[PATCH] simplify exec from init's subthread

I think it is enough to take tasklist_lock for reading while changing
child_reaper:

	Reparenting needs write_lock(tasklist_lock)

	Only one thread in a thread group can do exec()

	sighand->siglock garantees that get_signal_to_deliver()
	will not see a stale value of child_reaper.

This means that we can change child_reaper earlier, without calling
zap_other_threads() twice.

"child_reaper = current" is a NOOP when init does exec from main thread, we
don't care.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 10 insertions, 12 deletions

diff --git a/fs/exec.c b/fs/exec.c
index d0ecea0781f7..dd194923c52c 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -616,6 +616,15 @@ static int de_thread(struct task_struct *tsk)
                 kmem_cache_free(sighand_cachep, newsighand);
                 return -EAGAIN;
         }
+
+        /*
+         * child_reaper ignores SIGKILL, change it now.
+         * Reparenting needs write_lock on tasklist_lock,
+         * so it is safe to do it under read_lock.
+         */
+        if (unlikely(current->group_leader == child_reaper))
+                child_reaper = current;
+
         zap_other_threads(current);
         read_unlock(&tasklist_lock);
 
@@ -660,23 +669,12 @@ static int de_thread(struct task_struct *tsk)
                 struct dentry *proc_dentry1, *proc_dentry2;
                 unsigned long ptrace;
 
-                leader = current->group_leader;
-                /*
-                 * If our leader is the child_reaper become
-                 * the child_reaper and resend SIGKILL signal.
-                 */
-                if (unlikely(leader == child_reaper)) {
-                        write_lock(&tasklist_lock);
-                        child_reaper = current;
-                        zap_other_threads(current);
-                        write_unlock(&tasklist_lock);
-                }
-
                 /*
                  * Wait for the thread group leader to be a zombie.
                  * It should already be zombie at this point, most
                  * of the time.
                  */
+                leader = current->group_leader;
                 while (leader->exit_state != EXIT_ZOMBIE)
                         yield();