Re: [dpdk-dev] [PATCH v2] Change alarm cancel function to thread-safe:

[Neil Horman <nhorman@tuxdriver.com>]

On Thu, Sep 25, 2014 at 01:56:08PM +0100, Michal Jastrzebski wrote:
>     Change alarm cancel function to thread-safe.
>     It eliminates a race between threads using rte_alarm_cancel and
>     rte_alarm_set.
> 
> Signed-off-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
> Reviewed-by: Michal Jastrzebski <michalx.k.jastrzebski@intel.com>
> 
> ---
>  lib/librte_eal/common/include/rte_alarm.h |    3 +-
>  lib/librte_eal/linuxapp/eal/eal_alarm.c   |   68 ++++++++++++++++++-----------
>  2 files changed, 45 insertions(+), 26 deletions(-)
> 

> diff --git a/lib/librte_eal/common/include/rte_alarm.h b/lib/librte_eal/common/include/rte_alarm.h
> index d451522..e7cbaef 100644
> --- a/lib/librte_eal/common/include/rte_alarm.h
> +++ b/lib/librte_eal/common/include/rte_alarm.h
> @@ -76,7 +76,8 @@ typedef void (*rte_eal_alarm_callback)(void *arg);
>  int rte_eal_alarm_set(uint64_t us, rte_eal_alarm_callback cb, void *cb_arg);
>  
>  /**
> - * Function to cancel an alarm callback which has been registered before.
> + * Function to cancel an alarm callback which has been registered before. If
> + * used outside alarm callback it wait for all callbacks to finish its execution.
>   *
>   * @param cb_fn
>   *  alarm callback
> diff --git a/lib/librte_eal/linuxapp/eal/eal_alarm.c b/lib/librte_eal/linuxapp/eal/eal_alarm.c
> index 480f0cb..ea8dfb4 100644
> --- a/lib/librte_eal/linuxapp/eal/eal_alarm.c
> +++ b/lib/librte_eal/linuxapp/eal/eal_alarm.c
> @@ -69,7 +69,8 @@ struct alarm_entry {
>  	struct timeval time;
>  	rte_eal_alarm_callback cb_fn;
>  	void *cb_arg;
> -	volatile int executing;
> +	volatile uint8_t executing;
> +	volatile pthread_t executing_id;
>  };
>  
>  static LIST_HEAD(alarm_list, alarm_entry) alarm_list = LIST_HEAD_INITIALIZER();
> @@ -108,11 +109,13 @@ eal_alarm_callback(struct rte_intr_handle *hdl __rte_unused,
>  			(ap->time.tv_sec < now.tv_sec || (ap->time.tv_sec == now.tv_sec &&
>  						ap->time.tv_usec <= now.tv_usec))){
>  		ap->executing = 1;
> +		ap->executing_id = pthread_self();
How exactly does this work?  From my read all alarm callbacks are handled by the
thread created in rte_eal_intr_init (which runs forever in
eal_intr_thread_main()).  So every assignment to the above executing_id value
will be from that thread.  As such, anytime rte_eal_alarm_cancel is called from
within a callback we are guaranteed that:
a) the ap->executing flag is set to 1
b) the ap->executing_id value should equal whatever is returned from
pthread_self()

That will cause the executing counter local to the cancel function to get
incremented, meaning we will deadlock withing that do { ... } while (executing
!= 0) loop, no?

>  		rte_spinlock_unlock(&alarm_list_lk);
>  
>  		ap->cb_fn(ap->cb_arg);
>  
>  		rte_spinlock_lock(&alarm_list_lk);
> +
>  		LIST_REMOVE(ap, next);
>  		rte_free(ap);
>  	}
> @@ -145,7 +148,7 @@ rte_eal_alarm_set(uint64_t us, rte_eal_alarm_callback cb_fn, void *cb_arg)
>  	if (us < 1 || us > (UINT64_MAX - US_PER_S) || cb_fn == NULL)
>  		return -EINVAL;
>  
> -	new_alarm = rte_malloc(NULL, sizeof(*new_alarm), 0);
> +	new_alarm = rte_zmalloc(NULL, sizeof(*new_alarm), 0);
>  	if (new_alarm == NULL)
>  		return -ENOMEM;
>  
> @@ -156,7 +159,6 @@ rte_eal_alarm_set(uint64_t us, rte_eal_alarm_callback cb_fn, void *cb_arg)
>  	new_alarm->cb_arg = cb_arg;
>  	new_alarm->time.tv_usec = (now.tv_usec + us) % US_PER_S;
>  	new_alarm->time.tv_sec = now.tv_sec + ((now.tv_usec + us) / US_PER_S);
> -	new_alarm->executing = 0;
>  
This removes the only place where ->executing is cleared again.  If there is
only one change to this bits state (which is the case after this patch), it
seems that you can just use the executing bit as the test in the alarm_cancel
function, and remove all the pthread_self mess.  We still need to address the
deadlock question, but it seems using this single flag is easier than using
pthread_self.

Neil