commit b712bacd939466e66972337744983e180849c535
author Ian Lance Taylor <iant@golang.org> Fri Jul 21 10:12:24 2017 -0700
committer Ian Lance Taylor <iant@golang.org> Fri Jul 21 18:26:53 2017 +0000
tree ecf2676c91022d17570f1e83f1614d30b2ce7a28
parent a9f1aeced86691de891fbf2a8c97e848faf1962e

runtime: don't use runtime_lock in __go_get_backtrace_state

If getSiginfo does not know how to determine the PC, it will call
runtime_callers. That can happen in a thread that was started by
non-Go code, in which case the TLS variable g will not be set, in
which case runtime_lock will crash.

Avoid the problem by using atomic operations for the lock. This is OK
since creating a backtrace state is fast and never blocks.

The test case is TestCgoExternalThreadSIGPROF in the runtime package
on a system that getSiginfo doesn't handle specially.

Updates golang/go#20931

Change-Id: I6007c520e40ba2f00feabf404a7cce7df57fbcef
Reviewed-on: https://go-review.googlesource.com/50650
Reviewed-by: Than McIntosh <thanm@google.com>

libgo/runtime/go-caller.c

diff --git a/libgo/runtime/go-caller.c b/libgo/runtime/go-caller.c
index a35d8d7..360bae6 100644
--- a/libgo/runtime/go-caller.c
+++ b/libgo/runtime/go-caller.c
@@ -74,7 +74,7 @@
 
 /* A lock to control creating back_state.  */
 
-static Lock back_state_lock;
+static uint32 back_state_lock;
 
 /* The program arguments.  */
 
@@ -85,7 +85,15 @@
 struct backtrace_state *
 __go_get_backtrace_state ()
 {
-  runtime_lock (&back_state_lock);
+  uint32 set;
+
+  /* We may not have a g here, so we can't use runtime_lock.  */
+  set = 0;
+  while (!__atomic_compare_exchange_n (&back_state_lock, &set, 1, false, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
+    {
+      runtime_osyield ();
+      set = 0;
+    }
   if (back_state == NULL)
     {
       Slice args;
@@ -113,7 +121,7 @@
 
       back_state = backtrace_create_state (filename, 1, error_callback, NULL);
     }
-  runtime_unlock (&back_state_lock);
+  __atomic_store_n (&back_state_lock, 0, __ATOMIC_RELEASE);
   return back_state;
 }