runtime: prevent deadlock when profiling signal arrives during traceback

    
    Traceback routines, e.g. callers and funcentry, may call
    __go_get_backtrace_state. If a profiling signal arrives while we
    are in the critical section of __go_get_backtrace_state, it tries
    to do a traceback, which also calls __go_get_backtrace_state,
    which tries to enter the same critical section and will deadlock.
    Prevent this deadlock by setting up runtime_in_callers before
    calling __go_get_backtrace_state.
    
    Found while investigating golang/go#29448. Will add a test in the
    next CL.
    
    Updates golang/go#29448.
    
    Reviewed-on: https://go-review.googlesource.com/c/156037

From-SVN: r267590

gcc/go/gofrontend/MERGE

-0e482bef69d73b9381dbc543e200a1fe57275e81
+2ce291eaee427799bfcde256929dab89e0ab61eb
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/go-caller.c

@@ -137,7 +137,9 @@ __go_file_line (uintptr pc, int index, String *fn, String *file, intgo *line)
 
   runtime_memclr (&c, sizeof c);
   c.index = index;
+  runtime_xadd (&__go_runtime_in_callers, 1);
   state = __go_get_backtrace_state ();
+  runtime_xadd (&__go_runtime_in_callers, -1);
   backtrace_pcinfo (state, pc, callback, error_callback, &c);
   *fn = c.fn;
   *file = c.file;
@@ -169,8 +171,13 @@ syminfo_callback (void *data, uintptr_t pc __attribute__ ((unused)),
 static _Bool
 __go_symbol_value (uintptr pc, uintptr *val)
 {
+  struct backtrace_state *state;
+
   *val = 0;
-  backtrace_syminfo (__go_get_backtrace_state (), pc, syminfo_callback,
+  runtime_xadd (&__go_runtime_in_callers, 1);
+  state = __go_get_backtrace_state ();
+  runtime_xadd (&__go_runtime_in_callers, -1);
+  backtrace_syminfo (state, pc, syminfo_callback,
 		     error_callback, val);
   return *val != 0;
 }

libgo/runtime/go-callers.c

@@ -202,8 +202,8 @@ runtime_callers (int32 skip, Location *locbuf, int32 m, bool keep_thunks)
   data.index = 0;
   data.max = m;
   data.keep_thunks = keep_thunks;
-  state = __go_get_backtrace_state ();
   runtime_xadd (&__go_runtime_in_callers, 1);
+  state = __go_get_backtrace_state ();
   backtrace_full (state, 0, callback, error_callback, &data);
   runtime_xadd (&__go_runtime_in_callers, -1);