Re: [PATCH] Don't apply for write lock on tasklist_lock if parent doesn't ptrace other processes

[Oleg Nesterov <oleg@redhat.com>]

I am not surpized perf blaims tasklist, but I am really surpized this patch
adds 10% improvement...

On 07/21, Roland McGrath wrote:
>
> > > @@ -331,6 +331,9 @@ void exit_ptrace(struct task_struct *tra
> > >  	struct task_struct *p, *n;
> > >  	LIST_HEAD(ptrace_dead);
> > >
> > > +	if (list_empty(&tracer->ptraced))
> > > +		return;
> > > +
> > >  	write_lock_irq(&tasklist_lock);
> > >  	list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
> > >  		if (__ptrace_detach(tracer, p))
>
> I think we may have tried that before.  Oleg can tell us if it's really
> safe vs a race with PTRACE_TRACEME or something like that.

Yes, this can race with ptrace_traceme(). Without tasklist_lock in
exit_ptrace(), it is possible that ptrace_traceme() starts __ptrace_link()
before it sees PF_EXITING, and completes before the result of list_add()
is visible to the exiting parent. tasklist acts as a barrier.

So, this list_empty() check needs taskslit at least for reading. But, we
are going to take it for writing right after exit_ptrace() returns, afaics
we can add this fastpatch check for free.

Uncompiled/untested.

Oleg.

 kernel/ptrace.c |   10 +++++++---
 kernel/exit.c   |    3 ++-
 2 files changed, 9 insertions(+), 4 deletions(-)

--- x/kernel/ptrace.c
+++ x/kernel/ptrace.c
@@ -324,26 +324,30 @@ int ptrace_detach(struct task_struct *ch
 }
 
 /*
- * Detach all tasks we were using ptrace on.
+ * Detach all tasks we were using ptrace on. Called with tasklist held.
  */
 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)
--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -771,9 +771,10 @@ static void forget_original_parent(struc
 	struct task_struct *p, *n, *reaper;
 	LIST_HEAD(dead_children);
 
+	write_lock_irq(&tasklist_lock);
+
 	exit_ptrace(father);
 
-	write_lock_irq(&tasklist_lock);
 	reaper = find_new_reaper(father);
 
 	list_for_each_entry_safe(p, n, &father->children, sibling) {