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SubjectRe: [RFC git tree] Userspace RCU (urcu) for Linux (repost)
* Paul E. McKenney (paulmck@linux.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@linux.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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