Re: [PATCH 11/12] Use reinsert_breakpoint for vCont;s

Pedro Alves
2016-06-14T15:48:52Z

On 06/14/2016 02:14 PM, Yao Qi wrote:
> Pedro Alves <palves@redhat.com> writes:
>
>>> @@ -4293,7 +4313,7 @@ linux_resume_one_lwp_throw (struct lwp_info *lwp,
>>>
>>> step = maybe_hw_step (thread);
>>> }
>>> - else
>>> + else if (lwp->resume != NULL && lwp->resume->kind != resume_step)
>>> {
>>> /* If the thread isn't doing step-over, there shouldn't be any
>>> reinsert breakpoints. */
>>
>> Consider (non-stop RSP):
>>
>> -> vCont;s:1
>> <- OK
>> -> vCont;s:2
>> <- OK
>>
>> The handling of the second vCont sets thread 1's lwp->resume to NULL.
>
> If so, the assert won't be called for thread 1.
>
>> The lwp->resume pointer is only meaningful within linux_resume
>> and its callees. (But this function is called in other contexts.)
>>
>
> When I wrote the patch, it took me a while to think about this condition
> check. I wanted to remove this condition and assert, but finally
> decided to leave it there, as it is not harmful. If lwp->resume is only
> meaningful within linux_resume and its callees, how about remove the
> condition check and assert here?

Yes, if it's only for the assert, then let's remove it.

>
>>> @@ -5009,12 +5033,52 @@ linux_resume (struct thread_resume *resume_info, size_t n)
>>> debug_printf ("Resuming, no pending status or step over needed\n");
>>> }
>>>
>>> + /* Before we resume the threads, if resume_step is requested by GDB,
>>> + stop all threads and install reinsert breakpoints. */
>>
>> Looking again, I think the rationale for stopping threads should
>> be mentioned here, as it's not obvious.
>>
>
> How about this,
>
> /* Before we resume the threads, if resume_step is requested by GDB,
> we need to access the inferior memory to install reinsert
> breakpoints, so stop all threads. */

That doesn't tell the reader why we need to stop _all_ threads. The
threads that are about to be resumed are obviously stopped, and
thus we could already _access_ inferior memory through them.

I guess this is about flushing instruction caches?

>>> @@ -5176,6 +5241,30 @@ proceed_all_lwps (void)
>>> if (debug_threads)
>>> debug_printf ("Proceeding, no step-over needed\n");
>>>
>>> + /* Re-install the reinsert breakpoints on software single step target
>>> + if the client wants it step. */
>>> + if (can_software_single_step ())
>>
>> Not immediately obvious to why is this necessary. Where were they
>> removed in the first place? I'm it must be necessary, but maybe
>> extending the comment helps.
>
> How about this
>
> /* On software single step target, we removed reinsert breakpoints
> after we get any events from the inferior.

Is that all events, even internal events? From the patch, it seemed
like it was only before reporting an event to gdb.

> If the client wants
> thread step, re-install these reinsert breakpoints. */
>

If we only remove before reporting an event to gdb, then I don't
understand this. We already insert single-step breakpoints when
we process the resume request from gdb, no?

Thanks,
Pedro Alves