linux-kernel - Re: [PATCH 1/2] trace_events_filter: use rcu_assign_pointer() when setting ftrace_event

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]

Message-ID: <20111123160653.GB25780@google.com>
Date:	Wed, 23 Nov 2011 08:06:53 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] trace_events_filter: use rcu_assign_pointer() when
 setting ftrace_event_call->filter

On Wed, Nov 23, 2011 at 10:16:47AM -0500, Steven Rostedt wrote:
> [ Added Paul to Cc ]
> 
> On Tue, 2011-11-22 at 17:46 -0800, Tejun Heo wrote:
> > ftrace_event_call->filter is sched RCU protected but didn't use
> > rcu_assign_pointer().  Fix it.
> 
> Is it really needed? Maybe just for documentation but I'm not sure this
> use is required because all use cases have synchronize_sched() used,
> which is a big hammer compared to the rcu_assign_pointer().

Oh yeah, I think we do.  synchronize_sched() is to drain users of the
old pointer.  Whether synchronize_sched() or call_rcu() is used is
irrelevant to the synchronization of new pointer.

rcu_assign_pointer() is to synchronize against rcu_dereference()
dereferencing the new one.  More specifically, we need it for the
writer memory barrier, so that it matches the data dependency read
barrier in rcu_dereference() when it access the new pointer;
otherwise, it may fetch the old values from before the new area is
initialized on archs where data dependency barrier isn't noop.

> We update filter here, and then call synchronize_sched() before we free
> the filter_item->filter.
...
> Again you can see that synchronize_sched() is called here.

So, synchronized_sched() being called after isn't relevant.  We want
smp_wmb() between data structure initialization and assignment of the
new pointer.

Thank you.

-- 
tejun
--
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/