Re: [Qemu-devel] [6095] User-mode GDB stub improvements - handle fork

[Martin Mohring <martin.mohring@opensuse.org>]

I seem to have found the cause, why ARM user mode with nptl does not
work as before. see below.

Martin

Aurelien Jarno wrote:
> Revision: 6095
>           http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=6095
> Author:   aurel32
> Date:     2008-12-18 22:44:04 +0000 (Thu, 18 Dec 2008)
>
> Log Message:
> -----------
> User-mode GDB stub improvements - handle fork
>
> Close gdbserver in child processes, so that only one stub tries to talk
> to GDB at a time.  Updated from an earlier patch by Paul Brook.
>
> Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
>
> Modified Paths:
> --------------
>     trunk/gdbstub.c
>     trunk/gdbstub.h
>     trunk/linux-user/main.c
>     trunk/linux-user/syscall.c
>
> Modified: trunk/gdbstub.c
> ===================================================================
> --- trunk/gdbstub.c	2008-12-18 22:43:56 UTC (rev 6094)
> +++ trunk/gdbstub.c	2008-12-18 22:44:04 UTC (rev 6095)
> @@ -1996,6 +1996,18 @@
>      gdb_accept();
>      return 0;
>  }
> +
> +/* Disable gdb stub for child processes.  */
> +void gdbserver_fork(CPUState *env)
> +{
> +    GDBState *s = gdbserver_state;
> +    if (s->fd < 0)
> +      return;
> +    close(s->fd);
> +    s->fd = -1;
> +    cpu_breakpoint_remove_all(env, BP_GDB);
> +    cpu_watchpoint_remove_all(env, BP_GDB);
> +}
>   
In case of USE_NPTL is defined, it seems that either s or env can be
NULL, so I get a exception (yes, nptl works on arm in user mode).
My understanding is that this might be a race condition of the first
fork() call coming even before env or gdbserver_state is initialised
towards !NULL.

>  #else
>  static int gdb_chr_can_receive(void *opaque)
>  {
>
> Modified: trunk/gdbstub.h
> ===================================================================
> --- trunk/gdbstub.h	2008-12-18 22:43:56 UTC (rev 6094)
> +++ trunk/gdbstub.h	2008-12-18 22:44:04 UTC (rev 6095)
> @@ -13,6 +13,7 @@
>  int gdb_handlesig (CPUState *, int);
>  void gdb_exit(CPUState *, int);
>  int gdbserver_start(int);
> +void gdbserver_fork(CPUState *);
>  #else
>  int gdbserver_start(const char *port);
>  #endif
>
> Modified: trunk/linux-user/main.c
> ===================================================================
> --- trunk/linux-user/main.c	2008-12-18 22:43:56 UTC (rev 6094)
> +++ trunk/linux-user/main.c	2008-12-18 22:44:04 UTC (rev 6095)
> @@ -162,6 +162,7 @@
>          pthread_cond_init(&exclusive_cond, NULL);
>          pthread_cond_init(&exclusive_resume, NULL);
>          pthread_mutex_init(&tb_lock, NULL);
> +        gdbserver_fork(thread_env);
>   
This gdbserver_fork(thread_env) call then causes the exception.