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authorLinus Torvalds <torvalds@linux-foundation.org>2013-09-02 12:12:15 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-09-02 12:12:15 -0700
commitbc08b449ee14ace4d869adaa1bb35a44ce68d775 (patch)
tree133319a81192fad41aa999eac135e87905ab5bab
parent2f4f12e571c4e2f50f3818a3c2544929145f75dd (diff)
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lockref: implement lockless reference count updates using cmpxchg()
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>
-rw-r--r--arch/x86/Kconfig1
-rw-r--r--arch/x86/include/asm/spinlock.h5
-rw-r--r--include/linux/lockref.h11
-rw-r--r--lib/Kconfig10
-rw-r--r--lib/lockref.c60
5 files changed, 84 insertions, 3 deletions
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b32ebf9..67e0074 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -16,6 +16,7 @@ config X86_64
def_bool y
depends on 64BIT
select X86_DEV_DMA_OPS
+ select ARCH_USE_CMPXCHG_LOCKREF
### Arch settings
config X86
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index e3ddd7d..e0e6684 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -34,6 +34,11 @@
# define UNLOCK_LOCK_PREFIX
#endif
+static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
+{
+ return lock.tickets.head == lock.tickets.tail;
+}
+
/*
* Ticket locks are conceptually two parts, one indicating the current head of
* the queue, and the other indicating the current tail. The lock is acquired
diff --git a/include/linux/lockref.h b/include/linux/lockref.h
index 4c0af31..ca07b50 100644
--- a/include/linux/lockref.h
+++ b/include/linux/lockref.h
@@ -17,8 +17,15 @@
#include <linux/spinlock.h>
struct lockref {
- spinlock_t lock;
- unsigned int count;
+ union {
+#ifdef CONFIG_CMPXCHG_LOCKREF
+ aligned_u64 lock_count;
+#endif
+ struct {
+ spinlock_t lock;
+ unsigned int count;
+ };
+ };
};
extern void lockref_get(struct lockref *);
diff --git a/lib/Kconfig b/lib/Kconfig
index 71d9f81..6556171 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -48,6 +48,16 @@ config STMP_DEVICE
config PERCPU_RWSEM
boolean
+config ARCH_USE_CMPXCHG_LOCKREF
+ bool
+
+config CMPXCHG_LOCKREF
+ def_bool y if ARCH_USE_CMPXCHG_LOCKREF
+ depends on SMP
+ depends on !GENERIC_LOCKBREAK
+ depends on !DEBUG_SPINLOCK
+ depends on !DEBUG_LOCK_ALLOC
+
config CRC_CCITT
tristate "CRC-CCITT functions"
help
diff --git a/lib/lockref.c b/lib/lockref.c
index a9a4f4e..7819c2d 100644
--- a/lib/lockref.c
+++ b/lib/lockref.c
@@ -1,6 +1,33 @@
#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
@@ -10,6 +37,12 @@
*/
void lockref_get(struct lockref *lockref)
{
+ CMPXCHG_LOOP(
+ new.count++;
+ ,
+ return;
+ );
+
spin_lock(&lockref->lock);
lockref->count++;
spin_unlock(&lockref->lock);
@@ -23,9 +56,18 @@ EXPORT_SYMBOL(lockref_get);
*/
int lockref_get_not_zero(struct lockref *lockref)
{
- int retval = 0;
+ 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;
@@ -43,6 +85,14 @@ EXPORT_SYMBOL(lockref_get_not_zero);
*/
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;
@@ -59,6 +109,14 @@ EXPORT_SYMBOL(lockref_get_or_lock);
*/
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;