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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>,
	Fengguang Wu <fengguang.wu@intel.com>, LKP <lkp@01.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [rcu] kernel BUG at include/linux/pagemap.h:149!
Date: Mon, 21 Sep 2015 13:43:27 -0700
Message-ID: <20150921204327.GH4029@linux.vnet.ibm.com> (raw)
In-Reply-To: <20150921193045.GA13674@lerouge>

On Mon, Sep 21, 2015 at 09:30:49PM +0200, Frederic Weisbecker wrote:
> On Fri, Sep 11, 2015 at 10:19:47AM +0800, Boqun Feng wrote:
> > Subject: [PATCH 01/27] rcu: Don't disable preemption for Tiny and Tree RCU
> >  readers
> > 
> > Because preempt_disable() maps to barrier() for non-debug builds,
> > it forces the compiler to spill and reload registers.  Because Tree
> > RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
> > barrier() instances generate needless extra code for each instance of
> > rcu_read_lock() and rcu_read_unlock().  This extra code slows down Tree
> > RCU and bloats Tiny RCU.
> > 
> > This commit therefore removes the preempt_disable() and preempt_enable()
> > from the non-preemptible implementations of __rcu_read_lock() and
> > __rcu_read_unlock(), respectively.
> > 
> > For debug purposes, preempt_disable() and preempt_enable() are still
> > kept if CONFIG_PREEMPT_COUNT=y, which makes the detection of sleeping
> > inside atomic sections still work in non-preemptible kernels.
> > 
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> >  include/linux/rcupdate.h | 6 ++++--
> >  include/linux/rcutiny.h  | 1 +
> >  kernel/rcu/tree.c        | 9 +++++++++
> >  3 files changed, 14 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index d63bb77..6c3cece 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -297,12 +297,14 @@ void synchronize_rcu(void);
> >  
> >  static inline void __rcu_read_lock(void)
> >  {
> > -	preempt_disable();
> > +	if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
> > +		preempt_disable();
> 
> preempt_disable() is a no-op when !CONFIG_PREEMPT_COUNT, right?
> Or rather it's a barrier(), which is anyway implied by rcu_read_lock().
> 
> So perhaps we can get rid of the IS_ENABLED() check?

Actually, barrier() is not intended to be implied by rcu_read_lock().
In a non-preemptible RCU implementation, it doesn't help anything
to have the compiler flush its temporaries upon rcu_read_lock()
and rcu_read_unlock().

						Thanx, Paul


  reply index

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-09-10  0:57 Fengguang Wu
2015-09-10 10:25 ` Boqun Feng
2015-09-10 17:16   ` Paul E. McKenney
2015-09-11  2:19     ` Boqun Feng
     [not found]       ` <CAJzB8QG=1iZW3dQEie6ZSTLv8GZ3YSut0aL1VU7LLmiHQ1B1DQ@mail.gmail.com>
2015-09-11 21:59         ` Paul E. McKenney
2015-09-12  5:46           ` Boqun Feng
2015-09-21 19:30       ` Frederic Weisbecker
2015-09-21 20:43         ` Paul E. McKenney [this message]
2019-06-02  5:56           ` rcu_read_lock lost its compiler barrier Herbert Xu
2019-06-02 20:54             ` Linus Torvalds
2019-06-03  2:46               ` Herbert Xu
2019-06-03  3:47                 ` Paul E. McKenney
2019-06-03  4:01                   ` Herbert Xu
2019-06-03  4:17                     ` Herbert Xu
2019-06-03  7:23                     ` Paul E. McKenney
2019-06-03  8:42                       ` Paul E. McKenney
2019-06-03 15:26                         ` David Laight
2019-06-03 15:40                           ` Linus Torvalds
2019-06-03  5:26                   ` Herbert Xu
2019-06-03  6:42                     ` Boqun Feng
2019-06-03 20:03                       ` Paul E. McKenney
2019-06-04 14:44                         ` Alan Stern
2019-06-04 16:04                           ` Linus Torvalds
2019-06-04 17:00                             ` Alan Stern
2019-06-04 17:29                               ` Linus Torvalds
2019-06-07 14:09                             ` inet: frags: Turn fqdir->dead into an int for old Alphas Herbert Xu
2019-06-07 15:26                               ` Eric Dumazet
2019-06-07 15:32                                 ` Herbert Xu
2019-06-07 16:13                                   ` Eric Dumazet
2019-06-07 16:19                                 ` Linus Torvalds
2019-06-08 15:27                                   ` Paul E. McKenney
2019-06-08 17:42                                     ` Linus Torvalds
2019-06-08 17:50                                       ` Linus Torvalds
2019-06-08 18:50                                         ` Paul E. McKenney
2019-06-08 18:14                                       ` Paul E. McKenney
2019-06-06  4:51                           ` rcu_read_lock lost its compiler barrier Herbert Xu
2019-06-06  6:05                             ` Paul E. McKenney
2019-06-06  6:14                               ` Herbert Xu
2019-06-06  9:06                                 ` Paul E. McKenney
2019-06-06  9:28                                   ` Herbert Xu
2019-06-06 10:58                                     ` Paul E. McKenney
2019-06-06 13:38                                       ` Herbert Xu
2019-06-06 13:48                                         ` Paul E. McKenney
2019-06-06  8:16                           ` Andrea Parri
2019-06-06 14:19                             ` Alan Stern
2019-06-08 15:19                               ` Paul E. McKenney
2019-06-08 15:56                                 ` Alan Stern
2019-06-08 16:31                                   ` Paul E. McKenney
2019-06-03  9:35                     ` Paul E. McKenney
2019-06-06  8:38                 ` Andrea Parri
2019-06-06  9:32                   ` Herbert Xu
2019-06-03  0:06             ` Paul E. McKenney
2019-06-03  3:03               ` Herbert Xu
2019-06-03  9:27                 ` Paul E. McKenney
2019-06-03 15:55                 ` Linus Torvalds
2019-06-03 16:07                   ` Linus Torvalds
2019-06-03 19:53                     ` Paul E. McKenney
2019-06-03 20:24                       ` Linus Torvalds
2019-06-04 21:14                         ` Paul E. McKenney
2019-06-05  2:21                           ` Herbert Xu
2019-06-05  3:30                             ` Paul E. McKenney
2019-06-06  4:37                               ` Herbert Xu

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