[lttng-dev] [PATCH liburcu] Fix pthread_atfork() behaviour

[Mathieu Desnoyers]

----- Original Message -----
> From: "Keir Fraser" <keir at cohodata.com>
> To: "Mathieu Desnoyers" <mathieu.desnoyers at efficios.com>
> Cc: lttng-dev at lists.lttng.org, "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
> Sent: Friday, April 18, 2014 1:42:50 PM
> Subject: Re: [lttng-dev] [PATCH liburcu] Fix pthread_atfork() behaviour
> 
[...]
> 
> Worse, while reading the rcu_barrier() code I found a more subtle bug.
> rcu_barrier() can return as soon as completion.barrier_count==0, which
> frees the completion struct along with the rest of its stack frame. But
> its call_rcu callbacks may yet try to read and write completion.futex
> via the wake_up function.

Good catch! I think I've fixed this kind of problematic pattern elsewhere
already, but clearly rcu_barrier() slipped through.

> 
> The completion struct should be calloc()ed, reference-counted, and freed
> by the thread that decs the count to 0, imo. I will be cooking up a
> patch to do just that.

Yes, this should be a clean way to fix it.

> 
> There are other places in liburcu which protect a futex variable from
> premature freeing by the waiter polling a teardown flag, but I think
> refcounting is nicer :)

Makes sense to me,

Thanks!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com