[binutils-gdb/gdb-7.12-branch] Fix lwp_suspend/unsuspend imbalance in linux_wait_1

[Antoine Tremblay <hexa at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=305a161d11c4b5801a0e135fec5d77f3d64f121f

commit 305a161d11c4b5801a0e135fec5d77f3d64f121f
Author: Antoine Tremblay <antoine.tremblay@ericsson.com>
Date:   Wed Aug 31 13:46:22 2016 -0400

    Fix lwp_suspend/unsuspend imbalance in linux_wait_1
    
    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.

Diff:
---
 gdb/gdbserver/ChangeLog   |  4 ++++
 gdb/gdbserver/linux-low.c | 37 +++++++++++++++++++------------------
 2 files changed, 23 insertions(+), 18 deletions(-)

diff --git a/gdb/gdbserver/ChangeLog b/gdb/gdbserver/ChangeLog
index aecc0a9..a2031bf 100644
--- a/gdb/gdbserver/ChangeLog
+++ b/gdb/gdbserver/ChangeLog
@@ -1,3 +1,7 @@
+2016-08-31  Antoine Tremblay  <antoine.tremblay@ericsson.com>
+
+	* linux-low.c (linux_wait_1): Move event switch after unsuspend_lwps.
+
 2016-08-25  Adhemerval Zanella  <adhemerval.zanella@linaro.org>
 
 	PR server/20491
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 ();