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
next prev parent 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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