linux-kernel - Re: [PATCH] tracing: add trace_event_read

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 19 May 2009 10:05:21 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: add trace_event_read_lock()

Frederic Weisbecker wrote:
> On Mon, May 18, 2009 at 07:35:34PM +0800, Lai Jiangshan wrote:
>> I found that there is nothing to protect event_hash in
>> ftrace_find_event().
> 
> 
> 
> Actually, rcu protects it, but not enough. We have neither
> synchronize_rcu() nor rcu_read_lock.

We have no rcu_read_lock(), RCU can not protects it.

> 
> So we protect against concurrent hlist accesses.
> But the event can be removed when a module is unloaded,
> and that can happen between the time we get the event output
> callback and the time we actually use it.
> 

[...]

> It could be more fine grained.

I think it's fine-grained enough, write-side(modules loading/unloading)
is happened rarely. trace_event_read_lock() will not sleep very likely.

Thoughts?

> We could have a per event rwsem, and also place the
> protected read section only in trace_print_entry() which is the only racy window.
> 

print_trace_line() is the only racy window.
So I just protect print_trace_line()(except __ftrace_dump())

I protect loops which call print_trace_line(), it
reduces invoke-times:

trace_event_read_lock();
while (...) {
	...
	print_trace_line();
	...
}
trace_event_read_unlock();

Thanks!
Lai

> But I'm not sure it's that worthy since event removal is a rare thing.
> 
> So I guess this patch is fine.
> 
> 
> 
> 


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/