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authorOleg Nesterov <oleg@redhat.com>2010-08-10 18:03:07 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-08-11 08:59:19 -0700
commitc7e49c1488ab20342eaaf38f1ca35a207f4c051d (patch)
tree43876a69cafbb49ce86f71ba5db88018ca7cc036
parent13d7e3a2dba6a79589ed34dc0b9114d7b5ff9eab (diff)
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>
-rw-r--r--kernel/exit.c7
-rw-r--r--kernel/ptrace.c12
2 files changed, 14 insertions, 5 deletions
diff --git a/kernel/exit.c b/kernel/exit.c
index ceffc67b564a..671ed56e0a49 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) {
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 74a3d693c196..f34d798ef4a2 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -324,26 +324,32 @@ int ptrace_detach(struct task_struct *child, unsigned int data)
}
/*
- * Detach all tasks we were using ptrace on.
+ * Detach all tasks we were using ptrace on. Called with tasklist held
+ * for writing, and returns with it held too. But note it can release
+ * and reacquire the lock.
*/
void exit_ptrace(struct task_struct *tracer)
{
struct task_struct *p, *n;
LIST_HEAD(ptrace_dead);
- write_lock_irq(&tasklist_lock);
+ if (likely(list_empty(&tracer->ptraced)))
+ return;
+
list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
if (__ptrace_detach(tracer, p))
list_add(&p->ptrace_entry, &ptrace_dead);
}
- write_unlock_irq(&tasklist_lock);
+ write_unlock_irq(&tasklist_lock);
BUG_ON(!list_empty(&tracer->ptraced));
list_for_each_entry_safe(p, n, &ptrace_dead, ptrace_entry) {
list_del_init(&p->ptrace_entry);
release_task(p);
}
+
+ write_lock_irq(&tasklist_lock);
}
int ptrace_readdata(struct task_struct *tsk, unsigned long src, char __user *dst, int len)