[PATCH 1/2] Fix lwp_suspend/unsuspend imbalance in linux_wait_1

Antoine Tremblay
2016-08-31T17:14:21Z

This patch fixes imbalanced lwp_suspend/unsuspend calls caused by the
premature choosing of another event for fairness.

select_event_lwp would switch the event before a call to
unsuspend_all_lwps, thus it would be called with the wrong event.

This caused an assertion failure: unsuspend LWP xx, suspended=-1 when
testing gdb.threads/non-stop-fair-events.exp with ARM range stepping in
GDBServer.

This patch moves the switch of event after the unsuspend/unstop calls.

No regressions, tested on ubuntu 14.04 ARMv7 and x86.
With gdbserver-native.

gdb/gdbserver/ChangeLog:

* linux-low.c (linux_wait_1): Move event switch after unsuspend_lwps.
---
gdb/gdbserver/linux-low.c | 37 +++++++++++++++++++------------------
1 file changed, 19 insertions(+), 18 deletions(-)

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 45061ac..cdff436 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -3771,24 +3771,6 @@ linux_wait_1 (ptid_t ptid,
if (!non_stop)
stop_all_lwps (0, NULL);

- /* If we're not waiting for a specific LWP, choose an event LWP
- from among those that have had events. Giving equal priority
- to all LWPs that have had events helps prevent
- starvation. */
- if (ptid_equal (ptid, minus_one_ptid))
- {
- event_child->status_pending_p = 1;
- event_child->status_pending = w;
-
- select_event_lwp (&event_child);
-
- /* current_thread and event_child must stay in sync. */
- current_thread = get_lwp_thread (event_child);
-
- event_child->status_pending_p = 0;
- w = event_child->status_pending;
- }
-
if (step_over_finished)
{
if (!non_stop)
@@ -3813,6 +3795,25 @@ linux_wait_1 (ptid_t ptid,
}
}

+ /* If we're not waiting for a specific LWP, choose an event LWP
+ from among those that have had events. Giving equal priority
+ to all LWPs that have had events helps prevent
+ starvation. */
+ if (ptid_equal (ptid, minus_one_ptid))
+ {
+ event_child->status_pending_p = 1;
+ event_child->status_pending = w;
+
+ select_event_lwp (&event_child);
+
+ /* current_thread and event_child must stay in sync. */
+ current_thread = get_lwp_thread (event_child);
+
+ event_child->status_pending_p = 0;
+ w = event_child->status_pending;
+ }
+
+
/* Stabilize threads (move out of jump pads). */
if (!non_stop)
stabilize_threads ();
--
2.9.2