On 02.03.20 14:25, Igor Druzhinin wrote:
> 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.

Ah, okay, then I believe a combination of the two patches is needed.

Something like the attached version?


Juergen