[SPARC64]: Fix atomicity of TIF update in flush_thread()

Fix atomicity of TIF update in flush_thread() for sparc64

Fixes correctly the race by using *_ti_thread_flag.

Race :

parent process executing :
sys_ptrace()
 (lock_kernel())
 (ptrace_get_task_struct(pid))
 arch_ptrace()
   ptrace_detach()
     ptrace_disable(child);
       clear_singlestep(child);
         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
         (which clears the TIF_SINGLESTEP flag atomically from a different
          process)
 (put_task_struct(child))
 (unlock_kernel())

And at the same time, in the child process :
sys_execve()
 do_execve()
   search_binary_handler()
     load_elf_binary()
       flush_old_exec()
         flush_thread()
           doing a non-atomic thread flag update
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc64/kernel/process.c

@@ -413,8 +413,13 @@ void flush_thread(void)
 	struct thread_info *t = current_thread_info();
 	struct mm_struct *mm;
 
-	if (t->flags & _TIF_ABI_PENDING)
-		t->flags ^= (_TIF_ABI_PENDING | _TIF_32BIT);
+	if (test_ti_thread_flag(t, TIF_ABI_PENDING)) {
+		clear_ti_thread_flag(t, TIF_ABI_PENDING);
+		if (test_ti_thread_flag(t, TIF_32BIT))
+			clear_ti_thread_flag(t, TIF_32BIT);
+		else
+			set_ti_thread_flag(t, TIF_32BIT);
+	}
 
 	mm = t->task->mm;
 	if (mm)