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locking: Move common qspinlock helpers to a private header

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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>
This commit is contained in:
Kumar Kartikeya Dwivedi 2025-03-15 21:05:18 -07:00 committed by Alexei Starovoitov
parent 8707d1eedc
commit ac08f68f1f
2 changed files with 206 additions and 188 deletions
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193
kernel/locking/qspinlock.c
View File

@ -25,8 +25,9 @@
#include <trace/events/lock.h> #include <trace/events/lock.h>
/* /*
* Include queued spinlock statistics code * Include queued spinlock definitions and statistics code
*/ */
#include "qspinlock.h"
#include "qspinlock_stat.h" #include "qspinlock_stat.h"
/* /*
@ -67,36 +68,6 @@
*/ */
#include "mcs_spinlock.h" #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 * 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. * PV doubles the storage and uses the second cacheline for PV state.
*/ */
static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]); static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[_Q_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);
}
/* /*
* Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for * Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for
@ -410,7 +227,7 @@ pv_queue:
* any MCS node. This is not the most elegant solution, but is * any MCS node. This is not the most elegant solution, but is
* simple enough. * simple enough.
*/ */
if (unlikely(idx >= MAX_NODES)) { if (unlikely(idx >= _Q_MAX_NODES)) {
lockevent_inc(lock_no_node); lockevent_inc(lock_no_node);
while (!queued_spin_trylock(lock)) while (!queued_spin_trylock(lock))
cpu_relax(); cpu_relax();
@ -465,7 +282,7 @@ pv_queue:
* head of the waitqueue. * head of the waitqueue.
*/ */
if (old & _Q_TAIL_MASK) { if (old & _Q_TAIL_MASK) {
prev = decode_tail(old); prev = decode_tail(old, qnodes);
/* Link @node into the waitqueue. */ /* Link @node into the waitqueue. */
WRITE_ONCE(prev->next, node); WRITE_ONCE(prev->next, node);

201
kernel/locking/qspinlock.h Normal file
View File

@ -0,0 +1,201 @@
/* 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 __percpu *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 */