Do two parity flip in the writer to fix race condition

Do two parity flip in the writer to fix race condition

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@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>