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From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: linux-kernel@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Borislav Petkov <bp@alien8.de>,
	c0d1n61at3@gmail.com, "David S. Miller" <davem@davemloft.net>,
	edumazet@google.com,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Josh Triplett <josh@joshtriplett.org>,
	keescook@chromium.org, kernel-hardening@lists.openwall.com,
	kernel-team@android.com, Lai Jiangshan <jiangshanlai@gmail.com>,
	Len Brown <lenb@kernel.org>,
	linux-acpi@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-pci@vger.kernel.org, linux-pm@vger.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	neilb@suse.com, netdev@vger.kernel.org,
	Pavel Machek <pavel@ucw.cz>,
	peterz@infradead.org, "Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
	rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
	Tejun Heo <tj@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
	will@kernel.org,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>
Subject: Re: [PATCH v2 3/9] rcu/sync: Remove custom check for reader-section
Date: Fri, 12 Jul 2019 23:01:50 -0400
Message-ID: <20190713030150.GA246587@google.com> (raw)
In-Reply-To: <20190712233206.GZ26519@linux.ibm.com>

On Fri, Jul 12, 2019 at 04:32:06PM -0700, Paul E. McKenney wrote:
> On Fri, Jul 12, 2019 at 05:35:59PM -0400, Joel Fernandes wrote:
> > On Fri, Jul 12, 2019 at 01:00:18PM -0400, Joel Fernandes (Google) wrote:
> > > The rcu/sync code was doing its own check whether we are in a reader
> > > section. With RCU consolidating flavors and the generic helper added in
> > > this series, this is no longer need. We can just use the generic helper
> > > and it results in a nice cleanup.
> > > 
> > > Cc: Oleg Nesterov <oleg@redhat.com>
> > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > 
> > Hi Oleg,
> > Slightly unrelated to the patch,
> > I tried hard to understand this comment below in percpu_down_read() but no dice.
> > 
> > I do understand how rcu sync and percpu rwsem works, however the comment
> > below didn't make much sense to me. For one, there's no readers_fast anymore
> > so I did not follow what readers_fast means. Could the comment be updated to
> > reflect latest changes?
> > Also could you help understand how is a writer not able to change
> > sem->state and count the per-cpu read counters at the same time as the
> > comment tries to say?
> > 
> > 	/*
> > 	 * We are in an RCU-sched read-side critical section, so the writer
> > 	 * cannot both change sem->state from readers_fast and start checking
> > 	 * counters while we are here. So if we see !sem->state, we know that
> > 	 * the writer won't be checking until we're past the preempt_enable()
> > 	 * and that once the synchronize_rcu() is done, the writer will see
> > 	 * anything we did within this RCU-sched read-size critical section.
> > 	 */
> > 
> > Also,
> > I guess we could get rid of all of the gp_ops struct stuff now that since all
> > the callbacks are the same now. I will post that as a follow-up patch to this
> > series.
> 
> Hello, Joel,
> 
> Oleg has a set of patches updating this code that just hit mainline
> this week.  These patches get rid of the code that previously handled
> RCU's multiple flavors.  Or are you looking at current mainline and
> me just missing your point?
> 

Hi Paul,
You are right on point. I have a bad habit of not rebasing my trees. In this
case the feature branch of mine in concern was based on v5.1. Needless to
say, I need to rebase my tree.

Yes, this sync clean up patch does conflict when I rebase, but other patches
rebase just fine.

The 2 options I see are:
1. Let us drop this patch for now and I resend it later.
2. I resend all patches based on Linus's master branch.

thanks,

- Joel


  reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-12 17:00 [PATCH v2 0/9] Harden list_for_each_entry_rcu() and family Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 1/9] rcu/update: Remove useless check for debug_locks Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 2/9] rcu: Add support for consolidated-RCU reader checking Joel Fernandes (Google)
2019-07-16 18:22   ` Paul E. McKenney
2019-07-16 18:35     ` Joel Fernandes
2019-07-16 18:50       ` Paul E. McKenney
2019-07-12 17:00 ` [PATCH v2 3/9] rcu/sync: Remove custom check for reader-section Joel Fernandes (Google)
2019-07-12 21:35   ` Joel Fernandes
2019-07-12 23:32     ` Paul E. McKenney
2019-07-13  3:01       ` Joel Fernandes [this message]
2019-07-13  3:10         ` Joel Fernandes
2019-07-13  8:21           ` Paul E. McKenney
2019-07-13 13:30             ` Joel Fernandes
2019-07-13 14:41               ` Paul E. McKenney
2019-07-13 15:36                 ` Joel Fernandes
2019-07-13 15:50                   ` Paul E. McKenney
2019-07-13 16:13                     ` Joel Fernandes
2019-07-13 21:28                       ` Paul E. McKenney
2019-07-14 18:10                         ` Joel Fernandes
2019-07-14 18:38                           ` Joel Fernandes
2019-07-14 18:50                             ` Paul E. McKenney
2019-07-15  7:26           ` Oleg Nesterov
2019-07-16 18:26   ` Paul E. McKenney
2019-07-16 18:28     ` Paul E. McKenney
2019-07-12 17:00 ` [PATCH v2 4/9] ipv4: add lockdep condition to fix for_each_entry Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 5/9] driver/core: Convert to use built-in RCU list checking Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 6/9] workqueue: Convert for_each_wq to use built-in list check Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 7/9] x86/pci: Pass lockdep condition to pcm_mmcfg_list iterator Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 8/9] acpi: Use built-in RCU list checking for acpi_ioremaps list Joel Fernandes (Google)
2019-07-12 17:00 ` [PATCH v2 9/9] doc: Update documentation about list_for_each_entry_rcu Joel Fernandes (Google)

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