Re: [Xen-devel] [PATCH v6 1/4] xen/rcu: don't use stop_machine_run() for rcu_barrier()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.03.2020 13:06, Jürgen Groß wrote:
> On 17.03.20 14:56, Jan Beulich wrote:
>> On 13.03.2020 14:06, Juergen Gross wrote:
>>> +    cpumask_raise_softirq(&cpu_online_map, RCU_SOFTIRQ);
>>
>> Isn't there another barrier needed ahead of this, to order it wrt
>> the set?
> 
> No, I don't think so. cpumask_raise_softirq() needs to have appropriate
> ordering semantics as otherwise the softirq pending bit wouldn't be
> guaranteed to be seen by softirq processing.

You may have a point here, but I had given my comment after
looking at cpumask_raise_softirq() and not finding any such
barrier there. Oh, actually - set_bit() and test_and_set_bit()
differ in their barrier characteristics; I wasn't aware of
this.

Jan

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