[ltt-dev] [UST PATCH] libust: New transport mutex

[Mathieu Desnoyers]

* Nils Carlson (nils.carlson at ericsson.com) wrote:
> Make the locking around the transport list more intelligent, give
> it its own mutex.
> 
> Signed-off-by: Nils Carlson <nils.carlson at ericsson.com>

Hi Nils,

When adding this kind of new mutex, can you:

a) explain why it is needed (or what the advantage is)
b) comment, beside the mutex, what the nesting of the mutex with
   respect to other is.

In the past, I went for a very fine-grained locking scheme in the LTTng
control internals, only to find out that it complexified the code
needlessly and added race conditions here and there. At that point, I
went back to the big hammer (lock_traces()).

As long as adding more mutexes does not change anything in terms of
end-user scalability experience, I don't see any reason to add more.

Thanks,

Mathieu

> ---
>  libust/tracer.c |   13 ++++++++-----
>  1 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/libust/tracer.c b/libust/tracer.c
> index 100dec0..ceb5b74 100644
> --- a/libust/tracer.c
> +++ b/libust/tracer.c
> @@ -182,7 +182,7 @@ static enum ltt_channels get_channel_type_from_name(const char *name)
>  //ust// }
>  
>  static CDS_LIST_HEAD(ltt_transport_list);
> -
> +static DEFINE_MUTEX(ltt_transport_mutex);
>  /**
>   * ltt_transport_register - LTT transport registration
>   * @transport: transport structure
> @@ -204,9 +204,9 @@ void ltt_transport_register(struct ltt_transport *transport)
>  	 */
>  //ust//	vmalloc_sync_all();
>  
> -	ltt_lock_traces();
> +	pthread_mutex_lock(&ltt_transport_mutex);
>  	cds_list_add_tail(&transport->node, &ltt_transport_list);
> -	ltt_unlock_traces();
> +	pthread_mutex_unlock(&ltt_transport_mutex);
>  }
>  
>  /**
> @@ -215,9 +215,9 @@ void ltt_transport_register(struct ltt_transport *transport)
>   */
>  void ltt_transport_unregister(struct ltt_transport *transport)
>  {
> -	ltt_lock_traces();
> +	pthread_mutex_lock(&ltt_transport_mutex);
>  	cds_list_del(&transport->node);
> -	ltt_unlock_traces();
> +	pthread_mutex_unlock(&ltt_transport_mutex);
>  }
>  
>  static inline int is_channel_overwrite(enum ltt_channels chan,
> @@ -458,12 +458,15 @@ int ltt_trace_set_type(const char *trace_name, const char *trace_type)
>  		goto traces_error;
>  	}
>  
> +	pthread_mutex_lock(&ltt_transport_mutex);
>  	cds_list_for_each_entry(tran_iter, &ltt_transport_list, node) {
>  		if (!strcmp(tran_iter->name, trace_type)) {
>  			transport = tran_iter;
>  			break;
>  		}
>  	}
> +	pthread_mutex_unlock(&ltt_transport_mutex);
> +
>  	if (!transport) {
>  		ERR("Transport %s is not present", trace_type);
>  		err = -EINVAL;
> -- 
> 1.7.1
> 
> 
> _______________________________________________
> ltt-dev mailing list
> ltt-dev at lists.casi.polymtl.ca
> http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com