Re: [Xen-devel] [PATCH] xen: sched: avoid races on time values read from NOW()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.05.16 at 10:11, <dario.faggioli@xxxxxxxxxx> wrote:
> --- a/xen/common/sched_rt.c
> +++ b/xen/common/sched_rt.c
> @@ -1198,7 +1198,7 @@ static void
>  rt_vcpu_wake(const struct scheduler *ops, struct vcpu *vc)
>  {
>      struct rt_vcpu * const svc = rt_vcpu(vc);
> -    s_time_t now = NOW();
> +    s_time_t now;
>      bool_t missed;
>  
>      BUG_ON( is_idle_vcpu(vc) );
> @@ -1225,6 +1225,7 @@ rt_vcpu_wake(const struct scheduler *ops, struct vcpu 
> *vc)
>       * If a deadline passed while svc was asleep/blocked, we need new
>       * scheduling parameters (a new deadline and full budget).
>       */
> +    now = NOW();
>  
>      missed = ( now >= svc->cur_deadline );
>      if ( missed )
> @@ -1394,7 +1395,7 @@ rt_dom_cntl(
>   * from the replq and does the actual replenishment.
>   */
>  static void repl_timer_handler(void *data){
> -    s_time_t now = NOW();
> +    s_time_t now;
>      struct scheduler *ops = data;
>      struct rt_private *prv = rt_priv(ops);
>      struct list_head *replq = rt_replq(ops);
> @@ -1406,6 +1407,8 @@ static void repl_timer_handler(void *data){
>  
>      spin_lock_irq(&prv->lock);
>  
> +    now = NOW();
> +
>      /*
>       * Do the replenishment and move replenished vcpus
>       * to the temporary list to tickle.

While backporting this for 4.6 (which required substantial
adjustment to the sched_rt.c part) I noticed that there's another
of the cases mentioned in the description in rt_vcpu_insert(). The
commit message doesn't mention why this was left unchanged, so
was not fixing this perhaps just an oversight?

Jan


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