linux-kernel - Re: [PATCH] uprobes: Use synchronize_rcu() not synchronize

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Message-ID: <20180810113548.GA3677@redhat.com>
Date:   Fri, 10 Aug 2018 13:35:49 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] uprobes: Use synchronize_rcu() not synchronize_sched()

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 {