performance regression in ssl module under free-threading

On free-threading there is a large ~20% performance regression under asyncio_tcp_ssl benchmark. A large part of slowdown is from https://github.com/python/cpython/pull/124993 which added critical sections and locks for thread safety however 20% is large slowdown for the important single threaded use-case.

Critical sections are slow especially for extensions which are dynamically loaded because accessing thread states is slow and there are multiple function calls even for the fastpath of no contention for acquisition of critical section.

Comparing the assembly of _ssl_RAND_status in free-threading vs normal build:

• free-threading:

Dump of assembler code for function _ssl_RAND_status:
   0x00007ffff7b97090 <+0>:     push   %rbx
   0x00007ffff7b97091 <+1>:     sub    $0x10,%rsp
   0x00007ffff7b97095 <+5>:     lea    0xa(%rdi),%rbx
   0x00007ffff7b97099 <+9>:     mov    $0x1,%cl
   0x00007ffff7b9709b <+11>:    xor    %eax,%eax
   0x00007ffff7b9709d <+13>:    lock cmpxchg %cl,0xa(%rdi)
   0x00007ffff7b970a2 <+18>:    jne    0x7ffff7b970c5 <_ssl_RAND_status+53>
   0x00007ffff7b970a4 <+20>:    call   0x7ffff7b950e0 <_PyThreadState_GetCurrent@plt>
   0x00007ffff7b970a9 <+25>:    mov    %rbx,0x8(%rsp)
   0x00007ffff7b970ae <+30>:    mov    0xb0(%rax),%rcx
   0x00007ffff7b970b5 <+37>:    mov    %rcx,(%rsp)
   0x00007ffff7b970b9 <+41>:    mov    %rsp,%rcx
   0x00007ffff7b970bc <+44>:    mov    %rcx,0xb0(%rax)
   0x00007ffff7b970c3 <+51>:    jmp    0x7ffff7b970d0 <_ssl_RAND_status+64>
   0x00007ffff7b970c5 <+53>:    mov    %rsp,%rdi
   0x00007ffff7b970c8 <+56>:    mov    %rbx,%rsi
   0x00007ffff7b970cb <+59>:    call   0x7ffff7b95cc0 <_PyCriticalSection_BeginSlow@plt>
   0x00007ffff7b970d0 <+64>:    call   0x7ffff7b95c30 <RAND_status@plt>
   0x00007ffff7b970d5 <+69>:    movslq %eax,%rdi
   0x00007ffff7b970d8 <+72>:    call   0x7ffff7b96560 <PyBool_FromLong@plt>
   0x00007ffff7b970dd <+77>:    mov    %rax,%rbx
   0x00007ffff7b970e0 <+80>:    mov    0x8(%rsp),%rdi
   0x00007ffff7b970e5 <+85>:    test   %rdi,%rdi
   0x00007ffff7b970e8 <+88>:    je     0x7ffff7b97116 <_ssl_RAND_status+134>
   0x00007ffff7b970ea <+90>:    xor    %ecx,%ecx
   0x00007ffff7b970ec <+92>:    mov    $0x1,%al
   0x00007ffff7b970ee <+94>:    lock cmpxchg %cl,(%rdi)
   0x00007ffff7b970f2 <+98>:    je     0x7ffff7b970f9 <_ssl_RAND_status+105>
   0x00007ffff7b970f4 <+100>:   call   0x7ffff7b954a0 <PyMutex_Unlock@plt>
   0x00007ffff7b970f9 <+105>:   call   0x7ffff7b950e0 <_PyThreadState_GetCurrent@plt>
   0x00007ffff7b970fe <+110>:   mov    (%rsp),%rcx
   0x00007ffff7b97102 <+114>:   mov    %rcx,0xb0(%rax)
   0x00007ffff7b97109 <+121>:   test   $0x1,%cl
   0x00007ffff7b9710c <+124>:   je     0x7ffff7b97116 <_ssl_RAND_status+134>
   0x00007ffff7b9710e <+126>:   mov    %rax,%rdi
   0x00007ffff7b97111 <+129>:   call   0x7ffff7b95f90 <_PyCriticalSection_Resume@plt>
   0x00007ffff7b97116 <+134>:   mov    %rbx,%rax
   0x00007ffff7b97119 <+137>:   add    $0x10,%rsp
   0x00007ffff7b9711d <+141>:   pop    %rbx
   0x00007ffff7b9711e <+142>:   ret
End of assembler dump.

• normal build:

   0x00007ffff773bd00 <+0>:     push   %rax
   0x00007ffff773bd01 <+1>:     call   0x7ffff773ac20 <RAND_status@plt>
   0x00007ffff773bd06 <+6>:     movslq %eax,%rdi
   0x00007ffff773bd09 <+9>:     pop    %rax
   0x00007ffff773bd0a <+10>:    jmp    0x7ffff773b520 <PyBool_FromLong@plt>

Linked PRs

• gh-141406
• gh-143275
• gh-143491
• gh-146066

从 _ssl_RAND_status 入口点开始，在 free-threaded 和普通构建上复现 asyncio_tcp_ssl 基准测试。检查汇编中显示的临界区和线程状态操作，然后将链接的 PR 作为上下文参考。完成标准是在不影响线程安全的情况下实质性地减小回归，并由基准测试结果证明改进效果。