Re: [RFA]: Modified Watchthreads Patch

Eli Zaretskii
2004-12-10T23:01:18Z

> Date: Fri, 10 Dec 2004 14:10:15 -0500
> From: Daniel Jacobowitz <drow@false.org>
> Cc: gdb-patches@sources.redhat.com
>
> 1) I can see that it will be a bit of work to rearrange i386-linux to
> use this, but it should be doable. Do you know offhand of any
> i386-specific problems other than inserting watchpoints for all
> threads?

The design of the x86 watchpoint support explicitly assumes that
watchpoints are not thread-local. If we want to lift that limitation,
I think the x86-specific code needs to be redesigned. Someone who
knows way more than I do about x86 threads and how the debug registers
are handled by the relevant kernels in the presence of threads, should
present a clean replacement design that deals with thread-local
watchpoints. Small modifications like inserting watchpoints for all
threads and other similar patchwork will simply not cut it, IMHO.

Observe:

> 2) What should to_stopped_by_watchpoint do in the presence of multiple
> threads? It looks like it relies on inferior_ptid being the thread
> which stopped at a watchpoint; I'm worried that that may not be
> consistently true in a heavily threaded application. Maybe it should
> iterate over all threads.
>
> The to_stopped_data_address has its own problems with threads; but the
> case of handling hitting two watchpoints at once, I think, we can leave
> for another day.

These two are just the tip of the iceberg, but already you discovered
that the two cornerstones of the GDB watchpoint support do not work
reliably in multithreaded programs. We should redesign the x86
watchpoint support instead of taking the evolutionary approach, which
will leave us with messy, unmaintainable, and buggy code.