Re: RFA: lin-lwp bug with software-single-step or schedlock

[Michael Snyder <msnyder@redhat.com>]

Mark Kettenis wrote:
> 
>    Date: Wed, 23 Oct 2002 00:26:15 -0400
>    From: Daniel Jacobowitz <drow@mvista.com>
> 
>    This bug was noticed on MIPS, because MIPS GNU/Linux is
>    SOFTWARE_SINGLE_STEP_P.  There's a comment in lin_lwp_resume:
> 
>      /* Apparently the interpretation of PID is dependent on STEP: If
>         STEP is non-zero, a specific PID means `step only this process
>         id'.  But if STEP is zero, then PID means `continue *all*
>         processes, but give the signal only to this one'.  */
>      resume_all = (PIDGET (ptid) == -1) || !step;
> 
> I'm fairly certain it's not without reason that I wrote this comment
> as it is.

Uh, you didn't.  I did.  You copied it from "lin-thread.c".
And I wrote it because I found it empirically to be true at the time.


>    Now, I did some digging, and I believe this comment is completely
>    incorrect.  Saying "signal SIGWINCH" causes PIDGET (ptid) == -1,
>    and it is assumed the signal will be delivered to inferior_ptid.
>    There's some other problem there - I think I've discovered that we
>    will neglect to single-step over a breakpoint if we are told to
>    continue with a signal, which is a bit dubious of a decision - but
>    by and large it works as expected.
> 
> I don't see directly why, but I wouldn't be surprised by it.

The whole business is rather unplanned.  The exact meaning and 
connotation of those variables is nowhere defined in writing.
Mainly it's a matter of keeping consistent with what the code
in gdb expects, which you sometimes have to guess, or learn by
examining an older threaded architecture (eg. solaris).

>    So if STEP is 0, we always resume all processes.  STEP at this point _only_
>    refers to whether we want a PTRACE_SINGLESTEP or equivalent;
>    SOFTWARE_SINGLE_STEP has already been handled.  We can't make policy
>    decisions based on STEP any more.
> 
> Indeed, there's something wrong here.

Sounds like something has changed upstream.


>    I tried removing the || !step.  It's pretty hard to tell, since there are
>    still a few non-deterministic failures on my test systems (which is what I
>    was actually hunting when I found this!) but I believe testsuite results are
>    improved on i386.
> 
> There is one thing that might be affected.  Suppose you have a signal
> such as SIGUSR1 that stops the inferior but is also passed on to the
> inferior.  If a multi-threaded program gets this signal, GDB will
> stop.  If you now change the current thread to some other thread and
> try to single-step.  Will the signal be delivered to the origional
> thread?

That's what "prepare_to_proceed" is supposed to take care of.

> If your patch doesn't affect this, I think your patch is OK to check
> in.  Otherwise we'll have to think about this a bit more.

I'll pipe up and comment, as above, but I'll let you two guys
work out the decision between you.

Michael