linux-kernel - Re: [PATCH 1/1] rcu/sync: simplify the state machine

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: <20190429204041.GU3923@linux.ibm.com>
Date:   Mon, 29 Apr 2019 13:40:41 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] rcu/sync: simplify the state machine

On Mon, Apr 29, 2019 at 06:06:04PM +0200, Oleg Nesterov wrote:
> On 04/28, Paul E. McKenney wrote:
> >
> > And it still looks good after review, so I have pushed it.
> 
> Thanks!
> 
> > I did add
> > READ_ONCE() and WRITE_ONCE() to unprotected uses of ->gp_state, but
> > please let me know if I messed anything up.
> 
> Well, at least WRITE_ONCE()'s look certainly unneeded to me, gp_state
> is protected by rss_lock.
> 
> WARN_ON_ONCE(gp_state) can read gp_state lockless, but even in this case
> I do not understand what READ_ONCE() tries to prevent...
> 
> Nevermind, this won't hurt and as I already said I don't understand the
> _ONCE() magic anyway ;)

If I understand correctly, rcu_sync_is_idle() can be inline and returns
->gp_state.  Without the READ_ONCE(), the compiler might fuse reads from
consecutive calls to rcu_sync_is_idle() or (under register pressure)
re-read from it, getting inconsistent results.  For example, this:

	tmp = rcu_sync_is_idle(rsp);
	do_something(tmp);
	do_something_else(tmp);

Might become this:

	do_something(rcu_sync_is_idle(rsp));
	do_something_else(rcu_sync_is_idle(rsp));

This might actually be harmless given current calls, but it would be at
best an accident waiting to happen.

Or am I missing something here?

							Thanx, Paul