From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Masami Hiramatsu <mhiramat@kernel.org>,
Anders Roxell <anders.roxell@linaro.org>,
"Naveen N . Rao" <naveen.n.rao@linux.ibm.com>,
Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
David Miller <davem@davemloft.net>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe
Date: Thu, 5 Dec 2019 19:11:51 -0800 [thread overview]
Message-ID: <20191206031151.GY2889@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20191206011137.GB142442@google.com>
On Thu, Dec 05, 2019 at 08:11:37PM -0500, Joel Fernandes wrote:
> On Wed, Dec 04, 2019 at 08:12:39AM -0800, Paul E. McKenney wrote:
> > On Wed, Dec 04, 2019 at 11:05:50AM +0100, Ingo Molnar wrote:
> > >
> > > * Paul E. McKenney <paulmck@kernel.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?
> >
> > Potentially yes, if the READ_ONCE() constrains the compiler. Or not,
> > depending of course on the compiler and the surrounding code.
> >
> > > Also, the principle of passing warning flags around is problematic - but
> > > I can see the point in this specific case.
> >
> > Would it help to add an hlist_for_each_entry_protected() that expected
> > RCU-protected pointers and write-side protection, analogous to
> > rcu_dereference_protected()? Or would that expansion of the RCU API
> > outweigh any benefits?
>
> Personally, I like keeping the same API and using the optional argument like
> we did thus preventing too many APIs / new APIs.
Would you be willing to put together a prototype patch so that people
can see exactly how it would look?
Thanx, Paul
next prev parent reply other threads:[~2019-12-06 3:11 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-02 7:32 [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe Masami Hiramatsu
2019-12-02 15:17 ` Anders Roxell
2019-12-02 21:08 ` Joel Fernandes
2019-12-02 22:34 ` Masami Hiramatsu
2019-12-02 23:35 ` Joel Fernandes
2019-12-03 6:02 ` Masami Hiramatsu
2019-12-03 7:13 ` Ingo Molnar
2019-12-03 17:57 ` Paul E. McKenney
2019-12-04 10:05 ` Ingo Molnar
2019-12-04 16:12 ` Paul E. McKenney
2019-12-05 4:19 ` Masami Hiramatsu
2019-12-06 1:11 ` Joel Fernandes
2019-12-06 3:11 ` Paul E. McKenney [this message]
2019-12-08 0:08 ` Joel Fernandes
2019-12-09 3:39 ` Paul E. McKenney
2019-12-17 14:59 ` Masami Hiramatsu
2019-12-17 18:07 ` Joel Fernandes
2019-12-04 4:09 ` Joel Fernandes
2019-12-04 4:20 ` Joel Fernandes
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