linux-kernel - Re: [PATCH] RCU: implement rcu_read_[un]lock

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Message-Id: <1216188469.5232.0.camel@twins>
Date:	Wed, 16 Jul 2008 08:07:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul E McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] RCU: implement rcu_read_[un]lock_preempt()

On Mon, 2008-07-14 at 14:57 +0900, Tejun Heo wrote:
> With the introduction of preemptible RCU, RCU doesn't gurantee that
> its critical section runs on the CPU it started to run.  As there are
> cases where non-preemptible RCU critical section makes sense, create
> new RCU read lock variants which turns of preemption -
> rcu_read_[un]lock_preempt() which are identical to rcu_read_[un]lock()
> for classic implementation and have enclosing preempt disable/enable
> for preemptible RCU.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>

Sorry, NAK.

If you need preempt off you need it for other reasons than RCU, so
mixing it in the interface doesn't make sense to me.

> ---
> This will be used by following block layer updates.  If this and the
> block changes get acked, it'll be best to push this through block
> tree.
> 
> Thanks.
> 
>  include/linux/rcuclassic.h |    2 ++
>  include/linux/rcupdate.h   |   18 ++++++++++++++++++
>  include/linux/rcupreempt.h |    6 ++++--
>  3 files changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
> index b3aa05b..08c6153 100644
> --- a/include/linux/rcuclassic.h
> +++ b/include/linux/rcuclassic.h
> @@ -136,6 +136,8 @@ extern struct lockdep_map rcu_lock_map;
>  		__release(RCU); \
>  		preempt_enable(); \
>  	} while (0)
> +#define __rcu_read_lock_preempt()	__rcu_read_lock()
> +#define __rcu_read_unlock_preempt()	__rcu_read_unlock()
>  #define __rcu_read_lock_bh() \
>  	do { \
>  		local_bh_disable(); \
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index d42dbec..e0e3486 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -112,6 +112,24 @@ struct rcu_head {
>  #define rcu_read_unlock() __rcu_read_unlock()
>  
>  /**
> + * rcu_read_lock_preempt - mark the beginning of non-preemptible RCU
> + *			   critical section
> + *
> + * Identical to rcu_read_lock() but the critical section is guaranteed
> + * to be non-preemptible.  Note that this is identical to
> + * rcu_read_lock() on classic RCU implementation.
> + */
> +#define rcu_read_lock_preempt() __rcu_read_lock_preempt()
> +
> +/**
> + * rcu_read_unlock_preempt - mark the end of of non-preemptible RCU
> + *			     critical section
> + *
> + * See rcu_read_lock_preempt() for more information.
> + */
> +#define rcu_read_unlock_preempt() __rcu_read_unlock_preempt()
> +
> +/**
>   * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section
>   *
>   * This is equivalent of rcu_read_lock(), but to be used when updates
> diff --git a/include/linux/rcupreempt.h b/include/linux/rcupreempt.h
> index 8a05c7e..b263ceb 100644
> --- a/include/linux/rcupreempt.h
> +++ b/include/linux/rcupreempt.h
> @@ -49,8 +49,10 @@ extern void __rcu_read_unlock(void)	__releases(RCU);
>  extern int rcu_pending(int cpu);
>  extern int rcu_needs_cpu(int cpu);
>  
> -#define __rcu_read_lock_bh()	{ rcu_read_lock(); local_bh_disable(); }
> -#define __rcu_read_unlock_bh()	{ local_bh_enable(); rcu_read_unlock(); }
> +#define __rcu_read_lock_preempt()	{ rcu_read_lock(); preempt_disable(); }
> +#define __rcu_read_unlock_preempt()	{ preempt_enable(); rcu_read_unlock(); }
> +#define __rcu_read_lock_bh()		{ rcu_read_lock(); local_bh_disable(); }
> +#define __rcu_read_unlock_bh()		{ local_bh_enable(); rcu_read_unlock(); }
>  
>  extern void __synchronize_sched(void);
>  

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