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

Pedro Alves
2016-06-17T15:10:52Z

On 06/15/2016 05:41 PM, Yao Qi wrote:
> Pedro Alves <palves@redhat.com> writes:
>
>> 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.
>
> GDB may only resume some threads, and leave other threads running. In
> order to access inferior memory safely, we must stop all threads.

But what do you mean by "safely" ? What goes wrong if we
don't stop all threads?

>
>>
>> I guess this is about flushing instruction caches?
>>
>
> No, it is not about flushing instruction caches.

Then what is it about?

>
>>>>> @@ -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.
>>
>
> You are right, I though too much about supporting range-stepping.
> I rewrite the comments in the patch below,
>
>>> 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?
>
> We insert single-step breakpoints when we process the resume requests
> and threads are about to be resumed. If threads still have pending
> status, single-step breakpoints are not installed, so we need to install
> them in proceed_all_lwps.
>

> @@ -3518,6 +3521,23 @@ linux_wait_1 (ptid_t ptid,
> return ignore_event (ourstatus);
> }
>
> + /* Remove reinsert breakpoints ... */
> + if (can_software_single_step ()
> + && has_reinsert_breakpoints (current_thread)
> + /*... if GDB requests this thread doing resume_step or ...*/
> + && (current_thread->last_resume_kind == resume_step
> + /* GDBserver has already started the step-over for vCont;s,
> + but it gets some other signal, like SIGSTOP sent by
> + GDBserver for vCont;t or other signal program received. */
> + || !maybe_internal_trap))
> + {
> + stop_all_lwps (1, event_child);
> +
> + delete_reinsert_breakpoints (current_thread);
> +
> + unstop_all_lwps (1, event_child);
> + }

I'm re-looking at this and wondering if this is really the
right place to do this. If the thread hits a breakpoint
that ends up not reported to gdb (e.g., condition evals false),
then we'll remove the reinsert breakpoints here, and then
later reinsert them in proceed_all_lwps. The extra
stopping/unstopping everything is best avoided if possible.

Thus, couldn't we move this to after:

/* We found no reason GDB would want us to stop. We either hit one
of our own breakpoints, or finished an internal step GDB
shouldn't know about. */
if (!report_to_gdb)
{
...
}

?

- Also, if in all-stop mode, if gdb does:

vCont;s:1;c

thus setting thread 1 stepping, and all others continuing,
and then some other thread but thread 1 hits a breakpoint,
what is removing the reinsert breakpoint of thread 1?

> +
> /* Note that all addresses are always "out of the step range" when
> there's no range to begin with. */
> in_step_range = lwp_in_step_range (event_child);
> @@ -4281,12 +4301,6 @@ linux_resume_one_lwp_throw (struct lwp_info *lwp,
>
> step = maybe_hw_step (thread);
> }
> - else
> - {
> - /* If the thread isn't doing step-over, there shouldn't be any
> - reinsert breakpoints. */
> - gdb_assert (!has_reinsert_breakpoints (thread));
> - }
>
> if (fast_tp_collecting == 1)
> {
> @@ -4841,7 +4855,6 @@ linux_resume_one_thread (struct inferior_list_entry *entry, void *arg)
> {
> struct thread_info *thread = (struct thread_info *) entry;
> struct lwp_info *lwp = get_thread_lwp (thread);
> - int step;
> int leave_all_stopped = * (int *) arg;
> int leave_pending;
>
> @@ -4910,10 +4923,14 @@ linux_resume_one_thread (struct inferior_list_entry *entry, void *arg)
>
> if (!leave_pending)
> {
> + int step = 0;
> +
> if (debug_threads)
> debug_printf ("resuming LWP %ld\n", lwpid_of (thread));
>
> - step = (lwp->resume->kind == resume_step);
> + if (lwp->resume->kind == resume_step)
> + step = maybe_hw_step (thread);
> +
> linux_resume_one_lwp (lwp, step, lwp->resume->sig, NULL);
> }
> else
> @@ -4954,6 +4971,7 @@ linux_resume (struct thread_resume *resume_info, size_t n)
> struct thread_info *need_step_over = NULL;
> int any_pending;
> int leave_all_stopped;
> + int resume_step_is_handled = 0;
>
> if (debug_threads)
> {
> @@ -4997,12 +5015,55 @@ linux_resume (struct thread_resume *resume_info, size_t n)
> debug_printf ("Resuming, no pending status or step over needed\n");
> }
>
> + /* If resume_step is requested by GDB, install reinsert breakpoints
> + when the thread is about to be actually resumed. IOW, we don't
> + insert reinsert breakpoints if any thread has pending status. */
> + if (!leave_all_stopped && can_software_single_step ())
> + {
> + struct inferior_list_entry *inf, *tmp;
> +
> + if (debug_threads)
> + debug_printf ("Handle resume_step.\n");
> +
> + ALL_INFERIORS (&all_threads, inf, tmp)
> + {
> + struct thread_info *thread = (struct thread_info *) inf;
> + struct lwp_info *lwp = get_thread_lwp (thread);
> +
> + if (lwp->resume != NULL && lwp->resume->kind == resume_step)
> + {
> + if (!resume_step_is_handled)
> + {
> + /* We need to access the inferior memory to install
> + reinsert breakpoints, so stop all threads. */
> + stop_all_lwps (0, NULL);
> +
> + if (debug_threads)
> + debug_printf ("Done stopping all threads.\n");
> +
> + resume_step_is_handled = 1;
> + }
> +
> + install_software_single_step_breakpoints (lwp);
> +
> + if (debug_threads)
> + debug_printf ("Insert breakpoint for resume_step LWP %ld\n",
> + lwpid_of (thread));
> + }
> + }
> +
> + if (debug_threads)
> + debug_printf ("Handle resume_step. Done\n");
> + }
> +
> /* Even if we're leaving threads stopped, queue all signals we'd
> otherwise deliver. */
> find_inferior (&all_threads, linux_resume_one_thread, &leave_all_stopped);
>
> if (need_step_over)
> start_step_over (get_thread_lwp (need_step_over));
> + else if (resume_step_is_handled)
> + unstop_all_lwps (0, NULL);
>
> if (debug_threads)
> {
> @@ -5098,7 +5159,8 @@ proceed_one_lwp (struct inferior_list_entry *entry, void *except)
> if (debug_threads)
> debug_printf (" stepping LWP %ld, client wants it stepping\n",
> lwpid_of (thread));
> - step = 1;
> +
> + step = maybe_hw_step (thread);
> }
> else if (lwp->bp_reinsert != 0)
> {
> @@ -5164,6 +5226,33 @@ proceed_all_lwps (void)
> if (debug_threads)
> debug_printf ("Proceeding, no step-over needed\n");
>
> + if (can_software_single_step ())
> + {
> + struct inferior_list_entry *inf, *tmp;
> +
> + ALL_INFERIORS (&all_threads, inf, tmp)
> + {
> + struct thread_info *thread = (struct thread_info *) inf;
> +
> + /* On software single step target, we insert reinsert
> + breakpoints when the threads are about to be actually
> + resumed. IOW, we don't insert them if any thread has
> + pending status. Before we proceed threads, insert
> + reinsert breakpoints if the client wants it step. */
> + if (thread->last_resume_kind == resume_step)
> + {
> + struct lwp_info *lwp = get_thread_lwp (thread);
> +
> + if (!has_reinsert_breakpoints (thread))
> + install_software_single_step_breakpoints (lwp);
> +
> + if (debug_threads)
> + debug_printf ("Insert breakpoint for resume_step LWP %ld\n",
> + lwpid_of (thread));
> + }
> + }
> + }
> +
> find_inferior (&all_threads, proceed_one_lwp, NULL);
> }
>

Hmm, seeing that we need to handle installing the breakpoints in
both places, I wonder about making linux_resume just handle
setting up the last resume kind and queue signals, and then
end up calling proceed_all_lwps. If that works, I suspect it
would simplify things a good deal.

Thanks,
Pedro Alves