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

Yao Qi
2016-06-15T16:41:45Z

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.

>
> I guess this is about flushing instruction caches?
>

No, it is not 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.
>

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.

--
Yao (齐尧)
From f7237f92a32f0ab98ad5db91833f85c36eaf25a3 Mon Sep 17 00:00:00 2001
From: Yao Qi <yao.qi@linaro.org>
Date: Thu, 19 May 2016 17:07:49 +0100
Subject: [PATCH] Use reinsert_breakpoint for vCont;s

V2: fix spaces in changelog entry,
use maybe_hw_step,
cancel step-over if signal arrives (!maybe_internal_trap),

This patch is to teach GDBserver using software single step to handle
vCont;s. Simply speaking, if the thread's resume request is resume_step,
install reinsert breakpoint at the next pcs when GDBserver is about to
resume threads. These reinsert breakpoints of a thread are removed,
when GDBserver gets an event from that thread. Note that GDBserver may
or may not report this event back to GDB.

gdb/gdbserver:

2016-05-20 Yao Qi <yao.qi@linaro.org>

* linux-low.c (resume_stopped_resumed_lwps): If resume request
is resume_step, call maybe_hw_step.
(linux_wait_1): Stop all threads, remove reinsert breakpoints,
and unstop them.
(linux_resume_one_thread): If resume request is resume_step,
call maybe_hw_step.
(linux_resume): Install reinsert breakpoints if the thread is
requested to resume_step.
(proceed_one_lwp): If resume request is resume_step, call
maybe_hw_step.
(proceed_all_lwps): Install reinsert breakpoints if the thread is
requested to resume_step.

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 5d0eafc..dc28c0d 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -2567,7 +2567,10 @@ resume_stopped_resumed_lwps (struct inferior_list_entry *entry)
&& !lp->status_pending_p
&& thread->last_status.kind == TARGET_WAITKIND_IGNORE)
{
- int step = thread->last_resume_kind == resume_step;
+ int step = 0;
+
+ if (thread->last_resume_kind == resume_step)
+ step = maybe_hw_step (thread);

if (debug_threads)
debug_printf ("RSRL: resuming stopped-resumed LWP %s at %s: step=%d\n",
@@ -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);
+ }
+
/* 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);
}