=================================================================^M
^[[1m^[[31m==2291==ERROR: AddressSanitizer: heap-use-after-free on address 0x6160000999c4 at pc 0x000000826022 bp 0x7ffd28a8ff70 sp 0x7ffd28a8ff60^M
^[[1m^[[0m^[[1m^[[34mREAD of size 4 at 0x6160000999c4 thread T0^[[1m^[[0m^M
    #0 0x826021 in release_stop_context_cleanup ../../binutils-gdb/gdb/infrun.c:8203^M
    #1 0x72798a in do_my_cleanups ../../binutils-gdb/gdb/common/cleanups.c:154^M
    #2 0x727a32 in do_cleanups(cleanup*) ../../binutils-gdb/gdb/common/cleanups.c:176^M
    #3 0x826895 in normal_stop() ../../binutils-gdb/gdb/infrun.c:8381^M
    #4 0x815208 in fetch_inferior_event(void*) ../../binutils-gdb/gdb/infrun.c:4011^M
    #5 0x868aca in inferior_event_handler(inferior_event_type, void*) ../../binutils-gdb/gdb/inf-loop.c:44^M
....
^[[1m^[[32m0x6160000999c4 is located 68 bytes inside of 568-byte region [0x616000099980,0x616000099bb8)^M
^[[1m^[[0m^[[1m^[[35mfreed by thread T0 here:^[[1m^[[0m^M
    #0 0x7fb0bc1312ca in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x982ca)^M
    #1 0xb8c62f in xfree(void*) ../../binutils-gdb/gdb/common/common-utils.c:100^M
    #2 0x83df67 in free_thread ../../binutils-gdb/gdb/thread.c:207^M
    #3 0x83dfd2 in init_thread_list() ../../binutils-gdb/gdb/thread.c:223^M
    #4 0x805494 in kill_command ../../binutils-gdb/gdb/infcmd.c:2595^M
....

Detaching from program: /home/yao.qi/SourceCode/gnu/build-with-asan/gdb/testsuite/outputs/gdb.threads/threadapply/threadapply, process 2399^M
=================================================================^M
^[[1m^[[31m==2387==ERROR: AddressSanitizer: heap-use-after-free on address 0x6160000a98c0 at pc 0x00000083fd28 bp 0x7ffd401c3110 sp 0x7ffd401c3100^M
^[[1m^[[0m^[[1m^[[34mREAD of size 4 at 0x6160000a98c0 thread T0^[[1m^[[0m^M
    #0 0x83fd27 in thread_alive ../../binutils-gdb/gdb/thread.c:741^M
    #1 0x844277 in thread_apply_all_command ../../binutils-gdb/gdb/thread.c:1804^M
....
^M
^[[1m^[[32m0x6160000a98c0 is located 64 bytes inside of 568-byte region [0x6160000a9880,0x6160000a9ab8)^M
^[[1m^[[0m^[[1m^[[35mfreed by thread T0 here:^[[1m^[[0m^M
    #0 0x7f59a7e322ca in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x982ca)^M
    #1 0xb8c62f in xfree(void*) ../../binutils-gdb/gdb/common/common-utils.c:100^M
    #2 0x83df67 in free_thread ../../binutils-gdb/gdb/thread.c:207^M
    #3 0x83dfd2 in init_thread_list() ../../binutils-gdb/gdb/thread.c:223^M

This patch fixes the issue by always checking refcount before decreasing it.
If it is zero already, delete the thread_info.

gdb:

2017-03-28  Yao Qi  <yao.qi@linaro.org>

	PR gdb/19942
	* infrun.c (release_stop_context_cleanup): If refcount is zero
	delete thread, otherwise decrease the refcount.
	* thread.c (init_thread_list): Likewise.
	(restore_current_thread_cleanup_dtor): Likewise.
	(set_thread_refcount): Likewise.
---
 gdb/infrun.c |  7 ++++++-
 gdb/thread.c | 19 ++++++++++++++++---
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/gdb/infrun.c b/gdb/infrun.c
index c8c2d6e..f8eeddb 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -8182,7 +8182,12 @@ release_stop_context_cleanup (void *arg)
   struct stop_context *sc = (struct stop_context *) arg;
 
   if (sc->thread != NULL)
-    sc->thread->refcount--;
+    {
+      if (sc->thread->refcount == 0)
+	delete sc->thread;
+      else
+	sc->thread->refcount--;
+    }
   xfree (sc);
 }
 
diff --git a/gdb/thread.c b/gdb/thread.c
index 28907c5..b38c5bd 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -205,8 +205,11 @@ init_thread_list (void)
   for (tp = thread_list; tp; tp = tpnext)
     {
       tpnext = tp->next;
-      delete tp;
 
+      if (tp->refcount == 0)
+	delete tp;
+      else
+	tp->refcount--;
     }
 
   thread_list = NULL;
@@ -1636,7 +1639,12 @@ restore_current_thread_cleanup_dtor (void *arg)
 
   tp = find_thread_ptid (old->inferior_ptid);
   if (tp)
-    tp->refcount--;
+    {
+      if (tp->refcount == 0)
+	delete tp;
+      else
+	tp->refcount--;
+    }
   inf = find_inferior_id (old->inf_id);
   if (inf != NULL)
     inf->removable = old->was_removable;
@@ -1653,7 +1661,12 @@ set_thread_refcount (void *data)
     = (struct thread_array_cleanup *) data;
 
   for (k = 0; k != ta_cleanup->count; k++)
-    ta_cleanup->tp_array[k]->refcount--;
+    {
+      if (ta_cleanup->tp_array[k]->refcount == 0)
+	delete ta_cleanup->tp_array[k];
+      else
+	ta_cleanup->tp_array[k]->refcount--;
+    }
 }
 
 struct cleanup *
