[PATCH v2 1/2] gdb: keep record of thread numbers for reverse execution targets

Magne Hov via Gdb-patches
2023-07-07T16:25:19Z

Targets that support reverse execution may report threads that have
previously been seen to exit. Currently, GDB assigns a new thread
number every time it sees the same thread (re)appearing, but this is
problematic because it makes the output of `info threads` inconsistent,
and because thread-specific breakpoints depend on stable thread-numbers
in order to stop on the right thread.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23454
---
gdb/inferior.c | 1 +
gdb/inferior.h | 7 +++++++
gdb/thread.c | 38 ++++++++++++++++++++++++++++++++++++--
3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/gdb/inferior.c b/gdb/inferior.c
index eee4785fbf7..b6323110e69 100644
--- a/gdb/inferior.c
+++ b/gdb/inferior.c
@@ -258,6 +258,7 @@ inferior::clear_thread_list (bool silent)
delete thr;
});
ptid_thread_map.clear ();
+ ptid_thread_num_map.clear ();
}

/* Notify interpreters and observers that inferior INF was removed. */
diff --git a/gdb/inferior.h b/gdb/inferior.h
index caa8e4d494a..33eb857645e 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -569,6 +569,13 @@ class inferior : public refcounted_object,
/* The highest thread number this inferior ever had. */
int highest_thread_num = 0;

+ /* A map of ptid_t to global thread number and per-inferior thread
+ number, used for targets that support reverse execution. These
+ mappings are maintained for the lifetime of the inferior so that
+ thread numbers can be reused for threads that reappear after
+ having exited. */
+ std::unordered_map<ptid_t, std::pair<int, int>, hash_ptid> ptid_thread_num_map;
+
/* State of GDB control of inferior process execution.
See `struct inferior_control_state'. */
inferior_control_state control;
diff --git a/gdb/thread.c b/gdb/thread.c
index 7f7f035b5ab..ef3da68fc30 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -290,6 +290,15 @@ add_thread_silent (process_stratum_target *targ, ptid_t ptid)
inf->num, ptid.to_string ().c_str (),
targ->shortname ());

+ /* Targets that support reverse execution can see the same thread
+ being added multiple times. If the state for an exited thread is
+ still present in the inferior's thread list it must be cleaned up
+ now before we add a new non-exited entry for the same thread.
+ Targets without reverse execution are not affected by this because
+ they do not reuse thread numbers. */
+ if (target_can_execute_reverse ())
+ delete_exited_threads ();
+
/* We may have an old thread with the same id in the thread list.
If we do, it must be dead, otherwise we wouldn't be adding a new
thread with the same id. The OS is reusing this id --- delete
@@ -332,8 +341,33 @@ thread_info::thread_info (struct inferior *inf_, ptid_t ptid_)
{
gdb_assert (inf_ != NULL);

- this->global_num = ++highest_thread_num;
- this->per_inf_num = ++inf_->highest_thread_num;
+ /* Targets that support reverse execution may see the same thread be
+ created multiple times so a historical record must be maintained
+ and queried. For targets without reverse execution we don't look
+ up historical thread numbers because it leaves us vulnerable to
+ collisions between thread identifiers that have been recycled by
+ the target operating system. */
+ if (target_can_execute_reverse ())
+ {
+ auto pair = inf_->ptid_thread_num_map.find (ptid_);
+ if (pair != inf_->ptid_thread_num_map.end ())
+ {
+ this->global_num = pair->second.first;
+ this->per_inf_num = pair->second.second;
+ }
+ else
+ {
+ this->global_num = ++highest_thread_num;
+ this->per_inf_num = ++inf_->highest_thread_num;
+ inf_->ptid_thread_num_map[ptid_] = std::make_pair (this->global_num,
+ this->per_inf_num);
+ }
+ }
+ else
+ {
+ this->global_num = ++highest_thread_num;
+ this->per_inf_num = ++inf_->highest_thread_num;
+ }

/* Nothing to follow yet. */
this->pending_follow.set_spurious ();
--
2.40.1