[PATCH] elf_core_dump: don't take tasklist_lock

do_each_thread() is rcu-safe, and all tasks which use this ->mm must sleep
in wait_for_completion(&mm->core_done) at this point, so we can use RCU
locks.

Also, remove unneeded INIT_LIST_HEAD(new) before list_add(new, head).
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/binfmt_elf.c

@@ -1479,20 +1479,19 @@ static int elf_core_dump(long signr, struct pt_regs *regs, struct file *file)
 
 	if (signr) {
 		struct elf_thread_status *tmp;
-		read_lock(&tasklist_lock);
+		rcu_read_lock();
 		do_each_thread(g,p)
 			if (current->mm == p->mm && current != p) {
 				tmp = kzalloc(sizeof(*tmp), GFP_ATOMIC);
 				if (!tmp) {
-					read_unlock(&tasklist_lock);
+					rcu_read_unlock();
 					goto cleanup;
 				}
-				INIT_LIST_HEAD(&tmp->list);
 				tmp->thread = p;
 				list_add(&tmp->list, &thread_list);
 			}
 		while_each_thread(g,p);
-		read_unlock(&tasklist_lock);
+		rcu_read_unlock();
 		list_for_each(t, &thread_list) {
 			struct elf_thread_status *tmp;
 			int sz;