Re: [PATCH v8 10/19] x86/VPMU: Interface for setting PMU mode and flags

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 07/29/2014 10:31 AM, Boris Ostrovsky wrote:
> On 07/29/2014 02:19 AM, Jan Beulich wrote:
>>>>> On 28.07.14 at 19:13, <boris.ostrovsky@oracle.com> wrote:
>>> On 07/28/2014 12:36 PM, Jan Beulich wrote:
>>>>>>> On 28.07.14 at 18:29, <boris.ostrovsky@oracle.com> wrote:
>>>>> On 07/28/2014 11:22 AM, Jan Beulich wrote:
>>>>>>>>> On 01.07.14 at 16:37, <boris.ostrovsky@oracle.com> wrote:
>>>>>>> +    start = NOW();
>>>>>>> +    /*
>>>>>>> +     * Note that we may fail here if a CPU is hot-unplugged 
>>>>>>> while we are
>>>>>>> +     * waiting. We will then time out.
>>>>>>> +     */
>>>>>>> +    while ( atomic_read(&vpmu_sched_counter) != allbutself_num )
>>>>>>> +    {
>>>>>>> +        /* Give up after 5 seconds */
>>>>>>> +        if ( NOW() > start + SECONDS(5) )
>>>>>>> +        {
>>>>>>> +            printk("vpmu_unload_all: failed to sync\n");
>>>>>>> +            ret = -EBUSY;
>>>>>>> +            break;
>>>>>>> +        }
>>>>>>> +        cpu_relax();
>>>>>>> +        if ( hypercall_preempt_check() )
>>>>>>> +            return hypercall_create_continuation(
>>>>>>> +                __HYPERVISOR_xenpmu_op, "ih", XENPMU_mode_set, 
>>>>>>> arg);
>>>>>>> +    }
>>>>>> I wonder whether this is race free (wrt another CPU doing something
>>>>>> similar) and how you expect the 5s timeout above to ever be reached
>>>>>> (you're virtually guaranteed to get asked to preempt earlier).
>>>>> Race-wise there is xenpmu_mode_lock in the caller (quoted below).
>>>> That wasn't my point: I said "something similar" - imagine another
>>>> hypercall behaving this same way, and both hypercalls getting
>>>> run concurrently.
>>> Isn't it already possible to have two hypercalls doing continuations at
>>> the same time? (Assuming this was your concern)
>> Yes, it is, and no, this wasn't my concern. My concern is the waiting
>> for all CPUs by the code you add.
>
> Oh, I see.
>
> After I rework timeout tracking one of the competing hypercalls (this 
> one, for example), will error out after 5 seconds.
>

This sentence cannot be parsed.

After timeout tracking is fixed we should error out after 5 seconds 
regardless of what the other hypercall is doing.

-boris