Re: [Xen-devel] [PATCH v2 0/4] xen/rcu: let rcu work better with core scheduling

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 28/02/2020 07:10, Jürgen Groß wrote:
> 
> I think you are just narrowing the window of the race:
> 
> It is still possible to have two cpus entering rcu_barrier() and to
> make it into the if ( !initial ) clause.
> 
> Instead of introducing another atomic I believe the following patch
> instead of yours should do it:
> 
> diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
> index e6add0b120..0d5469a326 100644
> --- a/xen/common/rcupdate.c
> +++ b/xen/common/rcupdate.c
> @@ -180,23 +180,17 @@ static void rcu_barrier_action(void)
> 
>  void rcu_barrier(void)
>  {
> -    int initial = atomic_read(&cpu_count);
> -
>      while ( !get_cpu_maps() )
>      {
>          process_pending_softirqs();
> -        if ( initial && !atomic_read(&cpu_count) )
> +        if ( !atomic_read(&cpu_count) )
>              return;
> 
>          cpu_relax();
> -        initial = atomic_read(&cpu_count);
>      }
> 
> -    if ( !initial )
> -    {
> -        atomic_set(&cpu_count, num_online_cpus());
> +    if ( atomic_cmpxchg(&cpu_count, 0, num_online_cpus()) == 0 )
>          cpumask_raise_softirq(&cpu_online_map, RCU_SOFTIRQ);
> -    }
> 
>      while ( atomic_read(&cpu_count) )
>      {
> 
> Could you give that a try, please?

With this patch I cannot disable SMT at all.

The problem that my diff solved was a race between 2 consecutive
rcu_barrier operations on CPU0 (the pattern specific to SMT-on/off
operation) where some CPUs didn't exit the cpu_count checking loop
completely but cpu_count is already reinitialized on CPU0 - this
results in some CPUs being stuck in the loop.

Igor

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