Cygwin 3.7.0-0.569.g524d75ff regression: segfault on parallel open of /proc/PID/fd/*

[Christian Franke]

After 524d75ff ("Cygwin: open: Unlock fdtab before open_with_arch()"), 
CI workflow shows that 'stress-ng --procfs ...' now hangs.

Testcase:

$ uname -r # not with 3.7.0-0.567.g161e3cb999fb.x86_64
3.7.0-0.569.g524d75ff7398.x86_64

$ cat proc-self-fd.c
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

static void * threadfunc(void * arg)
{
   (void)arg;
   for (int i = 0; i < 100000; i++) {
     char path[32];
     snprintf(path, sizeof(path), "/proc/self/fd/%d", i % 10);
     int fd = open(path, O_RDONLY);
     if (fd < 0) {
       printf("%p#%06d: %s: errno=%d\n", pthread_self(), i, path, errno);
       continue;
     }
     printf("%p#%06d: %s: fd=%d\n", pthread_self(), i, path, fd);
     close(fd);
   }
   return NULL;
}

int main()
{
   pthread_t t = NULL;
   int ret = pthread_create(&t, NULL, threadfunc, NULL);
   if (ret)
     return 1;
   threadfunc(NULL);
   ret = pthread_join(t, NULL);
   printf("pthread_join(%p, .)=%d\n", t, ret);
   if (ret)
     return 1;
   return 0;
}

$ gcc -o proc-self-fd proc-self-fd.c

$ ./proc-self-fd
...
0xa00000500#001908: /proc/self/fd/8: errno=2
0xa00000010#001900: /proc/self/fd/0: fd=3
0xa00000500#001909: /proc/self/fd/9: errno=2
0xa00000010#001901: /proc/self/fd/1: errno=14<== EFAULT
[hangs, kill -9 does not work]

$ strace -o trace.log ./proc-self-fd
[taskkill /f /im proc-self-fd.exe in other window]

$ less trace.log
...
    25   21725 [main] proc-self-fd 14876 fhandler_pty_slave::open_setup: 
/dev/pty1 opened, usecount 3
    61   21786 [main] proc-self-fd 14876 build_fh_pc: found an archetype 
for (null)(136/1) io_handle 0x200
    66   21852 [main] proc-self-fd 14876 build_fh_pc: fh 0x800008E40, 
dev 00880001
    71   21923 [main] proc-self-fd 14876 fhandler_base::open_with_arch: 
line 471: /dev/pty1<0x8000086F0> usecount + 1 = 4
--- Process 3968 (pid: 14876), exception c0000008 at 00007ff81682416a
--- Process 3968 (pid: 14876), exception c0000008 at 00007ff81682416a
...


"... found an archetype for (null) ..." <== Hmm...

--
Regards,
Christian