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>
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 @@
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) {