Re: [PATCH] Fix setting watchpoints when current thread is running (PR gdb/31521)

[Pedro Alves <pedro@palves.net>]

On 2024-03-21 17:11, Eli Zaretskii wrote:
>> From: Pedro Alves <pedro@palves.net>
>> Date: Thu, 21 Mar 2024 16:43:31 +0000
>>
>> Currently, when the current thread is running, you can print global
>> variables.  However, if you try to set a watchpoint on the same
>> globals, GDB errors out, complaining that the selected thread is
>> running.  Like so:
>>
>>  (gdb) c&
>>  Continuing.
>>  (gdb) p global
>>  $1 = 1098377287
>>  (gdb) watch global
>>  Selected thread is running.
>>
>> This patch makes setting the watchpoint work.  The problem is that
>> update_watchpoint calls get_selected_frame unconditionally.  We can
>> skip it if the watchpoint expression is only watching globals.
> 
> What does this mean in practice?  E.g., suppose GDB sets the
> watchpoint when the selected thread is changing the variable to be
> watched -- will the thread stop or won't it?  IOW, I don't understand
> what happens with programming the debug registers while some thread is
> running.

GDB very briefly temporarily pauses all threads to program the debug registers,
and immediately resumes them.  But that is not user visible.  That part actually
always worked -- we needed to make that work for when the current thread you have
selected is stopped, but there are other threads in the program that are
running -- we need to arm the debug registers of those running threads too, so
the target backend needs to know to pause any running thread.

We just never reached that code when the _selected_ thread is running because very early
in the "watch" code, we tried to get information about the current frame, while when the
thread is running, there is no frame object to consult, thus GDB threw an error.

> 
>> New NEWS blurb added.  I don't think we need to change anything in the
>> manual.
> 
> If this is deemed a bugfix, then we don't need to say anything in
> NEWS, either, IMO.

OK, I was actually borderline with calling it a bug fix or a new feature.
I will just drop NEWS then.

Thanks!

Pedro Alves