Kernel 4.9.x-rt Fully Preemptible Kernel: Issue with gdb and unexpected SIGSTOP signals

["Koehrer Mathias (ETAS/ESW5)" <mathias.koehrer@etas.com>]

Hi all,

with the 4.9.0-rt1 and also the 4.9.4-rt2 kernel (x86, 64bit) I observe a strange issue when running my multithreaded real time executable within gdb.

Fairly often (in about 40% of all runs) the gdb stops executing with the message:
  Thread 8 "MDL07-Acknowled" received signal SIGSTOP, Stopped (signal).
  [Switching to Thread 0x7fffef141700 (LWP 9770)]
  0x00007ffff535b24d in read () at ../sysdeps/unix/syscall-template.S:84

Often it helps to enter "continue" a couple of times to continue the debugging.
However, sometimes it ends up in an endless loop of "continue" followed by a SIGSTOP, "continue", etc.
If this issue occurs, the SIGSTOP is complained for all threads in the executable.
Different system calls are affected: read, select, pselect, clone (via pthread_create), ...
Debugging is more or less impossible due to this issue.
No SIGSTOP is sent out by the application, the root cause for this signal is not clear to me.

When I reconfigured the kernel to use "Preemptible Kernel (Basic RT)" I do not observer this issue.
Also with kernel 3.18.42-rt44 (Fully Preemptible) it works fine. I never see this issue there.

Any hints on how to solve the issue are highly appreciated. 

Thanks

Best regards

Mathias