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/*
* Queued spinlock
*
* 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 2013-2015 Hewlett-Packard Development Company, L.P.
* (C) Copyright 2013-2014 Red Hat, Inc.
* (C) Copyright 2015 Intel Corp.
*
* Authors: Waiman Long <waiman.long@hp.com>
* Peter Zijlstra <peterz@infradead.org>
*/
#include <linux/smp.h>
#include <linux/bug.h>
#include <linux/cpumask.h>
#include <linux/percpu.h>
#include <linux/hardirq.h>
#include <linux/mutex.h>
#include <asm/qspinlock.h>
/*
* The basic principle of a queue-based spinlock can best be understood
* by studying a classic queue-based spinlock implementation called the
* MCS lock. The paper below provides a good description for this kind
* of lock.
*
* http://www.cise.ufl.edu/tr/DOC/REP-1992-71.pdf
*
* This queued spinlock implementation is based on the MCS lock, however to make
* it fit the 4 bytes we assume spinlock_t to be, and preserve its existing
* API, we must modify it somehow.
*
* In particular; where the traditional MCS lock consists of a tail pointer
* (8 bytes) and needs the next pointer (another 8 bytes) of its own node to
* unlock the next pending (next->locked), we compress both these: {tail,
* next->locked} into a single u32 value.
*
* Since a spinlock disables recursion of its own context and there is a limit
* to the contexts that can nest; namely: task, softirq, hardirq, nmi. As there
* are at most 4 nesting levels, it can be encoded by a 2-bit number. Now
* we can encode the tail by combining the 2-bit nesting level with the cpu
* number. With one byte for the lock value and 3 bytes for the tail, only a
* 32-bit word is now needed. Even though we only need 1 bit for the lock,
* we extend it to a full byte to achieve better performance for architectures
* that support atomic byte write.
*
* We also change the first spinner to spin on the lock bit instead of its
* node; whereby avoiding the need to carry a node from lock to unlock, and
* preserving existing lock API. This also makes the unlock code simpler and
* faster.
*/
#include "mcs_spinlock.h"
/*
* Per-CPU queue node structures; we can never have more than 4 nested
* contexts: task, softirq, hardirq, nmi.
*
* Exactly fits one 64-byte cacheline on a 64-bit architecture.
*/
static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[4]);
/*
* We must be able to distinguish between no-tail and the tail at 0:0,
* therefore increment the cpu number by one.
*/
static inline u32 encode_tail(int cpu, int idx)
{
u32 tail;
#ifdef CONFIG_DEBUG_SPINLOCK
BUG_ON(idx > 3);
#endif
tail = (cpu + 1) << _Q_TAIL_CPU_OFFSET;
tail |= idx << _Q_TAIL_IDX_OFFSET; /* assume < 4 */
return tail;
}
static inline 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(&mcs_nodes[idx], cpu);
}
/**
* queued_spin_lock_slowpath - acquire the queued spinlock
* @lock: Pointer to queued spinlock structure
* @val: Current value of the queued spinlock 32-bit word
*
* (queue tail, lock value)
*
* fast : slow : unlock
* : :
* uncontended (0,0) --:--> (0,1) --------------------------------:--> (*,0)
* : | ^--------. / :
* : v \ | :
* uncontended : (n,x) --+--> (n,0) | :
* queue : | ^--' | :
* : v | :
* contended : (*,x) --+--> (*,0) -----> (*,1) ---' :
* queue : ^--' :
*
*/
void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
{
struct mcs_spinlock *prev, *next, *node;
u32 new, old, tail;
int idx;
BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
node = this_cpu_ptr(&mcs_nodes[0]);
idx = node->count++;
tail = encode_tail(smp_processor_id(), idx);
node += idx;
node->locked = 0;
node->next = NULL;
/*
* trylock || xchg(lock, node)
*
* 0,0 -> 0,1 ; no tail, not locked -> no tail, locked.
* p,x -> n,x ; tail was p -> tail is n; preserving locked.
*/
for (;;) {
new = _Q_LOCKED_VAL;
if (val)
new = tail | (val & _Q_LOCKED_MASK);
old = atomic_cmpxchg(&lock->val, val, new);
if (old == val)
break;
val = old;
}
/*
* we won the trylock; forget about queueing.
*/
if (new == _Q_LOCKED_VAL)
goto release;
/*
* if there was a previous node; link it and wait until reaching the
* head of the waitqueue.
*/
if (old & ~_Q_LOCKED_MASK) {
prev = decode_tail(old);
WRITE_ONCE(prev->next, node);
arch_mcs_spin_lock_contended(&node->locked);
}
/*
* we're at the head of the waitqueue, wait for the owner to go away.
*
* *,x -> *,0
*/
while ((val = atomic_read(&lock->val)) & _Q_LOCKED_MASK)
cpu_relax();
/*
* claim the lock:
*
* n,0 -> 0,1 : lock, uncontended
* *,0 -> *,1 : lock, contended
*/
for (;;) {
new = _Q_LOCKED_VAL;
if (val != tail)
new |= val;
old = atomic_cmpxchg(&lock->val, val, new);
if (old == val)
break;
val = old;
}
/*
* contended path; wait for next, release.
*/
if (new != _Q_LOCKED_VAL) {
while (!(next = READ_ONCE(node->next)))
cpu_relax();
arch_mcs_spin_unlock_contended(&next->locked);
}
release:
/*
* release the node
*/
this_cpu_dec(mcs_nodes[0].count);
}
EXPORT_SYMBOL(queued_spin_lock_slowpath);
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