Re: [PATCH] Fix "PC register is not available" issue

[Eli Zaretskii <eliz at gnu dot org>]

> Date: Wed, 19 Mar 2014 10:06:51 +0000
> From: Pedro Alves <palves@redhat.com>
> CC: brobecker@adacore.com, gdb-patches@sourceware.org
> 
> On 03/19/2014 03:40 AM, Eli Zaretskii wrote:
> >> Date: Tue, 18 Mar 2014 17:33:16 +0000
> >> From: Pedro Alves <palves@redhat.com>
> >> CC: Joel Brobecker <brobecker@adacore.com>, gdb-patches@sourceware.org
> >>
> >> I see that the GetThreadContext call (do_windows_fetch_inferior_registers)
> >> doesn't check for errors (I think it should (*)).  It'd be interesting to know whether gdb can
> >> actually read the registers off of this thread
> > 
> > How to see those registers?
> 
> Just "info registers" ?

That's what I thought, but ...

> If we can't even read registers off of it, and GetThreadContext
> is failing, it means after your patch we'll be showing bogus
> register contents for these threads.

...how do you tell bogus register contents from correct contents?
It's not like I know which register should have what value at any
given time, do I?

> But I think GetThreadContext will indeed succeed for these threads.

Well, at least MSDN begs to differ:

  You cannot get a valid context for a running thread. Use the
  SuspendThread function to suspend the thread before calling
  GetThreadContext.

> AFAIK, we don't really need the SuspendThread calls when handling
> a debug event, given that when WaitForDebugEvent returns a
> stop event, all threads have already been stopped by the OS for us.

Yes, AFAIK that's true.

> We really only need to SuspendThread threads when we might want
> to leave most threads paused on the next resume, for e.g., when
> stepping over a breakpoint.  The suspend count handling in
> windows-nat.c is quite messy, and looking at the code, it doesn't
> look like we actually get that right, given we only SuspendThread
> threads if we try to read their registers, and so if nothing reads
> registers off all threads when e.g., handling a breakpoint that
> we decide needs to be stepped over (which we don't), then we end
> up resuming threads we shouldn't.

That's assuming that stepping resumes threads.  I'm not sure, but I
really don't know enough about debugging APIs on Windows.

> It'll likely show us the thread is stopped at some ntdll.dll function
> or some such, and from the function name we will likely
> be able to infer what/which thread is this, like, e.g., whether
> it's a thread injected with DebugBreakProcess or some such
> (internally by one of the system dlls or the dlls your app
> links with).

I'll see what I can find about that, but I doubt you'd see something
telltale in the backtrace.  (The thread started by Windows for
debugging is not part of this issue; I never saw the threads that are
to have any debug-related functions on their callstacks.)