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
> 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(a)gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck(a)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?
Hi Fengguang / LKP-folk,
Quick question -- how easy is it to add extra builds/tests/checks to
your marvellous 0-day kbuild system?
The reason I ask is that I've recently been exploring syzkaller ,
which is a system call fuzzer written by some of my colleagues here at
Google (cc'ed). Although it's fairly new, it has uncovered a bunch of
kernel bugs already  so I wondered if it might be a good candidate
for inclusion in the 0-day checks at some point.
(As an aside, I'm in the process of writing an article about syzkaller
for LWN, which might also expose it to more folk.)
What do you think?