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29671f22a8
rwsem_is_locked() tests ->activity without locks, so we should always keep ->activity consistent. However, the code in __rwsem_do_wake() breaks this rule, it updates ->activity after _all_ readers waken up, this may give some reader a wrong ->activity value, thus cause rwsem_is_locked() behaves wrong. Quote from Andrew: " - we have one or more processes sleeping in down_read(), waiting for access. - we wake one or more processes up without altering ->activity - they start to run and they do rwsem_is_locked(). This incorrectly returns "false", because the waker process is still crunching away in __rwsem_do_wake(). - the waker now alters ->activity, but it was too late. " So we need get a spinlock to protect this. And rwsem_is_locked() should not block, thus we use spin_trylock_irqsave(). [akpm@linux-foundation.org: simplify code] Reported-by: Brian Behlendorf <behlendorf1@llnl.gov> Cc: Ben Woodard <bwoodard@llnl.gov> Cc: David Howells <dhowells@redhat.com> Signed-off-by: WANG Cong <amwang@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
75 lines
2.2 KiB
C
75 lines
2.2 KiB
C
/* rwsem-spinlock.h: fallback C implementation
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*
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* Copyright (c) 2001 David Howells (dhowells@redhat.com).
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* - Derived partially from ideas by Andrea Arcangeli <andrea@suse.de>
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* - Derived also from comments by Linus
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*/
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#ifndef _LINUX_RWSEM_SPINLOCK_H
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#define _LINUX_RWSEM_SPINLOCK_H
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#ifndef _LINUX_RWSEM_H
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#error "please don't include linux/rwsem-spinlock.h directly, use linux/rwsem.h instead"
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#endif
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#include <linux/spinlock.h>
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#include <linux/list.h>
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#ifdef __KERNEL__
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#include <linux/types.h>
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struct rwsem_waiter;
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/*
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* the rw-semaphore definition
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* - if activity is 0 then there are no active readers or writers
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* - if activity is +ve then that is the number of active readers
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* - if activity is -1 then there is one active writer
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* - if wait_list is not empty, then there are processes waiting for the semaphore
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*/
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struct rw_semaphore {
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__s32 activity;
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spinlock_t wait_lock;
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struct list_head wait_list;
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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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#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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#define __RWSEM_INITIALIZER(name) \
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{ 0, __SPIN_LOCK_UNLOCKED(name.wait_lock), LIST_HEAD_INIT((name).wait_list) \
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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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extern void __down_read(struct rw_semaphore *sem);
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extern int __down_read_trylock(struct rw_semaphore *sem);
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extern void __down_write(struct rw_semaphore *sem);
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extern void __down_write_nested(struct rw_semaphore *sem, int subclass);
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extern int __down_write_trylock(struct rw_semaphore *sem);
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extern void __up_read(struct rw_semaphore *sem);
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extern void __up_write(struct rw_semaphore *sem);
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extern void __downgrade_write(struct rw_semaphore *sem);
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extern int rwsem_is_locked(struct rw_semaphore *sem);
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#endif /* __KERNEL__ */
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#endif /* _LINUX_RWSEM_SPINLOCK_H */
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