[PATCH bpf-next v2 02/26] locking: Move common qspinlock helpers to a private header

Kumar Kartikeya Dwivedi
2025-02-06T10:54:41Z

Move qspinlock helper functions that encode, decode tail word, set and
clear the pending and locked bits, and other miscellaneous definitions
and macros to a private header. To this end, create a qspinlock.h header
file in kernel/locking. Subsequent commits will introduce a modified
qspinlock slow path function, thus moving shared code to a private
header will help minimize unnecessary code duplication.

Reviewed-by: Barret Rhoden <brho@google.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
kernel/locking/qspinlock.c | 193 +----------------------------------
kernel/locking/qspinlock.h | 200 +++++++++++++++++++++++++++++++++++++
2 files changed, 205 insertions(+), 188 deletions(-)
create mode 100644 kernel/locking/qspinlock.h

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 7d96bed718e4..af8d122bb649 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -25,8 +25,9 @@
#include <trace/events/lock.h>

/*
- * Include queued spinlock statistics code
+ * Include queued spinlock definitions and statistics code
*/
+#include "qspinlock.h"
#include "qspinlock_stat.h"

/*
@@ -67,36 +68,6 @@
*/

#include "mcs_spinlock.h"
-#define MAX_NODES 4
-
-/*
- * On 64-bit architectures, the mcs_spinlock structure will be 16 bytes in
- * size and four of them will fit nicely in one 64-byte cacheline. For
- * pvqspinlock, however, we need more space for extra data. To accommodate
- * that, we insert two more long words to pad it up to 32 bytes. IOW, only
- * two of them can fit in a cacheline in this case. That is OK as it is rare
- * to have more than 2 levels of slowpath nesting in actual use. We don't
- * want to penalize pvqspinlocks to optimize for a rare case in native
- * qspinlocks.
- */
-struct qnode {
- struct mcs_spinlock mcs;
-#ifdef CONFIG_PARAVIRT_SPINLOCKS
- long reserved[2];
-#endif
-};
-
-/*
- * The pending bit spinning loop count.
- * This heuristic is used to limit the number of lockword accesses
- * made by atomic_cond_read_relaxed when waiting for the lock to
- * transition out of the "== _Q_PENDING_VAL" state. We don't spin
- * indefinitely because there's no guarantee that we'll make forward
- * progress.
- */
-#ifndef _Q_PENDING_LOOPS
-#define _Q_PENDING_LOOPS 1
-#endif

/*
* Per-CPU queue node structures; we can never have more than 4 nested
@@ -106,161 +77,7 @@ struct qnode {
*
* PV doubles the storage and uses the second cacheline for PV state.
*/
-static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]);
-
-/*
- * We must be able to distinguish between no-tail and the tail at 0:0,
- * therefore increment the cpu number by one.
- */
-
-static inline __pure u32 encode_tail(int cpu, int idx)
-{
- u32 tail;
-
- tail = (cpu + 1) << _Q_TAIL_CPU_OFFSET;
- tail |= idx << _Q_TAIL_IDX_OFFSET; /* assume < 4 */
-
- return tail;
-}
-
-static inline __pure struct mcs_spinlock *decode_tail(u32 tail)
-{
- int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
- int idx = (tail & _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
-
- return per_cpu_ptr(&qnodes[idx].mcs, cpu);
-}
-
-static inline __pure
-struct mcs_spinlock *grab_mcs_node(struct mcs_spinlock *base, int idx)
-{
- return &((struct qnode *)base + idx)->mcs;
-}
-
-#define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
-
-#if _Q_PENDING_BITS == 8
-/**
- * clear_pending - clear the pending bit.
- * @lock: Pointer to queued spinlock structure
- *
- * *,1,* -> *,0,*
- */
-static __always_inline void clear_pending(struct qspinlock *lock)
-{
- WRITE_ONCE(lock->pending, 0);
-}
-
-/**
- * clear_pending_set_locked - take ownership and clear the pending bit.
- * @lock: Pointer to queued spinlock structure
- *
- * *,1,0 -> *,0,1
- *
- * Lock stealing is not allowed if this function is used.
- */
-static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
-{
- WRITE_ONCE(lock->locked_pending, _Q_LOCKED_VAL);
-}
-
-/*
- * xchg_tail - Put in the new queue tail code word & retrieve previous one
- * @lock : Pointer to queued spinlock structure
- * @tail : The new queue tail code word
- * Return: The previous queue tail code word
- *
- * xchg(lock, tail), which heads an address dependency
- *
- * p,*,* -> n,*,* ; prev = xchg(lock, node)
- */
-static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
-{
- /*
- * We can use relaxed semantics since the caller ensures that the
- * MCS node is properly initialized before updating the tail.
- */
- return (u32)xchg_relaxed(&lock->tail,
- tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
-}
-
-#else /* _Q_PENDING_BITS == 8 */
-
-/**
- * clear_pending - clear the pending bit.
- * @lock: Pointer to queued spinlock structure
- *
- * *,1,* -> *,0,*
- */
-static __always_inline void clear_pending(struct qspinlock *lock)
-{
- atomic_andnot(_Q_PENDING_VAL, &lock->val);
-}
-
-/**
- * clear_pending_set_locked - take ownership and clear the pending bit.
- * @lock: Pointer to queued spinlock structure
- *
- * *,1,0 -> *,0,1
- */
-static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
-{
- atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
-}
-
-/**
- * xchg_tail - Put in the new queue tail code word & retrieve previous one
- * @lock : Pointer to queued spinlock structure
- * @tail : The new queue tail code word
- * Return: The previous queue tail code word
- *
- * xchg(lock, tail)
- *
- * p,*,* -> n,*,* ; prev = xchg(lock, node)
- */
-static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
-{
- u32 old, new;
-
- old = atomic_read(&lock->val);
- do {
- new = (old & _Q_LOCKED_PENDING_MASK) | tail;
- /*
- * We can use relaxed semantics since the caller ensures that
- * the MCS node is properly initialized before updating the
- * tail.
- */
- } while (!atomic_try_cmpxchg_relaxed(&lock->val, &old, new));
-
- return old;
-}
-#endif /* _Q_PENDING_BITS == 8 */
-
-/**
- * queued_fetch_set_pending_acquire - fetch the whole lock value and set pending
- * @lock : Pointer to queued spinlock structure
- * Return: The previous lock value
- *
- * *,*,* -> *,1,*
- */
-#ifndef queued_fetch_set_pending_acquire
-static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lock)
-{
- return atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
-}
-#endif
-
-/**
- * set_locked - Set the lock bit and own the lock
- * @lock: Pointer to queued spinlock structure
- *
- * *,*,0 -> *,0,1
- */
-static __always_inline void set_locked(struct qspinlock *lock)
-{
- WRITE_ONCE(lock->locked, _Q_LOCKED_VAL);
-}
-
+static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[_Q_MAX_NODES]);

/*
* Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for
@@ -410,7 +227,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
* any MCS node. This is not the most elegant solution, but is
* simple enough.
*/
- if (unlikely(idx >= MAX_NODES)) {
+ if (unlikely(idx >= _Q_MAX_NODES)) {
lockevent_inc(lock_no_node);
while (!queued_spin_trylock(lock))
cpu_relax();
@@ -465,7 +282,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
* head of the waitqueue.
*/
if (old & _Q_TAIL_MASK) {
- prev = decode_tail(old);
+ prev = decode_tail(old, qnodes);

/* Link @node into the waitqueue. */
WRITE_ONCE(prev->next, node);
diff --git a/kernel/locking/qspinlock.h b/kernel/locking/qspinlock.h
new file mode 100644
index 000000000000..d4ceb9490365
--- /dev/null
+++ b/kernel/locking/qspinlock.h
@@ -0,0 +1,200 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Queued spinlock defines
+ *
+ * This file contains macro definitions and functions shared between different
+ * qspinlock slow path implementations.
+ */
+#ifndef __LINUX_QSPINLOCK_H
+#define __LINUX_QSPINLOCK_H
+
+#include <asm-generic/percpu.h>
+#include <linux/percpu-defs.h>
+#include <asm-generic/qspinlock.h>
+#include <asm-generic/mcs_spinlock.h>
+
+#define _Q_MAX_NODES 4
+
+/*
+ * The pending bit spinning loop count.
+ * This heuristic is used to limit the number of lockword accesses
+ * made by atomic_cond_read_relaxed when waiting for the lock to
+ * transition out of the "== _Q_PENDING_VAL" state. We don't spin
+ * indefinitely because there's no guarantee that we'll make forward
+ * progress.
+ */
+#ifndef _Q_PENDING_LOOPS
+#define _Q_PENDING_LOOPS 1
+#endif
+
+/*
+ * On 64-bit architectures, the mcs_spinlock structure will be 16 bytes in
+ * size and four of them will fit nicely in one 64-byte cacheline. For
+ * pvqspinlock, however, we need more space for extra data. To accommodate
+ * that, we insert two more long words to pad it up to 32 bytes. IOW, only
+ * two of them can fit in a cacheline in this case. That is OK as it is rare
+ * to have more than 2 levels of slowpath nesting in actual use. We don't
+ * want to penalize pvqspinlocks to optimize for a rare case in native
+ * qspinlocks.
+ */
+struct qnode {
+ struct mcs_spinlock mcs;
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+ long reserved[2];
+#endif
+};
+
+/*
+ * We must be able to distinguish between no-tail and the tail at 0:0,
+ * therefore increment the cpu number by one.
+ */
+
+static inline __pure u32 encode_tail(int cpu, int idx)
+{
+ u32 tail;
+
+ tail = (cpu + 1) << _Q_TAIL_CPU_OFFSET;
+ tail |= idx << _Q_TAIL_IDX_OFFSET; /* assume < 4 */
+
+ return tail;
+}
+
+static inline __pure struct mcs_spinlock *decode_tail(u32 tail, struct qnode *qnodes)
+{
+ int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
+ int idx = (tail & _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
+
+ return per_cpu_ptr(&qnodes[idx].mcs, cpu);
+}
+
+static inline __pure
+struct mcs_spinlock *grab_mcs_node(struct mcs_spinlock *base, int idx)
+{
+ return &((struct qnode *)base + idx)->mcs;
+}
+
+#define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
+
+#if _Q_PENDING_BITS == 8
+/**
+ * clear_pending - clear the pending bit.
+ * @lock: Pointer to queued spinlock structure
+ *
+ * *,1,* -> *,0,*
+ */
+static __always_inline void clear_pending(struct qspinlock *lock)
+{
+ WRITE_ONCE(lock->pending, 0);
+}
+
+/**
+ * clear_pending_set_locked - take ownership and clear the pending bit.
+ * @lock: Pointer to queued spinlock structure
+ *
+ * *,1,0 -> *,0,1
+ *
+ * Lock stealing is not allowed if this function is used.
+ */
+static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
+{
+ WRITE_ONCE(lock->locked_pending, _Q_LOCKED_VAL);
+}
+
+/*
+ * xchg_tail - Put in the new queue tail code word & retrieve previous one
+ * @lock : Pointer to queued spinlock structure
+ * @tail : The new queue tail code word
+ * Return: The previous queue tail code word
+ *
+ * xchg(lock, tail), which heads an address dependency
+ *
+ * p,*,* -> n,*,* ; prev = xchg(lock, node)
+ */
+static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
+{
+ /*
+ * We can use relaxed semantics since the caller ensures that the
+ * MCS node is properly initialized before updating the tail.
+ */
+ return (u32)xchg_relaxed(&lock->tail,
+ tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
+}
+
+#else /* _Q_PENDING_BITS == 8 */
+
+/**
+ * clear_pending - clear the pending bit.
+ * @lock: Pointer to queued spinlock structure
+ *
+ * *,1,* -> *,0,*
+ */
+static __always_inline void clear_pending(struct qspinlock *lock)
+{
+ atomic_andnot(_Q_PENDING_VAL, &lock->val);
+}
+
+/**
+ * clear_pending_set_locked - take ownership and clear the pending bit.
+ * @lock: Pointer to queued spinlock structure
+ *
+ * *,1,0 -> *,0,1
+ */
+static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
+{
+ atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
+}
+
+/**
+ * xchg_tail - Put in the new queue tail code word & retrieve previous one
+ * @lock : Pointer to queued spinlock structure
+ * @tail : The new queue tail code word
+ * Return: The previous queue tail code word
+ *
+ * xchg(lock, tail)
+ *
+ * p,*,* -> n,*,* ; prev = xchg(lock, node)
+ */
+static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
+{
+ u32 old, new;
+
+ old = atomic_read(&lock->val);
+ do {
+ new = (old & _Q_LOCKED_PENDING_MASK) | tail;
+ /*
+ * We can use relaxed semantics since the caller ensures that
+ * the MCS node is properly initialized before updating the
+ * tail.
+ */
+ } while (!atomic_try_cmpxchg_relaxed(&lock->val, &old, new));
+
+ return old;
+}
+#endif /* _Q_PENDING_BITS == 8 */
+
+/**
+ * queued_fetch_set_pending_acquire - fetch the whole lock value and set pending
+ * @lock : Pointer to queued spinlock structure
+ * Return: The previous lock value
+ *
+ * *,*,* -> *,1,*
+ */
+#ifndef queued_fetch_set_pending_acquire
+static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lock)
+{
+ return atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
+}
+#endif
+
+/**
+ * set_locked - Set the lock bit and own the lock
+ * @lock: Pointer to queued spinlock structure
+ *
+ * *,*,0 -> *,0,1
+ */
+static __always_inline void set_locked(struct qspinlock *lock)
+{
+ WRITE_ONCE(lock->locked, _Q_LOCKED_VAL);
+}
+
+#endif /* __LINUX_QSPINLOCK_H */
--
2.43.5