Re: [PATCH] uprobes: Use synchronize_rcu() not synchronize_sched()

[Oleg Nesterov]

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.


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?

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 {