locking: Move common qspinlock helpers to a private header

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>
Link: https://lore.kernel.org/r/20250316040541.108729-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: kkdwvd

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);