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![Linus Torvalds](/assets/img/avatar_default.png)
Looking at the conditional lock acquire functions in the kernel due to the new sparse support (see commit4a557a5d1a
"sparse: introduce conditional lock acquire function attribute"), it became obvious that the lockref code has a couple of them, but they don't match the usual naming convention for the other ones, and their return value logic is also reversed. In the other very similar places, the naming pattern is '*_and_lock()' (eg 'atomic_put_and_lock()' and 'refcount_dec_and_lock()'), and the function returns true when the lock is taken. The lockref code is superficially very similar to the refcount code, only with the special "atomic wrt the embedded lock" semantics. But instead of the '*_and_lock()' naming it uses '*_or_lock()'. And instead of returning true in case it took the lock, it returns true if it *didn't* take the lock. Now, arguably the reflock code is quite logical: it really is a "either decrement _or_ lock" kind of situation - and the return value is about whether the operation succeeded without any special care needed. So despite the similarities, the differences do make some sense, and maybe it's not worth trying to unify the different conditional locking primitives in this area. But while looking at this all, it did become obvious that the 'lockref_get_or_lock()' function hasn't actually had any users for almost a decade. The only user it ever had was the shortlived 'd_rcu_to_refcount()' function, and it got removed and replaced with 'lockref_get_not_dead()' back in 2013 in commits0d98439ea3
("vfs: use lockred 'dead' flag to mark unrecoverably dead dentries") ande5c832d555
("vfs: fix dentry RCU to refcounting possibly sleeping dput()") In fact, that single use was removed less than a week after the whole function was introduced in commitb3abd80250
("lockref: add 'lockref_get_or_lock() helper") so this function has been around for a decade, but only had a user for six days. Let's just put this mis-designed and unused function out of its misery. We can think about the naming and semantic oddities of the remaining 'lockref_put_or_lock()' later, but at least that function has users. And while the naming is different and the return value doesn't match, that function matches the whole '{atomic,refcount}_dec_and_test()' pattern much better (ie the magic happens when the count goes down to zero, not when it is incremented from zero). Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
52 lines
1.5 KiB
C
52 lines
1.5 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __LINUX_LOCKREF_H
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#define __LINUX_LOCKREF_H
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/*
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* Locked reference counts.
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*
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* These are different from just plain atomic refcounts in that they
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* are atomic with respect to the spinlock that goes with them. In
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* particular, there can be implementations that don't actually get
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* the spinlock for the common decrement/increment operations, but they
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* still have to check that the operation is done semantically as if
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* the spinlock had been taken (using a cmpxchg operation that covers
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* both the lock and the count word, or using memory transactions, for
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* example).
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*/
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#include <linux/spinlock.h>
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#include <generated/bounds.h>
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#define USE_CMPXCHG_LOCKREF \
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(IS_ENABLED(CONFIG_ARCH_USE_CMPXCHG_LOCKREF) && \
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IS_ENABLED(CONFIG_SMP) && SPINLOCK_SIZE <= 4)
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struct lockref {
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union {
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#if USE_CMPXCHG_LOCKREF
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aligned_u64 lock_count;
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#endif
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struct {
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spinlock_t lock;
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int count;
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};
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};
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};
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extern void lockref_get(struct lockref *);
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extern int lockref_put_return(struct lockref *);
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extern int lockref_get_not_zero(struct lockref *);
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extern int lockref_put_not_zero(struct lockref *);
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extern int lockref_put_or_lock(struct lockref *);
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extern void lockref_mark_dead(struct lockref *);
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extern int lockref_get_not_dead(struct lockref *);
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/* Must be called under spinlock for reliable results */
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static inline bool __lockref_is_dead(const struct lockref *l)
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{
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return ((int)l->count < 0);
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}
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#endif /* __LINUX_LOCKREF_H */
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