linux-kernel - Re: [RFC PATCH v2] Add rcu user eqs exception hooks for async page fault

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: <1354264606.2552.39.camel@ThinkPad-T5421.cn.ibm.com>
Date:	Fri, 30 Nov 2012 16:36:46 +0800
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-next list <linux-next@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, paulmck@...ux.vnet.ibm.com,
	sasha.levin@...cle.com, avi@...hat.com
Subject: Re: [RFC PATCH v2] Add rcu user eqs exception hooks for async page
 fault

On Thu, 2012-11-29 at 19:25 +0200, Gleb Natapov wrote:
> On Thu, Nov 29, 2012 at 03:40:05PM +0100, Frederic Weisbecker wrote:
> > 2012/11/29 Li Zhong <zhong@...ux.vnet.ibm.com>:
> > > On Wed, 2012-11-28 at 13:55 +0100, Frederic Weisbecker wrote:
> > >> > With rcu_user_exit() at the beginning, now rcu_irq_enter() only protects
> > >> > the cpu idle eqs, but it's not good to call rcu_irq_exit() after the cpu
> > >> > halt and the page ready.
> > >>
> > >> Hmm, why is it not good?
> > >
> > > I think in this case, as cpu halts and waits for page ready, it is
> > > really in idle, and it's better for rcu to think it as rcu idle. But
> > > with rcu_irq_enter(), the rcu would not think this cpu as idle. And the
> > > rcu_irq_exit() is only called after this idle period to mark this cpu as
> > > rcu idle again.
> > 
> > 
> > Indeed. How about calling rcu_irq_exit() before native_safe_halt() and
> > rcu_irq_enter() right after?
> > 
> We can't. If exception happens in the middle of rcu read section while
> preemption is disabled then calling rcu_irq_exit() before halt is
> incorrect. We can do that only if exception happen during idle and this
> should be rare enough for us to not care.

If I understand correctly, maybe it doesn't cause problems.

I guess you mean in other(not idle) task's rcu read critical section, we
get an async page fault? In this case the rcu_idle_exit() won't make
this cpu to enter rcu idle state. As the task environment is rcu non
idle. This is the case we use rcu_irq_enter()/rcu_irq_exit() in code
path where it might be in rcu idle or rcu non idle, and they don't
change the rcu non-idle state if used in rcu non-idle state.

And it is also safe if it is rcu read critical section in idle, as it is
most probably wrapped in an outer rcu_irq_enter/exit(), so we have 

rcu idle
	rcu_irq_enter() - exit rcu idle, to protect the read section
	  exception
	  rcu_irq_enter() in the page not present path
		rcu_irq_exit() before halt 

so the halt is also safe here, as we have two rcu_irq_enter() and one
rcu_irq_exit().

I agree with Frederic that this is the safest and simplest way now, and
will try to update the code according to it.

Thanks, Zhong

> > >> > So I still want to remove it. And later if it shows that we really needs
> > >> > rcu somewhere in this code path, maybe we could use RCU_NONIDLE() to
> > >> > protect it. ( The suspicious RCU usage reported in commit
> > >> > c5e015d4949aa665 seems related to schedule(), which is not in the code
> > >> > path if we are in cpu idle eqs )
> > >>
> > >> Yes but if rcu_irq_*() calls are fine to be called there, and I
> > >> believe they are because exception_enter() exits the user mode, we
> > >> should start to protect there right now instead of waiting for a
> > >> potential future warning of illegal RCU use.
> > >
> > > I agree, but I think by only protecting the necessary code avoids
> > > marking the entire waiting period as rcu non idle.
> > 
> > If we use RCU_NONIDLE(), this assume we need to check all the code
> > there deeply for potential RCU uses and ensure it will never be
> > extended later to use RCU. We really don't scale enough for that.
> > There are too much subsystems involved there: waitqueue, spinlocks,
> > slab, per cpu, etc...
> > 
> > So I strongly suggest we use rcu_irq_*() APIs, and relax them around
> > native_safe_halt() like I suggested above. This seems to me the safest
> > solution.
> 
> --
> 			Gleb.
> --
> 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/
> 


--
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/