[POWERPC] Fix atomicity of TIF update in flush_thread()

Fix atomicity of TIF update in flush_thread() for powerpc

Fixes it correctly with *_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

Applies on 2.6.20.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index f3d4dd580dd6..e53b2988d1bf 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -465,8 +465,13 @@ void flush_thread(void)
 #ifdef CONFIG_PPC64
         struct thread_info *t = current_thread_info();
 
-        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);
+        }
 #endif
 
         discard_lazy_cpu_state();