ptrace: optimize exit_ptrace() for the likely case

exit_ptrace() takes tasklist_lock unconditionally.  We need this lock to
avoid the race with ptrace_traceme(), it acts as a barrier.

Change its caller, forget_original_parent(), to call exit_ptrace() under
tasklist_lock.  Change exit_ptrace() to drop and reacquire this lock if
needed.

This allows us to add the fastpath list_empty(ptraced) check.  In the
likely no-tracees case exit_ptrace() just returns and we avoid the lock()
+ unlock() sequence.

"Zhang, Yanmin" <yanmin_zhang@linux.intel.com> suggested to add this
check, and he reports that this change adds about 11% improvement in some
tests.

Suggested-and-tested-by: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index ceffc67..671ed56 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -771,9 +771,12 @@ static void forget_original_parent(struct task_struct *father)
 	struct task_struct *p, *n, *reaper;
 	LIST_HEAD(dead_children);
 
-	exit_ptrace(father);
-
 	write_lock_irq(&tasklist_lock);
+	/*
+	 * Note that exit_ptrace() and find_new_reaper() might
+	 * drop tasklist_lock and reacquire it.
+	 */
+	exit_ptrace(father);
 	reaper = find_new_reaper(father);
 
 	list_for_each_entry_safe(p, n, &father->children, sibling) {