为什么使用 std::mutex 的函数会对 pthread_key_create 的地址进行空检查?

Why do functions using std::mutex make a null check of the address of pthread_key_create?

本文关键字：地址 key create 检查 std mutex 函数 为什么 pthread      更新时间：2023-10-16

以这个简单的函数为例，该函数在std::mutex实现的锁下递增一个整数：

#include <mutex>
std::mutex m;
void inc(int& i) {
std::unique_lock<std::mutex> lock(m);
i++;
}

我希望这(内联后)以直接的方式编译为m.lock()i增量的调用，然后m.unlock().

检查生成的程序集以获取最新版本的gcc和clang，但是，我们看到了一个额外的复杂性。首先使用gcc版本：

inc(int&):
mov eax, OFFSET FLAT:__gthrw___pthread_key_create(unsigned int*, void (*)(void*))
test rax, rax
je .L2
push rbx
mov rbx, rdi
mov edi, OFFSET FLAT:m
call __gthrw_pthread_mutex_lock(pthread_mutex_t*)
test eax, eax
jne .L10
add DWORD PTR [rbx], 1
mov edi, OFFSET FLAT:m
pop rbx
jmp __gthrw_pthread_mutex_unlock(pthread_mutex_t*)
.L2:
add DWORD PTR [rdi], 1
ret
.L10:
mov edi, eax
call std::__throw_system_error(int)

前几行很有趣。组装的代码检查__gthrw___pthread_key_create的地址(这是pthread_key_create的实现 - 创建线程本地存储密钥的函数)，如果它为零，则分支到.L2，后者在单个指令中实现增量，而没有任何锁定。

如果它不为零，则按预期进行：锁定互斥锁、执行增量和解锁。

clang的功能更多：它会检查函数的地址两次，一次在lock之前，一次在unlock之前：

inc(int&): # @inc(int&)
push rbx
mov rbx, rdi
mov eax, __pthread_key_create
test rax, rax
je .LBB0_4
mov edi, m
call pthread_mutex_lock
test eax, eax
jne .LBB0_6
inc dword ptr [rbx]
mov eax, __pthread_key_create
test rax, rax
je .LBB0_5
mov edi, m
pop rbx
jmp pthread_mutex_unlock # TAILCALL
.LBB0_4:
inc dword ptr [rbx]
.LBB0_5:
pop rbx
ret
.LBB0_6:
mov edi, eax
call std::__throw_system_error(int)

此检查的目的是什么？

也许是为了支持这样的情况，即目标文件最终被编译成没有 pthreads 支持的二进制文件，然后在该情况下回退到没有锁定的版本？我找不到有关此行为的任何文档。

你的猜测看起来是正确的。 从 gcc 源存储库 (https://github.com/gcc-mirror/gcc.git) 中的libgcc/gthr-posix.h文件：

/* For a program to be multi-threaded the only thing that it certainly must
be using is pthread_create.  However, there may be other libraries that
intercept pthread_create with their own definitions to wrap pthreads
functionality for some purpose.  In those cases, pthread_create being
defined might not necessarily mean that libpthread is actually linked
in.
For the GNU C library, we can use a known internal name.  This is always
available in the ABI, but no other library would define it.  That is
ideal, since any public pthread function might be intercepted just as
pthread_create might be.  __pthread_key_create is an "internal"
implementation symbol, but it is part of the public exported ABI.  Also,
it's among the symbols that the static libpthread.a always links in
whenever pthread_create is used, so there is no danger of a false
negative result in any statically-linked, multi-threaded program.
For others, we choose pthread_cancel as a function that seems unlikely
to be redefined by an interceptor library.  The bionic (Android) C
library does not provide pthread_cancel, so we do use pthread_create
there (and interceptor libraries lose).  */
#ifdef __GLIBC__
__gthrw2(__gthrw_(__pthread_key_create),
__pthread_key_create,
pthread_key_create)
# define GTHR_ACTIVE_PROXY  __gthrw_(__pthread_key_create)
#elif defined (__BIONIC__)
# define GTHR_ACTIVE_PROXY  __gthrw_(pthread_create)
#else
# define GTHR_ACTIVE_PROXY  __gthrw_(pthread_cancel)
#endif
static inline int
__gthread_active_p (void)
{
static void *const __gthread_active_ptr
= __extension__ (void *) &GTHR_ACTIVE_PROXY;
return __gthread_active_ptr != 0;
}

然后，在文件的其余部分中，许多 pthread API 都包装在对__gthread_active_p()函数的检查中。如果__gthread_active_p()返回 0，则不执行任何操作并返回成功。

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