deadlock on console mutex in gdb

[Takashi Yano]

On Wed, 22 Dec 2021 18:17:20 -0400
David McFarland wrote:
> ===== test.c
> #include <pthread.h>
> #include <stdio.h>
> 
> void *thread(void* p) {
>         printf("thread %p\n", p);
>         return 0;
> }
> 
> int main() {
>         pthread_t ids[100];
>         for (int i = 0; i < 100; ++i) {
>                 pthread_create(&ids[i], 0, &thread, &ids[i]);
>         }
>         for (int i = 0; i < 100; ++i) {
>                 pthread_join(ids[i], 0);
>         }
>         return 0;
> }
> =====
> 
> If I compile that with gcc and run it under gdb, it hangs almost
> immediately. 5/5 attempts using a fresh cygwin with latest packages.

Thanks for the test case. I could reproduce your problem.

I looked into this problem and found the mechanism causing
the issue.

GDB inferior may be suspended while the inferior grabs mutex.
When the inferior creates new thread, GDB receives
CREATE_THREAD_DEBUG_EVENT and inferior is suspended even if
the inferior is grabing the mutex. This causes deadlock in
terminal I/O.

I think there is no other way than not to wait mutex in the
debugger process. If anyone have any other idea, please let
me know.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>