Bug 503130

Looking at mcinvokeWS

Valgrind runs and ends waiting in vgModuleLocal_do_syscall_for_client_WRK for sys_pollsys.
progB is the invoker script which (on a quick reading) will try 60 times and does a poll with a 0 timeout (nonblocking).

The Valgrind process is here

│      143         syscall
│      144
│      145 2:      /* In the range [2, 3), the syscall result is in %rax and %rdx and C,
│      146            but hasn't been committed to the thread state.  If we get
│      147            interrupted in this section then we'll just use values saved in the
│      148            ucontext structure.
│      149
│      150            Important note for this and the following section: Don't add here
│      151            any code that alters the carry flag or worse, call any function.
│      152            That would completely break the fixup after an interrupt. */
│  >   153         movq    -40(%rbp), %rcx

That movq triggers a SIGSEGV. That should be getting the pointer to VexGuestArchState, not something that should be going wrong.