linux-kernel - Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule be Tasks RCU quiescent state

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 for Android: free password hash cracker in your pocket

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

Date:   Fri, 29 Sep 2017 13:44:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Sasha Levin <levinsasha928@...il.com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "dipankar@...ibm.com" <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Fr??d??ric Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "bobby.prani@...il.com" <bobby.prani@...il.com>,
        Radim Kr??m???? <rkrcmar@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule
 be Tasks RCU quiescent state

On 29/09/2017 12:34, Peter Zijlstra wrote:
> On Fri, Sep 29, 2017 at 12:01:24PM +0200, Paolo Bonzini wrote:
>>> Does this mean whenever we get a page fault in a RCU read-side critical
>>> section, we may hit this?
>>>
>>> Could we simply avoid to schedule() in kvm_async_pf_task_wait() if the
>>> fault process is in a RCU read-side critical section as follow?
>>>
>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>> index aa60a08b65b1..291ea13b23d2 100644
>>> --- a/arch/x86/kernel/kvm.c
>>> +++ b/arch/x86/kernel/kvm.c
>>> @@ -140,7 +140,7 @@ void kvm_async_pf_task_wait(u32 token)
>>>  
>>>  	n.token = token;
>>>  	n.cpu = smp_processor_id();
>>> -	n.halted = is_idle_task(current) || preempt_count() > 1;
>>> +	n.halted = is_idle_task(current) || preempt_count() > 1 || rcu_preempt_depth();
>>>  	init_swait_queue_head(&n.wq);
>>>  	hlist_add_head(&n.link, &b->list);
>>>  	raw_spin_unlock(&b->lock);
>>>
>>> (Add KVM folks and list Cced)
>>
>> Yes, that would work.  Mind to send it as a proper patch?
> 
> I'm confused, why would we do an ASYNC PF at all here? Thing is, a
> printk() shouldn't trigger a major fault _ever_. At worst it triggers
> something like a vmalloc minor fault. And I'm thinking we should not do
> the whole ASYNC machinery for minor faults.

Async page faults are page faults _on the host_ side, and you cannot
control what the host pages out.  Of course the hypervisor filters out
some cases itself (e.g. IF=0) but in general you could get one at any time.

Paolo