linux-kernel - Re: [RFC git tree] Userspace RCU (urcu) for Linux (repost)

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Message-ID: <20090208001956.GB5282@Krystal>
Date:	Sat, 7 Feb 2009 19:19:56 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	ltt-dev@...ts.casi.polymtl.ca, linux-kernel@...r.kernel.org,
	Robert Wisniewski <bob@...son.ibm.com>
Subject: Re: [RFC git tree] Userspace RCU (urcu) for Linux (repost)

* Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> On Sat, Feb 07, 2009 at 07:10:28AM -0800, Paul E. McKenney wrote:
> > So, how to fix?  Here are some approaches:
> > 
> > o	Make urcu_publish_content() do two parity flips rather than one.
> > 	I use this approach in my rcu_rcpg, rcu_rcpl, and rcu_rcpls
> > 	algorithms in CodeSamples/defer.
> > 
> > o	Use a single free-running counter, in a manner similar to rcu_nest,
> > 	as suggested earlier.  This one is interesting, as I rely on a
> > 	read-side memory barrier to handle the long-preemption case.
> > 	However, if you believe that any thread that waits several minutes
> > 	between executing adjacent instructions must have been preempted
> > 	(which the memory barriers that are required to do a context
> > 	switch), then a compiler barrier suffices.  ;-)
> > 
> > Of course, the probability of seeing this failure during test is quite
> > low, since it is unlikely that thread 0 would run just long enough to
> > execute its signal handler.  However, it could happen.  And if you were
> > to adapt this algorithm for use in a real-time application, then priority
> > boosting could cause this to happen naturally.
> 
> And here is a patch, taking the first approach.  It also exposes a
> synchronize_rcu() API that is used by the existing urcu_publish_content()
> API.  This allows easier handling of structures that are referenced by
> more than one pointer.  And should also allow to be more easily plugged
> into my rcutorture test.  ;-)
> 

Merged, thanks !

Mathieu

> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
> 
>  urcu.c |   39 ++++++++++++++++++++++++++-------------
>  1 file changed, 26 insertions(+), 13 deletions(-)
> 
> diff --git a/urcu.c b/urcu.c
> index e401d8d..1a276ce 100644
> --- a/urcu.c
> +++ b/urcu.c
> @@ -113,13 +113,35 @@ void wait_for_quiescent_state(int parity)
>  	force_mb_all_threads();
>  }
>  
> +static void switch_qparity(void)
> +{
> +	int prev_parity;
> +
> +	/* All threads should read qparity before accessing data structure. */
> +	/* Write ptr before changing the qparity */
> +	force_mb_all_threads();
> +	prev_parity = switch_next_urcu_qparity();
> +
> +	/*
> +	 * Wait for previous parity to be empty of readers.
> +	 */
> +	wait_for_quiescent_state(prev_parity);
> +}
> +
> +void synchronize_rcu(void)
> +{
> +	rcu_write_lock();
> +	switch_qparity();
> +	switch_qparity();
> +	rcu_write_unlock();
> +}
> +
>  /*
>   * Return old pointer, OK to free, no more reference exist.
>   * Called under rcu_write_lock.
>   */
>  void *urcu_publish_content(void **ptr, void *new)
>  {
> -	int prev_parity;
>  	void *oldptr;
>  
>  	/*
> @@ -134,19 +156,10 @@ void *urcu_publish_content(void **ptr, void *new)
>  	 */
>  	oldptr = *ptr;
>  	*ptr = new;
> -	/* All threads should read qparity before ptr */
> -	/* Write ptr before changing the qparity */
> -	force_mb_all_threads();
> -	prev_parity = switch_next_urcu_qparity();
>  
> -	/*
> -	 * Wait for previous parity to be empty of readers.
> -	 */
> -	wait_for_quiescent_state(prev_parity);
> -	/*
> -	 * Deleting old data is ok !
> -	 */
> -	
> +	switch_qparity();
> +	switch_qparity();
> +
>  	return oldptr;
>  }
>  
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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