Linux线程同步(三)---互斥锁源码分析
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一 源码分析
1.linux中用户态的mutex实现在哪里？
下载Glibc源码后解压，mutex相关实现函数在：/work/tools/glibc-2.34/nptl中，头文件在glibc-2.34/sysdeps/nptl/bits路径下。
2.pthread_mutex_t联合体定
#/glibc-2.34/sysdeps/nptl/bits/pthreadtypes.h
typedef union
{
struct __pthread_mutex_s __data;
char __size[__SIZEOF_PTHREAD_MUTEX_T];
long int __align;
} pthread_mutex_t;
pthread_mutex_t主要有三个成员变量，其中最关键的是__pthread_mutex_s
#/glibc-2.34/sysdeps/nptl/bits/struct_mutex.h
/* Generic struct for both POSIX and C11 mutexes. New ports are expected
to use the default layout, however architecture can redefine it to
add arch-specific extension (such as lock-elision). The struct have
a size of 32 bytes on LP32 and 40 bytes on LP64 architectures. */
struct __pthread_mutex_s
{
int __lock __LOCK_ALIGNMENT;
unsigned int __count;
int __owner;
#if __WORDSIZE == 64
unsigned int __nusers;
#endif
/* KIND must stay at this position in the structure to maintain
binary compatibility with static initializers.
Concurrency notes:
The __kind of a mutex is initialized either by the static
PTHREAD_MUTEX_INITIALIZER or by a call to pthread_mutex_init.
After a mutex has been initialized, the __kind of a mutex is usually not
changed. BUT it can be set to -1 in pthread_mutex_destroy or elision can
be enabled. This is done concurrently in the pthread_mutex_*lock
functions by using the macro FORCE_ELISION. This macro is only defined
for architectures which supports lock elision.
For elision, there are the flags PTHREAD_MUTEX_ELISION_NP and
PTHREAD_MUTEX_NO_ELISION_NP which can be set in addition to the already
set type of a mutex. Before a mutex is initialized, only
PTHREAD_MUTEX_NO_ELISION_NP can be set with pthread_mutexattr_settype.
After a mutex has been initialized, the functions pthread_mutex_*lock can
enable elision - if the mutex-type and the machine supports it - by
setting the flag PTHREAD_MUTEX_ELISION_NP. This is done concurrently.
Afterwards the lock / unlock functions are using specific elision
code-paths. */
int __kind;
#if __WORDSIZE != 64
unsigned int __nusers;
#endif
#if __WORDSIZE == 64
int __spins;
__pthread_list_t __list;
# define __PTHREAD_MUTEX_HAVE_PREV 1
#else
__extension__ union
{
int __spins;
__pthread_slist_t __list;
};
# define __PTHREAD_MUTEX_HAVE_PREV 0
#endif
};
一般地，__pthread_mutex_s中最重要的成员是如下四个（其他成员先忽略）
struct __pthread_mutex_s
{
int __lock __LOCK_ALIGNMENT;
unsigned int __count;
int __owner;
unsigned int __nusers;
};
1.lock表示当前mutex的状态，0表示初始化没有被持有的状态，此时可以对mutex执行lock操作，lock为1时表示当前mutex已经被持有，并且没有其他线程在等待它的释放，当lock > 1时，表示mutex被某个线程持有并且有另外的线程在等待它的释放。
2.count表示当前被持有的次数，一般来说对不可重入的锁，这个值只可能是0和1，对于可重入的锁，比如递归锁，这个值会大于1。
3.owner用来记录持有当前mutex的线程id，如果没有线程持有，这个值为0。
4.nusers用来记录当前有多少线程持有该互斥体，一般来说，这个值只能是0和1，但是对于读写锁来说，多个读线程是可以共同持有mutex的，因此用nusers来记录线程的数量。
3.___pthread_mutex_init初始化函数
#/glibc-2.34/nptl/pthread_mutex_init.c
int
___pthread_mutex_init (pthread_mutex_t *mutex,
const pthread_mutexattr_t *mutexattr)
{
const struct pthread_mutexattr *imutexattr;
ASSERT_TYPE_SIZE (pthread_mutex_t, __SIZEOF_PTHREAD_MUTEX_T);
/* __kind is the only field where its offset should be checked to
avoid ABI breakage with static initializers. */
ASSERT_PTHREAD_INTERNAL_OFFSET (pthread_mutex_t, __data.__kind,
__PTHREAD_MUTEX_KIND_OFFSET);
ASSERT_PTHREAD_INTERNAL_MEMBER_SIZE (pthread_mutex_t, __data.__kind, int);
imutexattr = ((const struct pthread_mutexattr *) mutexattr
?: &default_mutexattr);
/* Sanity checks. */
switch (__builtin_expect (imutexattr->mutexkind
& PTHREAD_MUTEXATTR_PROTOCOL_MASK,
PTHREAD_PRIO_NONE
<< PTHREAD_MUTEXATTR_PROTOCOL_SHIFT))
{
case PTHREAD_PRIO_NONE << PTHREAD_MUTEXATTR_PROTOCOL_SHIFT:
break;
case PTHREAD_PRIO_INHERIT << PTHREAD_MUTEXATTR_PROTOCOL_SHIFT:
if (__glibc_unlikely (prio_inherit_missing ()))
return ENOTSUP;
break;
default:
/* XXX: For now we don't support robust priority protected mutexes. */
if (imutexattr->mutexkind & PTHREAD_MUTEXATTR_FLAG_ROBUST)
return ENOTSUP;
break;
}
/* Clear the whole variable. */
memset (mutex, '\0', __SIZEOF_PTHREAD_MUTEX_T);
/* Copy the values from the attribute. */
int mutex_kind = imutexattr->mutexkind & ~PTHREAD_MUTEXATTR_FLAG_BITS;
if ((imutexattr->mutexkind & PTHREAD_MUTEXATTR_FLAG_ROBUST) != 0)
{
#ifndef __ASSUME_SET_ROBUST_LIST
if ((imutexattr->mutexkind & PTHREAD_MUTEXATTR_FLAG_PSHARED) != 0
&& !__nptl_set_robust_list_avail)
return ENOTSUP;
#endif
mutex_kind |= PTHREAD_MUTEX_ROBUST_NORMAL_NP;
}
switch (imutexattr->mutexkind & PTHREAD_MUTEXATTR_PROTOCOL_MASK)
{
case PTHREAD_PRIO_INHERIT << PTHREAD_MUTEXATTR_PROTOCOL_SHIFT:
mutex_kind |= PTHREAD_MUTEX_PRIO_INHERIT_NP;
break;
case PTHREAD_PRIO_PROTECT << PTHREAD_MUTEXATTR_PROTOCOL_SHIFT:
mutex_kind |= PTHREAD_MUTEX_PRIO_PROTECT_NP;
int ceiling = (imutexattr->mutexkind
& PTHREAD_MUTEXATTR_PRIO_CEILING_MASK)
>> PTHREAD_MUTEXATTR_PRIO_CEILING_SHIFT;
if (! ceiling)
{
/* See __init_sched_fifo_prio. */
if (atomic_load_relaxed (&__sched_fifo_min_prio) == -1)
__init_sched_fifo_prio ();
if (ceiling < atomic_load_relaxed (&__sched_fifo_min_prio))
ceiling = atomic_load_relaxed (&__sched_fifo_min_prio);
}
mutex->__data.__lock = ceiling << PTHREAD_MUTEX_PRIO_CEILING_SHIFT;
break;
default:
break;
}
/* The kernel when waking robust mutexes on exit never uses
FUTEX_PRIVATE_FLAG FUTEX_WAKE. */
if ((imutexattr->mutexkind & (PTHREAD_MUTEXATTR_FLAG_PSHARED
| PTHREAD_MUTEXATTR_FLAG_ROBUST)) != 0)
mutex_kind |= PTHREAD_MUTEX_PSHARED_BIT;
/* See concurrency notes regarding __kind in struct __pthread_mutex_s
in sysdeps/nptl/bits/thread-shared-types.h. */
atomic_store_relaxed (&(mutex->__data.__kind), mutex_kind);
/* Default values: mutex not used yet. */
// mutex->__count = 0; already done by memset
// mutex->__owner = 0; already done by memset
// mutex->__nusers = 0; already done by memset
// mutex->__spins = 0; already done by memset
// mutex->__next = NULL; already done by memset
LIBC_PROBE (mutex_init, 1, mutex);
return 0;
}
init函数就比较简单了，将mutex结构体清零，设置结构体中__kind属性。函数的前三行部分是参数合法性判断
ASSERT_TYPE_SIZE (pthread_mutex_t, __SIZEOF_PTHREAD_MUTEX_T);
/* __kind is the only field where its offset should be checked to
avoid ABI breakage with static initializers. */
ASSERT_PTHREAD_INTERNAL_OFFSET (pthread_mutex_t, __data.__kind,
__PTHREAD_MUTEX_KIND_OFFSET);
ASSERT_PTHREAD_INTERNAL_MEMBER_SIZE (pthread_mutex_t, __data.__kind, int);
紧接着的一段语句都是为了设置mutex_kind的值
imutexattr = ((const struct pthread_mutexattr *) mutexattr
?: &default_mutexattr);
/* Sanity checks. */
switch (__builtin_expect (imutexattr->mutexkind
& PTHREAD_MUTEXATTR_PROTOCOL_MASK,
PTHREAD_PRIO_NONE
<< PTHREAD_MUTEXATTR_PROTOCOL_SHIFT))
{
case PTHREAD_PRIO_NONE << PTHREAD_MUTEXATTR_PROTOCOL_SHIFT:
break;
case PTHREAD_PRIO_INHERIT << PTHREAD_MUTEXATTR_PROTOCOL_SHIFT:
if (__glibc_unlikely (prio_inherit_missing ()))
return ENOTSUP;
break;
default:
/* XXX: For now we don't support robust priority protected mutexes. */
if (imutexattr->mutexkind & PTHREAD_MUTEXATTR_FLAG_ROBUST)
return ENOTSUP;
break;
}
/* Clear the whole variable. */
memset (mutex, '\0', __SIZEOF_PTHREAD_MUTEX_T);
/* Copy the values from the attribute. */
int mutex_kind = imutexattr->mutexkind & ~PTHREAD_MUTEXATTR_FLAG_BITS;
if ((imutexattr->mutexkind & PTHREAD_MUTEXATTR_FLAG_ROBUST) != 0)
{
#ifndef __ASSUME_SET_ROBUST_LIST
if ((imutexattr->mutexkind & PTHREAD_MUTEXATTR_FLAG_PSHARED) != 0
&& !__nptl_set_robust_list_avail)
return ENOTSUP;
#endif
mutex_kind |= PTHREAD_MUTEX_ROBUST_NORMAL_NP;
}
switch (imutexattr->mutexkind & PTHREAD_MUTEXATTR_PROTOCOL_MASK)
{
case PTHREAD_PRIO_INHERIT << PTHREAD_MUTEXATTR_PROTOCOL_SHIFT:
mutex_kind |= PTHREAD_MUTEX_PRIO_INHERIT_NP;
break;
case PTHREAD_PRIO_PROTECT << PTHREAD_MUTEXATTR_PROTOCOL_SHIFT:
mutex_kind |= PTHREAD_MUTEX_PRIO_PROTECT_NP;
int ceiling = (imutexattr->mutexkind
& PTHREAD_MUTEXATTR_PRIO_CEILING_MASK)
>> PTHREAD_MUTEXATTR_PRIO_CEILING_SHIFT;
if (! ceiling)
{
/* See __init_sched_fifo_prio. */
if (atomic_load_relaxed (&__sched_fifo_min_prio) == -1)
__init_sched_fifo_prio ();
if (ceiling < atomic_load_relaxed (&__sched_fifo_min_prio))
ceiling = atomic_load_relaxed (&__sched_fifo_min_prio);
}
mutex->__data.__lock = ceiling << PTHREAD_MUTEX_PRIO_CEILING_SHIFT;
break;
default:
break;
}
/* The kernel when waking robust mutexes on exit never uses
FUTEX_PRIVATE_FLAG FUTEX_WAKE. */
if ((imutexattr->mutexkind & (PTHREAD_MUTEXATTR_FLAG_PSHARED
| PTHREAD_MUTEXATTR_FLAG_ROBUST)) != 0)
mutex_kind |= PTHREAD_MUTEX_PSHARED_BIT;
设置__kind的值
/* See concurrency notes regarding __kind in struct __pthread_mutex_s
in sysdeps/nptl/bits/thread-shared-types.h. */
atomic_store_relaxed (&(mutex->__data.__kind), mutex_kind);
/* Default values: mutex not used yet. */
// mutex->__count = 0; already done by memset
// mutex->__owner = 0; already done by memset
// mutex->__nusers = 0; already done by memset
// mutex->__spins = 0; already done by memset
// mutex->__next = NULL; already done by memset
LIBC_PROBE (mutex_init, 1, mutex);
有人可能会有疑问，我们在用户态调用的函数是pthread_mutex_init，但是上面的函数名称是___pthread_mutex_init，这两个是同样的函数吗？
答案是肯定的，glibc中做了如下的声明，有关这部分声明大家可以追一下代码看一下，这里就不做详细分析。
4.___pthread_mutex_lock函数
同样地，我们在app层调用的函数时pthread_mutex_lock，它在glibc中的声明如下，也是和pthread_mutex_init相同的方式。
#/glibc-2.34/nptl/pthread_mutex_lock.c
#if PTHREAD_MUTEX_VERSIONS
libc_hidden_ver (___pthread_mutex_lock, __pthread_mutex_lock)
# ifndef SHARED
strong_alias (___pthread_mutex_lock, __pthread_mutex_lock)
# endif
//这里声明了pthread_mutex_lock，实际上就是___pthread_mutex_lock
versioned_symbol (libpthread, ___pthread_mutex_lock, pthread_mutex_lock, GLIBC_2_0);
# if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_34)
compat_symbol (libpthread, ___pthread_mutex_lock, __pthread_mutex_lock,GLIBC_2_0);
# endif
#endif /* PTHREAD_MUTEX_VERSIONS */
接下来，我们看___pthread_mutex_lock函数，它的声明如下，___pthread_mutex_lock函数实际上就是PTHREAD_MUTEX_LOCK。
# define PTHREAD_MUTEX_LOCK ___pthread_mutex_lock
再看PTHREAD_MUTEX_LOCK，完整实现部分如下
int
PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
{
/* See concurrency notes regarding mutex type which is loaded from __kind
in struct __pthread_mutex_s in sysdeps/nptl/bits/thread-shared-types.h. */
unsigned int type = PTHREAD_MUTEX_TYPE_ELISION (mutex);
LIBC_PROBE (mutex_entry, 1, mutex);
if (__builtin_expect (type & ~(PTHREAD_MUTEX_KIND_MASK_NP
| PTHREAD_MUTEX_ELISION_FLAGS_NP), 0))
return __pthread_mutex_lock_full (mutex);
if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_NP))
{
FORCE_ELISION (mutex, goto elision);
simple:
/* Normal mutex. */
LLL_MUTEX_LOCK_OPTIMIZED (mutex);
assert (mutex->__data.__owner == 0);
}
#if ENABLE_ELISION_SUPPORT
else if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_ELISION_NP))
{
elision: __attribute__((unused))
/* This case can never happen on a system without elision,
as the mutex type initialization functions will not
allow to set the elision flags. */
/* Don't record owner or users for elision case. This is a
tail call. */
return LLL_MUTEX_LOCK_ELISION (mutex);
}
#endif
else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
== PTHREAD_MUTEX_RECURSIVE_NP, 1))
{
/* Recursive mutex. */
pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
/* Check whether we already hold the mutex. */
if (mutex->__data.__owner == id)
{
/* Just bump the counter. */
if (__glibc_unlikely (mutex->__data.__count + 1 == 0))
/* Overflow of the counter. */
return EAGAIN;
++mutex->__data.__count;
return 0;
}
/* We have to get the mutex. */
LLL_MUTEX_LOCK_OPTIMIZED (mutex);
assert (mutex->__data.__owner == 0);
mutex->__data.__count = 1;
}
else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
== PTHREAD_MUTEX_ADAPTIVE_NP, 1))
{
if (LLL_MUTEX_TRYLOCK (mutex) != 0)
{
int cnt = 0;
int max_cnt = MIN (max_adaptive_count (),
mutex->__data.__spins * 2 + 10);
do
{
if (cnt++ >= max_cnt)
{
LLL_MUTEX_LOCK (mutex);
break;
}
atomic_spin_nop ();
}
while (LLL_MUTEX_TRYLOCK (mutex) != 0);
mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
}
assert (mutex->__data.__owner == 0);
}
else
{
pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
assert (PTHREAD_MUTEX_TYPE (mutex) == PTHREAD_MUTEX_ERRORCHECK_NP);
/* Check whether we already hold the mutex. */
if (__glibc_unlikely (mutex->__data.__owner == id))
return EDEADLK;
goto simple;
}
pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
/* Record the ownership. */
mutex->__data.__owner = id;
#ifndef NO_INCR
++mutex->__data.__nusers;
#endif
LIBC_PROBE (mutex_acquired, 1, mutex);
return 0;
}
我们逐步分析这个函数中的实现，如下是参数检查
LIBC_PROBE (mutex_entry, 1, mutex);
如下是普通mutex（mutexkind = PTHREAD_MUTEX_NORMAL，也就是我们在电泳pthread_mutex_init设置的mutex类型）的调用，也是用户最常用的mutex类型，我们主要关注这段实现
if (__glibc_likely (type == PTHREAD_MUTEX_TIMED_NP))
{
FORCE_ELISION (mutex, goto elision);
simple:
/* Normal mutex. */
LLL_MUTEX_LOCK_OPTIMIZED (mutex);
assert (mutex->__data.__owner == 0);
}
显然这个if分支中，主要是调用了宏LLL_MUTEX_LOCK_OPTIMIZED，现在我们重点关注一下这个宏
# define LLL_MUTEX_LOCK_OPTIMIZED(mutex) lll_mutex_lock_optimized (mutex)
static inline void
lll_mutex_lock_optimized (pthread_mutex_t *mutex)
{
/* The single-threaded optimization is only valid for private
mutexes. For process-shared mutexes, the mutex could be in a
shared mapping, so synchronization with another process is needed
even without any threads. If the lock is already marked as
acquired, POSIX requires that pthread_mutex_lock deadlocks for
normal mutexes, so skip the optimization in that case as
well. */
int private = PTHREAD_MUTEX_PSHARED (mutex);
if (private == LLL_PRIVATE && SINGLE_THREAD_P && mutex->__data.__lock == 0)
mutex->__data.__lock = 1;
else
lll_lock (mutex->__data.__lock, private);
}
显然这个函数中主要是调用了lll_lock，我们继续看lll_lock宏的实现
#define lll_lock(futex, private) \
__lll_lock (&(futex), private)
继续看__lll_lock宏定义，如下
/* This is an expression rather than a statement even though its value is
void, so that it can be used in a comma expression or as an expression
that's cast to void. */
/* The inner conditional compiles to a call to __lll_lock_wait_private if
private is known at compile time to be LLL_PRIVATE, and to a call to
__lll_lock_wait otherwise. */
/* If FUTEX is 0 (not acquired), set to 1 (acquired with no waiters) and
return. Otherwise, ensure that it is >1 (acquired, possibly with waiters)
and then block until we acquire the lock, at which point FUTEX will still be
>1. The lock is always acquired on return. */
#define __lll_lock(futex, private) \
((void) \
({ \
int *__futex = (futex); \
if (__glibc_unlikely \
(atomic_compare_and_exchange_bool_acq (__futex, 1, 0))) \
{ \
if (__builtin_constant_p (private) && (private) == LLL_PRIVATE) \
__lll_lock_wait_private (__futex); \
else \
__lll_lock_wait (__futex, private); \
} \
}))
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