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
    
    Reviewed-on: https://go-review.googlesource.com/50650

From-SVN: r250439

1 files changed, 11 insertions, 3 deletions

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 @@ static void *back_state;
 
 /* A lock to control creating back_state.  */
 
-static Lock back_state_lock;
+static uint32 back_state_lock;
 
 /* The program arguments.  */
 
@@ -85,7 +85,15 @@ extern Slice runtime_get_args(void);
 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 @@ __go_get_backtrace_state ()
 
       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;
 }