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Message-ID: <20090207221627.GA26162@linux.vnet.ibm.com>
Date: Sat, 7 Feb 2009 14:16:27 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Mathieu Desnoyers <compudj@...stal.dyndns.org>
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)
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. ;-)
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;
}
--
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