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SubjectRe: [PATCH] uprobes: Use synchronize_rcu() not synchronize_sched()
On Fri, 10 Aug 2018 13:35:49 +0200
Oleg Nesterov <oleg@redhat.com> wrote:

> On 08/09, Steven Rostedt wrote:
> >
> > --- a/kernel/trace/trace_uprobe.c
> > +++ b/kernel/trace/trace_uprobe.c
> > @@ -952,7 +952,7 @@ probe_event_disable(struct trace_uprobe *tu, struct trace_event_file *file)
> >
> > list_del_rcu(&link->list);
> > /* synchronize with u{,ret}probe_trace_func */
> > - synchronize_sched();
> > + synchronize_rcu();
>
> Can't we change uprobe_trace_func() and uretprobe_trace_func() to use
> rcu_read_lock_sched() instead? It is more cheap.

Is it? rcu_read_lock_sched() is a preempt_disable(), where
rcu_read_lock() may just be a task counter increment.

>
>
> Hmm. probe_event_enable() does list_del + kfree on failure, this doesn't
> look right... Not only because kfree() can race with list_for_each_entry_rcu(),
> we should not put the 1st link on list until uprobe_buffer_enable().
>
> Does the patch below make sense or I am confused?

I guess the question is, if it isn't enabled, are there any users or
even past users still running. If not, then I think the current code is
OK, as there shouldn't be anything happening to race with it.

-- Steve

>
> Oleg.
>
>
> --- x/kernel/trace/trace_uprobe.c
> +++ x/kernel/trace/trace_uprobe.c
> @@ -896,8 +896,6 @@ probe_event_enable(struct trace_uprobe *
> return -ENOMEM;
>
> link->file = file;
> - list_add_tail_rcu(&link->list, &tu->tp.files);
> -
> tu->tp.flags |= TP_FLAG_TRACE;
> } else {
> if (tu->tp.flags & TP_FLAG_TRACE)
> @@ -909,7 +907,7 @@ probe_event_enable(struct trace_uprobe *
> WARN_ON(!uprobe_filter_is_empty(&tu->filter));
>
> if (enabled)
> - return 0;
> + goto add;
>
> ret = uprobe_buffer_enable();
> if (ret)
> @@ -920,7 +918,8 @@ probe_event_enable(struct trace_uprobe *
> ret = uprobe_register(tu->inode, tu->offset, &tu->consumer);
> if (ret)
> goto err_buffer;
> -
> + add:
> + list_add_tail_rcu(&link->list, &tu->tp.files);
> return 0;
>
> err_buffer:
> @@ -928,7 +927,6 @@ probe_event_enable(struct trace_uprobe *
>
> err_flags:
> if (file) {
> - list_del(&link->list);
> kfree(link);
> tu->tp.flags &= ~TP_FLAG_TRACE;
> } else {

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