performance (1.1%) in this particular case. Maybe it was caused by the elimination of a branch or just a testing noise. Turning on the CONFIG_DEBUG_RWSEMS option also had less than the expected impact on performance. The least significant 2 bits of the owner value are now used to designate the rwsem is readers owned and the owners are anonymous. Signed-off-by: Waiman Long <longman@redhat.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will.deacon@arm.com> Link: http://lkml.kernel.org/r/1536265114-10842-1-git-send-email-longman@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
196 lines
6 KiB
C
196 lines
6 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/* rwsem.h: R/W semaphores, public interface
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*
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* Written by David Howells (dhowells@redhat.com).
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* Derived from asm-i386/semaphore.h
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*/
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#ifndef _LINUX_RWSEM_H
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#define _LINUX_RWSEM_H
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#include <linux/linkage.h>
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#include <linux/types.h>
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#include <linux/kernel.h>
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#include <linux/list.h>
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#include <linux/spinlock.h>
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#include <linux/atomic.h>
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#include <linux/err.h>
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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#include <linux/osq_lock.h>
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#endif
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struct rw_semaphore;
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#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
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#include <linux/rwsem-spinlock.h> /* use a generic implementation */
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#define __RWSEM_INIT_COUNT(name) .count = RWSEM_UNLOCKED_VALUE
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#else
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/* All arch specific implementations share the same struct */
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struct rw_semaphore {
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atomic_long_t count;
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struct list_head wait_list;
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raw_spinlock_t wait_lock;
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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struct optimistic_spin_queue osq; /* spinner MCS lock */
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/*
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* Write owner. Used as a speculative check to see
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* if the owner is running on the cpu.
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*/
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struct task_struct *owner;
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#endif
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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struct lockdep_map dep_map;
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#endif
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};
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/*
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* Setting bit 1 of the owner field but not bit 0 will indicate
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* that the rwsem is writer-owned with an unknown owner.
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*/
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#define RWSEM_OWNER_UNKNOWN ((struct task_struct *)-2L)
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extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
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extern struct rw_semaphore *rwsem_down_read_failed_killable(struct rw_semaphore *sem);
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extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
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extern struct rw_semaphore *rwsem_down_write_failed_killable(struct rw_semaphore *sem);
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extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *);
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extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
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/* Include the arch specific part */
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#include <asm/rwsem.h>
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/* In all implementations count != 0 means locked */
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static inline int rwsem_is_locked(struct rw_semaphore *sem)
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{
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return atomic_long_read(&sem->count) != 0;
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}
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#define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
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#endif
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/* Common initializer macros and functions */
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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# define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
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#else
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# define __RWSEM_DEP_MAP_INIT(lockname)
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#endif
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED, .owner = NULL
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#else
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#define __RWSEM_OPT_INIT(lockname)
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#endif
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#define __RWSEM_INITIALIZER(name) \
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{ __RWSEM_INIT_COUNT(name), \
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.wait_list = LIST_HEAD_INIT((name).wait_list), \
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.wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
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__RWSEM_OPT_INIT(name) \
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__RWSEM_DEP_MAP_INIT(name) }
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#define DECLARE_RWSEM(name) \
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struct rw_semaphore name = __RWSEM_INITIALIZER(name)
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extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key);
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#define init_rwsem(sem) \
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do { \
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static struct lock_class_key __key; \
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\
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__init_rwsem((sem), #sem, &__key); \
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} while (0)
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/*
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* This is the same regardless of which rwsem implementation that is being used.
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* It is just a heuristic meant to be called by somebody alreadying holding the
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* rwsem to see if somebody from an incompatible type is wanting access to the
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* lock.
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*/
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static inline int rwsem_is_contended(struct rw_semaphore *sem)
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{
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return !list_empty(&sem->wait_list);
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}
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/*
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* lock for reading
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*/
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extern void down_read(struct rw_semaphore *sem);
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extern int __must_check down_read_killable(struct rw_semaphore *sem);
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/*
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* trylock for reading -- returns 1 if successful, 0 if contention
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*/
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extern int down_read_trylock(struct rw_semaphore *sem);
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/*
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* lock for writing
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*/
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extern void down_write(struct rw_semaphore *sem);
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extern int __must_check down_write_killable(struct rw_semaphore *sem);
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/*
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* trylock for writing -- returns 1 if successful, 0 if contention
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*/
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extern int down_write_trylock(struct rw_semaphore *sem);
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/*
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* release a read lock
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*/
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extern void up_read(struct rw_semaphore *sem);
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/*
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* release a write lock
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*/
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extern void up_write(struct rw_semaphore *sem);
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/*
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* downgrade write lock to read lock
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*/
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extern void downgrade_write(struct rw_semaphore *sem);
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* nested locking. NOTE: rwsems are not allowed to recurse
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* (which occurs if the same task tries to acquire the same
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* lock instance multiple times), but multiple locks of the
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* same lock class might be taken, if the order of the locks
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* is always the same. This ordering rule can be expressed
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* to lockdep via the _nested() APIs, but enumerating the
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* subclasses that are used. (If the nesting relationship is
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* static then another method for expressing nested locking is
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* the explicit definition of lock class keys and the use of
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* lockdep_set_class() at lock initialization time.
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* See Documentation/locking/lockdep-design.txt for more details.)
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*/
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extern void down_read_nested(struct rw_semaphore *sem, int subclass);
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extern void down_write_nested(struct rw_semaphore *sem, int subclass);
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extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
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extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
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# define down_write_nest_lock(sem, nest_lock) \
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do { \
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typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
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_down_write_nest_lock(sem, &(nest_lock)->dep_map); \
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} while (0);
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/*
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* Take/release a lock when not the owner will release it.
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*
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* [ This API should be avoided as much as possible - the
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* proper abstraction for this case is completions. ]
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*/
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extern void down_read_non_owner(struct rw_semaphore *sem);
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extern void up_read_non_owner(struct rw_semaphore *sem);
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#else
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# define down_read_nested(sem, subclass) down_read(sem)
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# define down_write_nest_lock(sem, nest_lock) down_write(sem)
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# define down_write_nested(sem, subclass) down_write(sem)
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# define down_write_killable_nested(sem, subclass) down_write_killable(sem)
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# define down_read_non_owner(sem) down_read(sem)
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# define up_read_non_owner(sem) up_read(sem)
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#endif
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#endif /* _LINUX_RWSEM_H */
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