This patch introduces a new spinlock_refcount.h header file to be
included by kernel code that want to do a lockless update of reference
count protected by a spinlock.
To try to locklessly update the reference count while lock isn't
acquired by others, the 32-bit count and 32-bit raw spinlock can be
combined into a single 64-bit word to be updated atomically whenever
the following conditions are true:
1. The lock is not taken, i.e. spin_can_lock() returns true.
2. The value of the count isn't equal to the given non-negative
threshold value.
To maximize the chance of doing lockless update, the inlined
__lockcnt_add_unless() function calls spin_unlock_wait() before trying
to do the update.
The new code also attempts to do lockless atomic update twice before
falling back to the old code path of acquring a lock before doing
the update. It is because there will still be some fair amount of
contention with only one attempt.
After including the header file, the LOCK_WITH_REFCOUNT() macro
should be used to define the spinlock with reference count combo in
an embedding data structure. Then the __lockcnt_add_unless() inlined
function can be used to locklessly update the reference count if the
lock hasn't be acquired by others.
Build and boot tests of the new code and the associated dcache changes
were conducted for the following configurations:
1. x86 64-bit SMP, CONFIG_DEBUG_SPINLOCK=n
2. x86 64-bit SMP, CONFIG_DEBUG_SPINLOCK=y
3. x86 32-bit UP , CONFIG_DEBUG_SPINLOCK=n
4. x86 32-bit SMP, CONFIG_DEBUG_SPINLOCK=n
5. x86 32-bit SMP, CONFIG_DEBUG_SPINLOCK=y
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
include/linux/spinlock_refcount.h | 145 +++++++++++++++++++++++++++++++++++++
include/linux/spinlock_types.h | 19 +++++
2 files changed, 164 insertions(+), 0 deletions(-)
create mode 100644 include/linux/spinlock_refcount.h
diff --git a/include/linux/spinlock_refcount.h b/include/linux/spinlock_refcount.h
new file mode 100644
index 0000000..eaf4897
--- /dev/null
+++ b/include/linux/spinlock_refcount.h
@@ -0,0 +1,145 @@
+/*
+ * Spinlock with reference count combo
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * (C) Copyright 2013 Hewlett-Packard Development Company, L.P.
+ *
+ * Authors: Waiman Long <waiman.long@xxxxxx>
+ */
+#include <linux/spinlock.h>
+
+/*
+ * The LOCK_WITH_REFCOUNT() macro defines the combined spinlock with reference
+ * count data structure to be embedded in a larger structure. With unnamed
+ * structure, the lock and count names can be accessed directly if no field
+ * name is assigned to the structure. Otherwise, they will have to be accessed
+ * indirectly via the assigned field name of the combined structure.
+ *
+ * The combined data structure is 8-byte aligned. So proper placement of this
+ * structure in the larger embedding data structure is needed to ensure that
+ * there is no hole in it.
+ *
+ * @lock: Name of the spinlock
+ * @count: Name of the reference count
+ * @u_name: Name of combined data structure union (can be empty for unnamed
+ * union)
+ */
+#ifndef CONFIG_SMP
+#define LOCK_WITH_REFCOUNT(lock, count, u_name) \
+ unsigned int count; \
+ spinlock_t lock
+
+#elif defined(__SPINLOCK_HAS_REFCOUNT)
+#define LOCK_WITH_REFCOUNT(lock, count, u_name) \
+ union u_name { \
+ u64 __lock_count; \
+ spinlock_t lock; \
+ struct { \
+ arch_spinlock_t __raw_lock; \
+ unsigned int count; \
+ }; \
+ }
+
+#else /* __SPINLOCK_HAS_REFCOUNT */
+#define LOCK_WITH_REFCOUNT(lock, count, u_name) \
+ union u_name { \
+ u64 __lock_count; \
+ struct { \
+ unsigned int count; \
+ spinlock_t lock; \
+ }; \
+ }
+
+#endif /* __SPINLOCK_HAS_REFCOUNT */
+
+#ifdef CONFIG_SMP
+#define LOCKCNT_COMBO_PTR(s) (&(s)->__lock_count)
+
+/*
+ * Define a "union _lock_refcnt" structure to be used by the helper function
+ */
+LOCK_WITH_REFCOUNT(lock, count, __lock_refcnt);
+
+/**
+ *
+ * __lockcnt_add_unless - atomically add the given value to the count unless
+ * the lock was acquired or the count equals to the
+ * given threshold value.
+ *
+ * @plockcnt : pointer to the combined lock and count 8-byte data
+ * @plock : pointer to the spinlock
+ * @pcount : pointer to the reference count
+ * @value : value to be added
+ * @threshold: threshold value for acquiring the lock
+ * Return : 1 if operation succeed, 0 otherwise
+ *
+ * If the lock was not acquired, __lockcnt_add_unless() atomically adds the
+ * given value to the reference count unless the given threshold is reached.
+ * If the lock was acquired or the threshold was reached, 0 is returned and
+ * the caller will have to acquire the lock and update the count accordingly
+ * (can be done in a non-atomic way).
+ */
+static inline int __lockcnt_add_unless(u64 *plockcnt, spinlock_t *plock,
+ unsigned int *pcount, int value,
+ int threshold)
+{
+ union __lock_refcnt old, new;
+ int get_lock;
+
+ /*
+ * Code doesn't work if raw spinlock is larger than 4 bytes
+ * or is empty.
+ */
+ BUG_ON((sizeof(arch_spinlock_t) > 4) || (sizeof(arch_spinlock_t) == 0));
+
+ spin_unlock_wait(plock); /* Wait until lock is released */
+ old.__lock_count = ACCESS_ONCE(*plockcnt);
+ get_lock = ((threshold >= 0) && (old.count == threshold));
+ if (likely(!get_lock && spin_can_lock(&old.lock))) {
+ new.__lock_count = old.__lock_count;
+ new.count += value;
+ if (cmpxchg64(plockcnt, old.__lock_count, new.__lock_count)
+ == old.__lock_count)
+ return 1;
+ cpu_relax();
+ /*
+ * Try one more time
+ */
+ old.__lock_count = ACCESS_ONCE(*plockcnt);
+ get_lock = ((threshold >= 0) && (old.count == threshold));
+ if (likely(!get_lock && spin_can_lock(&old.lock))) {
+ new.__lock_count = old.__lock_count;
+ new.count += value;
+ if (cmpxchg64(plockcnt, old.__lock_count,
+ new.__lock_count) == old.__lock_count)
+ return 1;
+ cpu_relax();
+ }
+ }
+ return 0; /* The caller will need to acquire the lock */
+}
+#else /* CONFIG_SMP */
+#define LOCKCNT_COMBO_PTR(s) NULL
+
+/*
+ * Just add the value as the spinlock is a no-op
+ */
+static inline int __lockcnt_add_unless(u64 *plockcnt, spinlock_t *plock,
+ unsigned int *pcount, int value,
+ int threshold)
+{
+ if ((threshold >= 0) && (*pcount == threshold))
+ return 0;
+ (*pcount) += value;
+ return 1;
+}
+#endif /* CONFIG_SMP */
diff --git a/include/linux/spinlock_types.h b/include/linux/spinlock_types.h
index 73548eb..cc107ad 100644
--- a/include/linux/spinlock_types.h
+++ b/include/linux/spinlock_types.h
@@ -17,8 +17,27 @@
#include <linux/lockdep.h>
+/*
+ * The presence of either one of the CONFIG_DEBUG_SPINLOCK or
+ * CONFIG_DEBUG_LOCK_ALLOC configuration parameter will force the
+ * spinlock_t structure to be 8-byte aligned.
+ *
+ * To support the spinlock/reference count combo data type for 64-bit SMP
+ * environment with spinlock debugging turned on, the reference count has
+ * to be integrated into the spinlock_t data structure in this special case.
+ * The spinlock_t data type will be 8 bytes larger if CONFIG_GENERIC_LOCKBREAK
+ * is also defined.
+ */
+#if defined(CONFIG_64BIT) && (defined(CONFIG_DEBUG_SPINLOCK) ||\
+ defined(CONFIG_DEBUG_LOCK_ALLOC))
+#define __SPINLOCK_HAS_REFCOUNT 1
+#endif
+
typedef struct raw_spinlock {
arch_spinlock_t raw_lock;
+#ifdef __SPINLOCK_HAS_REFCOUNT
+ unsigned int count;
+#endif
#ifdef CONFIG_GENERIC_LOCKBREAK
unsigned int break_lock;
#endif
--
1.7.1
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