linux-kernel - Re: [PATCH 2/9] rcu: Add support for consolidated-RCU reader checking (v3)

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Message-ID: <20190717000757.GQ14271@linux.ibm.com>
Date:   Tue, 16 Jul 2019 17:07:57 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Borislav Petkov <bp@...en8.de>, c0d1n61at3@...il.com,
        "David S. Miller" <davem@...emloft.net>, edumazet@...gle.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>, keescook@...omium.org,
        kernel-hardening@...ts.openwall.com, kernel-team@...roid.com,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-pm@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        neilb@...e.com, netdev@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        peterz@...radead.org, "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, will@...nel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH 2/9] rcu: Add support for consolidated-RCU reader
 checking (v3)

On Tue, Jul 16, 2019 at 06:02:05PM -0400, Joel Fernandes wrote:
> On Tue, Jul 16, 2019 at 11:53:03AM -0700, Paul E. McKenney wrote:
> [snip]
> > > > A few more things below.
> > > > > ---
> > > > >  include/linux/rculist.h  | 28 ++++++++++++++++++++-----
> > > > >  include/linux/rcupdate.h |  7 +++++++
> > > > >  kernel/rcu/Kconfig.debug | 11 ++++++++++
> > > > >  kernel/rcu/update.c      | 44 ++++++++++++++++++++++++----------------
> > > > >  4 files changed, 67 insertions(+), 23 deletions(-)
> > > > > 
> > > > > diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> > > > > index e91ec9ddcd30..1048160625bb 100644
> > > > > --- a/include/linux/rculist.h
> > > > > +++ b/include/linux/rculist.h
> > > > > @@ -40,6 +40,20 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
> > > > >   */
> > > > >  #define list_next_rcu(list)	(*((struct list_head __rcu **)(&(list)->next)))
> > > > >  
> > > > > +/*
> > > > > + * Check during list traversal that we are within an RCU reader
> > > > > + */
> > > > > +
> > > > > +#ifdef CONFIG_PROVE_RCU_LIST
> > > > 
> > > > This new Kconfig option is OK temporarily, but unless there is reason to
> > > > fear malfunction that a few weeks of rcutorture, 0day, and -next won't
> > > > find, it would be better to just use CONFIG_PROVE_RCU.  The overall goal
> > > > is to reduce the number of RCU knobs rather than grow them, must though
> > > > history might lead one to believe otherwise.  :-/
> > > 
> > > If you want, we can try to drop this option and just use PROVE_RCU however I
> > > must say there may be several warnings that need to be fixed in a short
> > > period of time (even a few weeks may be too short) considering the 1000+
> > > uses of RCU lists.
> > Do many people other than me build with CONFIG_PROVE_RCU?  If so, then
> > that would be a good reason for a temporary CONFIG_PROVE_RCU_LIST,
> > as in going away in a release or two once the warnings get fixed.
> 
> PROVE_RCU is enabled by default with PROVE_LOCKING, so it is used quite
> heavilty.
> 
> > > But I don't mind dropping it and it may just accelerate the fixing up of all
> > > callers.
> > 
> > I will let you decide based on the above question.  But if you have
> > CONFIG_PROVE_RCU_LIST, as noted below, it needs to depend on RCU_EXPERT.
> 
> Ok, will make it depend. But yes for temporary purpose, I will leave it as a
> config and remove it later.

Very good, thank you!  Plus you got another ack.  ;-)

							Thanx, Paul