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

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Date:   Sat, 11 Jan 2020 21:05:37 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     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

On Sat, Jan 11, 2020 at 08:35:07AM +0900, Masami Hiramatsu wrote:
> Hi Joel and Paul,
> 
> On Fri, 10 Jan 2020 16:14:38 -0500
> Joel Fernandes <joel@...lfernandes.org> wrote:
> 
> > On Tue, Jan 07, 2020 at 09:15:35PM +0900, Masami Hiramatsu wrote:
> > > Hello,
> > > 
> > > Anyone have any comment on this series?
> > > Without this series, I still see the suspicious RCU warning for kprobe on -tip tree.
> > 
> > +Paul since RCU.
> > 
> > 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?

thanks,

 - Joel

> Thank you,
> 
> -- 
> Masami Hiramatsu <mhiramat@...nel.org>