linux-kernel - Re: [PATCH -tip] kprobes: Lock rcu_read

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Message-ID: <20191204100549.GB114697@gmail.com>
Date:   Wed, 4 Dec 2019 11:05:50 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Anders Roxell <anders.roxell@...aro.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] kprobes: Lock rcu_read_lock() while searching kprobe


* Paul E. McKenney <paulmck@...nel.org> wrote:

> >  * This list-traversal primitive may safely run concurrently with
> >  * the _rcu list-mutation primitives such as hlist_add_head_rcu()
> >  * as long as the traversal is guarded by rcu_read_lock().
> >  */
> > #define hlist_for_each_entry_rcu(pos, head, member, cond...)            \
> > 
> > is actively harmful. Why is it there?
> 
> For cases where common code might be invoked both from the reader
> (with RCU protection) and from the updater (protected by some
> lock).  This common code can then use the optional argument to
> hlist_for_each_entry_rcu() to truthfully tell lockdep that it might be
> called with either form of protection in place.
> 
> This also combines with the __rcu tag used to mark RCU-protected
> pointers, in which case sparse complains when a non-RCU API is applied
> to these pointers, to get back to your earlier question about use of
> hlist_for_each_entry_rcu() within the update-side lock.
> 
> But what are you seeing as actively harmful about all of this?
> What should we be doing instead?

Yeah, so basically in the write-locked path hlist_for_each_entry() 
generates (slightly) more efficient code than hlist_for_each_entry_rcu(), 
correct?

Also, the principle of passing warning flags around is problematic - but 
I can see the point in this specific case.

Thanks,

	Ingo