Linux内核源码分析 -- 同步原语 -- 互斥锁 mutex
/*
* Simple, straightforward mutexes with strict semantics:
*
* - only one task can hold the mutex at a time (同一时间仅能被一个进程持有)
* - only the owner can unlock the mutex (只有锁的持有者才能进行解锁操作)
* - multiple unlocks are not permitted (不能进行多次解锁)
* - recursive locking is not permitted (不能进行递归加锁)
* - a mutex object must be initialized via the API (mutex 结构只能通过API 区初始化)
* - a mutex object must not be initialized via memset or copying (mutex 不能通过 memset 或者拷贝进行初始化)
* - task may not exit with mutex held (获取互斥锁后进程可能不能退出)
* - memory areas where held locks reside must not be freed
* - held mutexes must not be reinitialized (被持有的互斥锁不能进行再次初始化)
* - mutexes may not be used in hardware or software interrupt (互斥锁不能用在硬件或者软件上下文)
* contexts such as tasklets and timers
*
* These semantics are fully enforced when DEBUG_MUTEXES is
* enabled. Furthermore, besides enforcing the above rules, the mutex
* debugging code also implements a number of additional features
* that make lock debugging easier and faster:
*
* - uses symbolic names of mutexes, whenever they are printed in debug output
* - point-of-acquire tracking, symbolic lookup of function names
* - list of all locks held in the system, printout of them
* - owner tracking
* - detects self-recursing locks and prints out all relevant info
* - detects multi-task circular deadlocks and prints out all affected
* locks and tasks (and only those tasks)
*/
struct mutex {
atomic_long_t owner;
spinlock_t wait_lock;
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
struct optimistic_spin_queue osq; /* Spinner MCS lock */
#endif
struct list_head wait_list;
#ifdef CONFIG_DEBUG_MUTEXES
void *magic;
#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
};
owner -- 原子计数，用于指向锁持有者的 task_struct 结构，当 owner 等于 0 时表明锁没有被持有，当 owner 不等于 0 时表明锁被其他进程持有
wait_list -- 等待队列
wait_lock -- 一个自旋锁,用来保护 wait_list (等待队列)
初始化互斥锁 -- mutex_init
方法 1：
DEFINE_MUTEX(mutexname)
#define DEFINE_MUTEX(mutexname)
struct mutex mutexname = __MUTEX_INITIALIZER(mutexname)
__MUTEX_INITIALIZER(lockname)
#define __MUTEX_INITIALIZER(lockname)
{ .owner = ATOMIC_LONG_INIT(0)
, .wait_lock = __SPIN_LOCK_UNLOCKED(lockname.wait_lock)
, .wait_list = LIST_HEAD_INIT(lockname.wait_list)
__DEBUG_MUTEX_INITIALIZER(lockname)
__DEP_MAP_MUTEX_INITIALIZER(lockname) }
owner 初始化为 0，表明这个锁刚开始是 unlock
wait_lock 初始化保护等待队列的锁，是个自旋锁
wait_list 初始化 等待队列，是一个链表
方法2：
mutex_init
#define mutex_init(mutex)
do {
static struct lock_class_key __key;
__mutex_init((mutex), #mutex, &__key);
} while (0)
__mutex_init
void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
atomic_long_set(&lock->owner, 0); // 一样调用 atomic_long_set 把 &lock->owner 置为 0
spin_lock_init(&lock->wait_lock); // 初始化保护等待队列的自旋锁
INIT_LIST_HEAD(&lock->wait_list); // 初始化等待队列
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
osq_lock_init(&lock->osq);
#endif
debug_mutex_init(lock, name, key);
}
EXPORT_SYMBOL(__mutex_init);
请求持有互斥锁 -- mutex_lock
mutex_lock
请求一个 互斥锁
/**
* mutex_lock - acquire the mutex
* @lock: the mutex to be acquired
*
* Lock the mutex exclusively for this task. If the mutex is not
* available right now, it will sleep until it can get it.
*
* The mutex must later on be released by the same task that
* acquired it. Recursive locking is not allowed. The task
* may not exit without first unlocking the mutex. Also, kernel
* memory where the mutex resides must not be freed with
* the mutex still locked. The mutex must first be initialized
* (or statically defined) before it can be locked. memset()-ing
* the mutex to 0 is not allowed.
*
* (The CONFIG_DEBUG_MUTEXES .config option turns on debugging
* checks that will enforce the restrictions and will also do
* deadlock debugging)
*
* This function is similar to (but not equivalent to) down().
*/
void __sched mutex_lock(struct mutex *lock)
{
might_sleep();
if (!__mutex_trylock_fast(lock)) // 尝试先从 fastpath 去获取锁
__mutex_lock_slowpath(lock); // 失败的话从 midpath 去获取锁
}
EXPORT_SYMBOL(mutex_lock);
当进程试图获取互斥锁时，有两种可能的路径，选择哪一种主要取决于锁的当前状态
1.fastpath -- 这是最简单的情况，就是锁没有被任何进程持有
获取锁的时候就是让 owner 等于获取锁的进程的 task_struct 的地址 (原子性操作)
__mutex_trylock_fast
/*
* Optimistic trylock that only works in the uncontended case. Make sure to
* follow with a __mutex_trylock() before failing.
*/
static __always_inline bool __mutex_trylock_fast(struct mutex *lock)
{
// current 宏其实就是获取当前进程的 task_struct 的地址
unsigned long curr = (unsigned long)current;
unsigned long zero = 0UL;
// 把 &lock->owner 设置成 curr (原子操作)
if (atomic_long_try_cmpxchg_acquire(&lock->owner, &zero, curr))
return true;
return false;
}
2.slowpath -- 当请求的锁处于被持有状态的时候则会走这条路
__mutex_lock_slowpath
@lock -- 要请求的锁
static noinline void __sched
__mutex_lock_slowpath(struct mutex *lock)
{
__mutex_lock(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
}
这里说一下 _RET_IP_ 其实这个是个宏
(unsigned long)__builtin_return_address(0)
__builtin_return_address 是 gcc 的内建函数，其实就是用来获取函数的返回地址，这里是 0
__mutex_lock
static int __sched
__mutex_lock(struct mutex *lock, long state, unsigned int subclass,
struct lockdep_map *nest_lock, unsigned long ip)
{
return __mutex_lock_common(lock, state, subclass, nest_lock, ip, NULL, false);
}
__mutex_lock_common
阻塞进程真正获取锁的地方
struct ww_mutex {
struct mutex base;
struct ww_acquire_ctx *ctx;
#ifdef CONFIG_DEBUG_MUTEXES
struct ww_class *ww_class;
#endif
};
/*
* Lock a mutex (possibly interruptible), slowpath:
*/
static __always_inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
struct lockdep_map *nest_lock, unsigned long ip,
struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
{
struct mutex_waiter waiter;
bool first = false;
struct ww_mutex *ww;
int ret;
might_sleep();
#ifdef CONFIG_DEBUG_MUTEXES
DEBUG_LOCKS_WARN_ON(lock->magic != lock);
#endif
// 获取 lock 对应的 ww_mutex 结构的地址（因为 mutex 结构在 ww_mutex 对应的是 base 字段。在 Linux 内核里面这是个很出名的用的范围也广的宏，还有 offsetof）
ww = container_of(lock, struct ww_mutex, base);
if (use_ww_ctx && ww_ctx) {
if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
return -EALREADY;
/*
* Reset the wounded flag after a kill. No other process can
* race and wound us here since they can't have a valid owner
* pointer if we don't have any locks held.
*/
if (ww_ctx->acquired == 0)
ww_ctx->wounded = 0;
}
preempt_disable(); // 禁止内核抢占
mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
// 尝试获取锁
if (__mutex_trylock(lock) ||
mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, NULL)) {
/* got the lock, yay! */
// 执行到这里说明获取锁成功
lock_acquired(&lock->dep_map, ip);
if (use_ww_ctx && ww_ctx)
ww_mutex_set_context_fastpath(ww, ww_ctx);
preempt_enable(); // 恢复内核抢占
return 0;
}
spin_lock(&lock->wait_lock); // 上锁等待队列
/*
* After waiting to acquire the wait_lock, try again.
*/
// 再次尝试
if (__mutex_trylock(lock)) {
if (use_ww_ctx && ww_ctx)
__ww_mutex_check_waiters(lock, ww_ctx);
goto skip_wait;
}
debug_mutex_lock_common(lock, &waiter);
lock_contended(&lock->dep_map, ip);
if (!use_ww_ctx) {
/* add waiting tasks to the end of the waitqueue (FIFO): */
__mutex_add_waiter(lock, &waiter, &lock->wait_list);
#ifdef CONFIG_DEBUG_MUTEXES
waiter.ww_ctx = MUTEX_POISON_WW_CTX;
#endif
} else {
/*
* Add in stamp order, waking up waiters that must kill
* themselves.
*/
ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
if (ret)
goto err_early_kill;
waiter.ww_ctx = ww_ctx;
}
waiter.task = current;
// 设置的状态取决于在调用 __mutex_lock 函数
// 设置当前进程的状态（当是 __mutex_lock_slowpath 调用过来的话就是把当前进程状态设置成不可中断）
set_current_state(state);
for (;;) {
/*
* Once we hold wait_lock, we're serialized against
* mutex_unlock() handing the lock off to us, do a trylock
* before testing the error conditions to make sure we pick up
* the handoff.
*/
if (__mutex_trylock(lock))
goto acquired;
/*
* Check for signals and kill conditions while holding
* wait_lock. This ensures the lock cancellation is ordered
* against mutex_unlock() and wake-ups do not go missing.
*/
if (signal_pending_state(state, current)) {
ret = -EINTR;
goto err;
}
if (use_ww_ctx && ww_ctx) {
ret = __ww_mutex_check_kill(lock, &waiter, ww_ctx);
if (ret)
goto err;
}
spin_unlock(&lock->wait_lock);
schedule_preempt_disabled();
/*
* ww_mutex needs to always recheck its position since its waiter
* list is not FIFO ordered.
*/
if ((use_ww_ctx && ww_ctx) || !first) {
first = __mutex_waiter_is_first(lock, &waiter);
if (first)
__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
}
set_current_state(state);
/*
* Here we order against unlock; we must either see it change
* state back to RUNNING and fall through the next schedule(),
* or we must see its unlock and acquire.
*/
if (__mutex_trylock(lock) ||
(first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, &waiter)))
break;
spin_lock(&lock->wait_lock);
}
spin_lock(&lock->wait_lock);
acquired:
__set_current_state(TASK_RUNNING);
if (use_ww_ctx && ww_ctx) {
/*
* Wound-Wait; we stole the lock (!first_waiter), check the
* waiters as anyone might want to wound us.
*/
if (!ww_ctx->is_wait_die &&
!__mutex_waiter_is_first(lock, &waiter))
__ww_mutex_check_waiters(lock, ww_ctx);
}
mutex_remove_waiter(lock, &waiter, current);
if (likely(list_empty(&lock->wait_list)))
__mutex_clear_flag(lock, MUTEX_FLAGS);
debug_mutex_free_waiter(&waiter);
skip_wait:
/* got the lock - cleanup and rejoice! */
lock_acquired(&lock->dep_map, ip);
if (use_ww_ctx && ww_ctx)
ww_mutex_lock_acquired(ww, ww_ctx);
spin_unlock(&lock->wait_lock);
preempt_enable();
return 0;
err:
__set_current_state(TASK_RUNNING);
mutex_remove_waiter(lock, &waiter, current);
err_early_kill:
spin_unlock(&lock->wait_lock);
debug_mutex_free_waiter(&waiter);
mutex_release(&lock->dep_map, ip);
preempt_enable();
return ret;
}
__mutex_trylock
先说一下 mutex 标志位（task_struxt 的 低 3 bits）
Since task_struct pointers are aligned at least L1_CACHE_BYTES, we have low bits to store extra state.
由于 task_struct 指针向 L1_CACHE_BYTES 对齐（至于数值是多少的看架构，位于 arch/xxxx/include/asm/cache.h 里面），这样用低 3 bits 做标识，并不会有影响
/*
* @owner: contains: 'struct task_struct *' to the current lock owner,
* NULL means not owned. Since task_struct pointers are aligned at
* at least L1_CACHE_BYTES, we have low bits to store extra state.
*
* Bit0 indicates a non-empty waiter list; unlock must issue a wakeup.
* Bit1 indicates unlock needs to hand the lock to the top-waiter
* Bit2 indicates handoff has been done and we're waiting for pickup.
*/
#define MUTEX_FLAG_WAITERS 0x01
#define MUTEX_FLAG_HANDOFF 0x02
#define MUTEX_FLAG_PICKUP 0x04
#define MUTEX_FLAGS 0x07
Bit0：表明 等待队列非空，解锁时要执行唤醒（wakeup）操作（唤醒等待的进程）
Bit1：释放锁时要把锁交给等待队列的第一个等待进程（top-waiter）
Bit2：锁的状态切换已经完成，等待被获取
__mutex_trylock_or_owner
尝试上锁，成功的话返回持有者的 task_struct 指针
/*
* Trylock variant that retuns the owning task on failure.
*/
static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock)
{
// 获取当前进程的 task_struct
unsigned long owner, curr = (unsigned long)current;
// 获取 lock 的持有者
owner = atomic_long_read(&lock->owner);
// 有点像自旋锁的操作
for (;;) { /* must loop, can race against a flag */
// 取出标志位
unsigned long old, flags = __owner_flags(owner); // owner & 0x7
// 获得真正的 task_struxt 的地址（屏蔽掉 mutex 的 flags）
unsigned long task = owner & ~MUTEX_FLAGS; // MUTEX_FLAGS = 0x7
if (task) {
// 如果锁的持有者不是当前进程直接跳出循环
if (likely(task != curr))
break;
// 如果锁现在处于不可获取的状态跳出循环
if (likely(!(flags & MUTEX_FLAG_PICKUP)))
break;
// 把 flags 的 MUTEX_FLAG_PICKUP 置反
flags &= ~MUTEX_FLAG_PICKUP;
} else {
#ifdef CONFIG_DEBUG_MUTEXES
DEBUG_LOCKS_WARN_ON(flags & MUTEX_FLAG_PICKUP);
#endif
}
/*
* We set the HANDOFF bit, we must make sure it doesn't live
* past the point where we acquire it. This would be possible
* if we (accidentally) set the bit on an unlocked mutex.
*/
// 设置 状态切换位 （HANDOFF bit）
flags &= ~MUTEX_FLAG_HANDOFF;
// &lock->owner = (curr | flags); return (curr | flags); (原子操作)
old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);
// 检查更改 锁 的所有者是否成功
if (old == owner)
return NULL;
// owner = (curr | flags)，仔细看吧
owner = old;
}
// 返回指向锁的持有者的，屏蔽 flags 后的 task_struct 指针
return __owner_task(owner);
}
static inline struct task_struct *__owner_task(unsigned long owner)
{
return (struct task_struct *)(owner & ~MUTEX_FLAGS);
}
mutex_optimistic_spin
/*
* Optimistic spinning.
*
* We try to spin for acquisition when we find that the lock owner
* is currently running on a (different) CPU and while we don't
* need to reschedule. The rationale is that if the lock owner is
* running, it is likely to release the lock soon.
*
* The mutex spinners are queued up using MCS lock so that only one
* spinner can compete for the mutex. However, if mutex spinning isn't
* going to happen, there is no point in going through the lock/unlock
* overhead.
*
* Returns true when the lock was taken, otherwise false, indicating
* that we need to jump to the slowpath and sleep.
*
* The waiter flag is set to true if the spinner is a waiter in the wait
* queue. The waiter-spinner will spin on the lock directly and concurrently
* with the spinner at the head of the OSQ, if present, until the owner is
* changed to itself.
*/
static __always_inline bool
mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
const bool use_ww_ctx, struct mutex_waiter *waiter)
{
if (!waiter) {
/*
* The purpose of the mutex_can_spin_on_owner() function is
* to eliminate the overhead of osq_lock() and osq_unlock()
* in case spinning isn't possible. As a waiter-spinner
* is not going to take OSQ lock anyway, there is no need
* to call mutex_can_spin_on_owner().
*/
//
if (!mutex_can_spin_on_owner(lock))
goto fail;
/*
* In order to avoid a stampede of mutex spinners trying to
* acquire the mutex all at once, the spinners need to take a
* MCS (queued) lock first before spinning on the owner field.
*/
if (!osq_lock(&lock->osq))
goto fail;
}
for (;;) {
struct task_struct *owner;
/* Try to acquire the mutex... */
// 获取锁的持有者
owner = __mutex_trylock_or_owner(lock);
if (!owner)
break;
/*
* There's an owner, wait for it to either
* release the lock or go to sleep.
*/
if (!mutex_spin_on_owner(lock, owner, ww_ctx, waiter))
goto fail_unlock;
/*
* The cpu_relax() call is a compiler barrier which forces
* everything in this loop to be re-loaded. We don't need
* memory barriers as we'll eventually observe the right
* values at the cost of a few extra spins.
*/
cpu_relax();
}
if (!waiter)
osq_unlock(&lock->osq);
return true;
fail_unlock:
if (!waiter)
osq_unlock(&lock->osq);
fail:
/*
* If we fell out of the spin path because of need_resched(),
* reschedule now, before we try-lock the mutex. This avoids getting
* scheduled out right after we obtained the mutex.
*/
if (need_resched()) {
/*
* We _should_ have TASK_RUNNING here, but just in case
* we do not, make it so, otherwise we might get stuck.
*/
__set_current_state(TASK_RUNNING);
schedule_preempt_disabled();
}
return false;
}
mutex_can_spin_on_owner
/*
* Initial check for entering the mutex spinning loop
*/
static inline int mutex_can_spin_on_owner(struct mutex *lock)
{
struct task_struct *owner;
int retval = 1;
if (need_resched())
return 0;
rcu_read_lock();
// 获取锁的持有者
owner = __mutex_owner(lock);
/*
* As lock holder preemption issue, we both skip spinning if task is not
* on cpu or its cpu is preempted
*/
// 如果任务不在 cpu 上或其 cpu 被抢占，都会跳过旋转
if (owner)
retval = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
rcu_read_unlock();
/*
* If lock->owner is not set, the mutex has been released. Return true
* such that we'll trylock in the spin path, which is a faster option
* than the blocking slow path.
*/
// 如果 lock->owner 没有设置，就说明锁是 释放（released） 状态
// 这样的话我们尝试走 spin path 去获取锁，这是一个比阻塞 slow path 更快的选项。
return retval;
}
mutex_spin_on_owner
/*
* Look out! "owner" is an entirely speculative pointer access and not
* reliable.
*
* "noinline" so that this function shows up on perf profiles.
*/
static noinline
bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
struct ww_acquire_ctx *ww_ctx, struct mutex_waiter *waiter)
{
bool ret = true;
rcu_read_lock();
while (__mutex_owner(lock) == owner) {
/*
* Ensure we emit the owner->on_cpu, dereference _after_
* checking lock->owner still matches owner. If that fails,
* owner might point to freed memory. If it still matches,
* the rcu_read_lock() ensures the memory stays valid.
*/
// 内存屏障
barrier();
/*
* Use vcpu_is_preempted to detect lock holder preemption issue.
*/
if (!owner->on_cpu || need_resched() ||
vcpu_is_preempted(task_cpu(owner))) {
ret = false;
break;
}
if (ww_ctx && !ww_mutex_spin_on_owner(lock, ww_ctx, waiter)) {
ret = false;
break;
}
cpu_relax();
}
rcu_read_unlock();
return ret;
}
mutex_unlock -- 释放互斥锁
这个也分两个路径
释放一个互斥锁
/**
* mutex_unlock - release the mutex
* @lock: the mutex to be released
*
* Unlock a mutex that has been locked by this task previously.
*
* This function must not be used in interrupt context. Unlocking
* of a not locked mutex is not allowed.
*
* This function is similar to (but not equivalent to) up().
*/
void __sched mutex_unlock(struct mutex *lock)
{
#ifndef CONFIG_DEBUG_LOCK_ALLOC
if (__mutex_unlock_fast(lock))
return;
#endif
__mutex_unlock_slowpath(lock, _RET_IP_);
}
EXPORT_SYMBOL(mutex_unlock);
__mutex_unlock_fast
通过 fastpath 去释放锁
static __always_inline bool __mutex_unlock_fast(struct mutex *lock)
{
// 获取当前进程的task_struct 的地址
unsigned long curr = (unsigned long)current;
// &lock->owner = 0; (原子操作)
if (atomic_long_cmpxchg_release(&lock->owner, curr, 0UL) == curr)
return true;
return false;
}
__mutex_unlock_slowpath
通过 slowpath 去释放锁
/*
* Release the lock, slowpath:
*/
static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)
{
struct task_struct *next = NULL;
DEFINE_WAKE_Q(wake_q);
unsigned long owner;
mutex_release(&lock->dep_map, ip);
/*
* Release the lock before (potentially) taking the spinlock such that
* other contenders can get on with things ASAP.
*
* Except when HANDOFF, in that case we must not clear the owner field,
* but instead set it to the top waiter.
*/
owner = atomic_long_read(&lock->owner);
for (;;) {
unsigned long old;
#ifdef CONFIG_DEBUG_MUTEXES
DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);
#endif
if (owner & MUTEX_FLAG_HANDOFF)
break;
old = atomic_long_cmpxchg_release(&lock->owner, owner,
__owner_flags(owner));
if (old == owner) {
if (owner & MUTEX_FLAG_WAITERS)
break;
return;
}
owner = old;
}
spin_lock(&lock->wait_lock);
debug_mutex_unlock(lock);
if (!list_empty(&lock->wait_list)) {
/* get the first entry from the wait-list: */
struct mutex_waiter *waiter =
list_first_entry(&lock->wait_list,
struct mutex_waiter, list);
next = waiter->task;
debug_mutex_wake_waiter(lock, waiter);
wake_q_add(&wake_q, next);
}
if (owner & MUTEX_FLAG_HANDOFF)
__mutex_handoff(lock, next);
spin_unlock(&lock->wait_lock);
wake_up_q(&wake_q);
}