linux-kernel - Re: [PATCH -tip V2 0/2] kprobes: Fix RCU warning and cleanup

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Date:   Mon, 13 Jan 2020 22:09:53 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Ingo Molnar <mingo@...nel.org>,
        Anders Roxell <anders.roxell@...aro.org>, paulmck@...nel.org,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -tip V2 0/2] kprobes: Fix RCU warning and cleanup

Hi Joel,

On Mon, 13 Jan 2020 12:16:40 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > > 
> > > > Hi Masami,
> > > > 
> > > > I believe I had commented before that I don't agree with this patch:
> > > > https://lore.kernel.org/lkml/157535318870.16485.6366477974356032624.stgit@devnote2/
> > > > 
> > > > The rationale you used is to replace RCU-api with non-RCU api just to avoid
> > > > warnings. I think a better approach is to use RCU api and pass the optional
> > > > expression to silence the false-positive warnings by informing the RCU API
> > > > about the fact that locks are held (similar to what we do for
> > > > rcu_dereference_protected()). The RCU API will do additional checking
> > > > (such as making sure preemption is disabled for safe RCU usage etc) as well.
> > > 
> > > Yes, that is what I did in [1/2] for get_kprobe().
> > > Let me clarify the RCU list usage in [2/2].
> > > 
> > > With the careful check, other list traversals never be done in non-sleepable
> > > context, those are always runs with kprobe_mutex held.
> > > If I correctly understand the Documentation/RCU/listRCU.rst, we should/can use
> > > non-RCU api for those cases, or do I miss something?
> > 
> > Yes, that is fine. However personally I prefer not to mix usage of
> > list_for_each_entry_rcu() and list_for_each_entry() on the same pointer
> > (kprobe_table). I think it is more confusing and error prone. Just use
> > list_for_each_entry_rcu() everywhere and pass the appropriate lockdep
> > expression, instead of calling lockdep_assert_held() independently. Is this
> > not doable?
> 
> Hmm, but isn't it more confusing that user just take a mutex but
> no rcu_read_lock() with list_for_each_entry_rcu()? In that case,
> sometimes it might sleep inside list_for_each_entry_rcu(), I thought
> that might be more confusing mind model for users...

I meant, do we always need to do something like below?

{
	mutex_lock(&lock);
	list_for_each_entry_rcu(list, ..., lockdep_is_held(&lock)) {
		...
	}
	mutex_unlock(&lock);
}

BTW, I found another problem on this policy, since we don't have
list_for_each_*_safe() equivalents for RCU, we can not do a safe
loop on it. Should we call a find function for each time?

Thank you, 

-- 
Masami Hiramatsu <mhiramat@...nel.org>