[lttng-dev] Horrible hack to make C++ link to liburcu

[Mathieu Desnoyers]

----- On Mar 4, 2016, at 3:32 PM, Paul E. McKenney paulmck at linux.vnet.ibm.com wrote:

> Hello!
> 
> Still working with the C++ standards committee to add RCU to C++, and
> of course one step on that path is to make C++ programs build against
> liburcu.  The following hack-patch makes this work by disabling C++ access
> to wfcqueue and by making C++ see the rcu_head ->next field as a void *.
> 
> This supports prototyping, but what would be the real fix?

One fix would be:

diff --git a/urcu/wfcqueue.h b/urcu/wfcqueue.h
index 83ec219..c0bb289 100644
--- a/urcu/wfcqueue.h
+++ b/urcu/wfcqueue.h
@@ -43,7 +43,7 @@ extern "C" {
  * McKenney.
  */
 
-#define CDS_WFCQ_WOULDBLOCK    ((void *) -1UL)
+#define CDS_WFCQ_WOULDBLOCK    ((struct cds_wfcq_node *) -1UL)
 
 enum cds_wfcq_ret {
        CDS_WFCQ_RET_WOULDBLOCK =       -1,

In addition, g++ does not support the __transparent_union__ gcc attribute
(it just ignores it). So we might want to rethink our use of this attribute
within wfcqueue.h. We current use it for

/*
 * Do not put head and tail on the same cache-line if concurrent
 * enqueue/dequeue are expected from many CPUs. This eliminates
 * false-sharing between enqueue and dequeue.
 */
struct __cds_wfcq_head {
        struct cds_wfcq_node node;
};

struct cds_wfcq_head {
        struct cds_wfcq_node node;
        pthread_mutex_t lock;
};

/*
 * The transparent union allows calling functions that work on both
 * struct cds_wfcq_head and struct __cds_wfcq_head on any of those two
 * types.
 */
typedef union {
        struct __cds_wfcq_head *_h;
        struct cds_wfcq_head *h;
} __attribute__((__transparent_union__)) cds_wfcq_head_ptr_t;

Thoughts ?

Thanks,

Mathieu


> 
>							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/urcu-call-rcu.h b/urcu-call-rcu.h
> index 339ebacc3f7c..d37c49e9d958 100644
> --- a/urcu-call-rcu.h
> +++ b/urcu-call-rcu.h
> @@ -32,7 +32,9 @@
> #include <stdlib.h>
> #include <pthread.h>
> 
> +#ifndef __cplusplus
> #include <urcu/wfcqueue.h>
> +#endif
> 
> #ifdef __cplusplus
> extern "C" {
> @@ -57,7 +59,11 @@ struct call_rcu_data;
>  */
> 
> struct rcu_head {
> +#ifdef __cplusplus
> +	void *next;
> +#else
> 	struct cds_wfcq_node next;
> +#endif
> 	void (*func)(struct rcu_head *head);
>  };

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