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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.07.14 at 18:29, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 07/28/2014 11:22 AM, Jan Beulich wrote:
>>>>> On 01.07.14 at 16:37, <boris.ostrovsky@xxxxxxxxxx> 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.

>>> +long do_xenpmu_op(int op, XEN_GUEST_HANDLE_PARAM(xen_pmu_params_t) arg)
>>> +{
>>> +    int ret = -EINVAL;
>>> +    xen_pmu_params_t pmu_params;
>>> +
>>> +    switch ( op )
>>> +    {
>>> +    case XENPMU_mode_set:
>>> +    {
>>> +        static DEFINE_SPINLOCK(xenpmu_mode_lock);
>>> +        uint32_t current_mode;
>>> +
>>> +        if ( !is_control_domain(current->domain) )
>>> +            return -EPERM;
>>> +
>>> +        if ( copy_from_guest(&pmu_params, arg, 1) )
>>> +            return -EFAULT;
>>> +
>>> +        if ( pmu_params.val & ~XENPMU_MODE_ON )
>>> +            return -EINVAL;
>>> +
>>> +        if ( !spin_trylock(&xenpmu_mode_lock) )
>>> +            return -EAGAIN;
>> Wouldn't it be better for this to also set a continuation, rather than
>> having the caller do the retry?
> 
> I actually want the caller (who is most likely the administrator doing 
> 'echo off > /sys/hypervisor/pmu/pmu_mode') see the error since this 
> indicates two people trying to change system-wise settings at the same time.

Then please state this in a code comment, or someone (like me)
might end up "cleaning" this up.

Jan


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