mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-09-12 05:48:40 +00:00
![Linus Torvalds](/assets/img/avatar_default.png)
Instead of taking the spinlock, the lockless versions atomically check that the lock is not taken, and do the reference count update using a cmpxchg() loop. This is semantically identical to doing the reference count update protected by the lock, but avoids the "wait for lock" contention that you get when accesses to the reference count are contended. Note that a "lockref" is absolutely _not_ equivalent to an atomic_t. Even when the lockref reference counts are updated atomically with cmpxchg, the fact that they also verify the state of the spinlock means that the lockless updates can never happen while somebody else holds the spinlock. So while "lockref_put_or_lock()" looks a lot like just another name for "atomic_dec_and_lock()", and both optimize to lockless updates, they are fundamentally different: the decrement done by atomic_dec_and_lock() is truly independent of any lock (as long as it doesn't decrement to zero), so a locked region can still see the count change. The lockref structure, in contrast, really is a *locked* reference count. If you hold the spinlock, the reference count will be stable and you can modify the reference count without using atomics, because even the lockless updates will see and respect the state of the lock. In order to enable the cmpxchg lockless code, the architecture needs to do three things: (1) Make sure that the "arch_spinlock_t" and an "unsigned int" can fit in an aligned u64, and have a "cmpxchg()" implementation that works on such a u64 data type. (2) define a helper function to test for a spinlock being unlocked ("arch_spin_value_unlocked()") (3) select the "ARCH_USE_CMPXCHG_LOCKREF" config variable in its Kconfig file. This enables it for x86-64 (but not 32-bit, we'd need to make sure cmpxchg() turns into the proper cmpxchg8b in order to enable it for 32-bit mode). Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
127 lines
2.6 KiB
C
127 lines
2.6 KiB
C
#include <linux/export.h>
|
|
#include <linux/lockref.h>
|
|
|
|
#ifdef CONFIG_CMPXCHG_LOCKREF
|
|
|
|
/*
|
|
* Note that the "cmpxchg()" reloads the "old" value for the
|
|
* failure case.
|
|
*/
|
|
#define CMPXCHG_LOOP(CODE, SUCCESS) do { \
|
|
struct lockref old; \
|
|
BUILD_BUG_ON(sizeof(old) != 8); \
|
|
old.lock_count = ACCESS_ONCE(lockref->lock_count); \
|
|
while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) { \
|
|
struct lockref new = old, prev = old; \
|
|
CODE \
|
|
old.lock_count = cmpxchg(&lockref->lock_count, \
|
|
old.lock_count, new.lock_count); \
|
|
if (likely(old.lock_count == prev.lock_count)) { \
|
|
SUCCESS; \
|
|
} \
|
|
} \
|
|
} while (0)
|
|
|
|
#else
|
|
|
|
#define CMPXCHG_LOOP(CODE, SUCCESS) do { } while (0)
|
|
|
|
#endif
|
|
|
|
/**
|
|
* lockref_get - Increments reference count unconditionally
|
|
* @lockcnt: pointer to lockref structure
|
|
*
|
|
* This operation is only valid if you already hold a reference
|
|
* to the object, so you know the count cannot be zero.
|
|
*/
|
|
void lockref_get(struct lockref *lockref)
|
|
{
|
|
CMPXCHG_LOOP(
|
|
new.count++;
|
|
,
|
|
return;
|
|
);
|
|
|
|
spin_lock(&lockref->lock);
|
|
lockref->count++;
|
|
spin_unlock(&lockref->lock);
|
|
}
|
|
EXPORT_SYMBOL(lockref_get);
|
|
|
|
/**
|
|
* lockref_get_not_zero - Increments count unless the count is 0
|
|
* @lockcnt: pointer to lockref structure
|
|
* Return: 1 if count updated successfully or 0 if count was zero
|
|
*/
|
|
int lockref_get_not_zero(struct lockref *lockref)
|
|
{
|
|
int retval;
|
|
|
|
CMPXCHG_LOOP(
|
|
new.count++;
|
|
if (!old.count)
|
|
return 0;
|
|
,
|
|
return 1;
|
|
);
|
|
|
|
spin_lock(&lockref->lock);
|
|
retval = 0;
|
|
if (lockref->count) {
|
|
lockref->count++;
|
|
retval = 1;
|
|
}
|
|
spin_unlock(&lockref->lock);
|
|
return retval;
|
|
}
|
|
EXPORT_SYMBOL(lockref_get_not_zero);
|
|
|
|
/**
|
|
* lockref_get_or_lock - Increments count unless the count is 0
|
|
* @lockcnt: pointer to lockref structure
|
|
* Return: 1 if count updated successfully or 0 if count was zero
|
|
* and we got the lock instead.
|
|
*/
|
|
int lockref_get_or_lock(struct lockref *lockref)
|
|
{
|
|
CMPXCHG_LOOP(
|
|
new.count++;
|
|
if (!old.count)
|
|
break;
|
|
,
|
|
return 1;
|
|
);
|
|
|
|
spin_lock(&lockref->lock);
|
|
if (!lockref->count)
|
|
return 0;
|
|
lockref->count++;
|
|
spin_unlock(&lockref->lock);
|
|
return 1;
|
|
}
|
|
EXPORT_SYMBOL(lockref_get_or_lock);
|
|
|
|
/**
|
|
* lockref_put_or_lock - decrements count unless count <= 1 before decrement
|
|
* @lockcnt: pointer to lockref structure
|
|
* Return: 1 if count updated successfully or 0 if count <= 1 and lock taken
|
|
*/
|
|
int lockref_put_or_lock(struct lockref *lockref)
|
|
{
|
|
CMPXCHG_LOOP(
|
|
new.count--;
|
|
if (old.count <= 1)
|
|
break;
|
|
,
|
|
return 1;
|
|
);
|
|
|
|
spin_lock(&lockref->lock);
|
|
if (lockref->count <= 1)
|
|
return 0;
|
|
lockref->count--;
|
|
spin_unlock(&lockref->lock);
|
|
return 1;
|
|
}
|
|
EXPORT_SYMBOL(lockref_put_or_lock);
|