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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/28/2014 12:36 PM, Jan Beulich wrote:
> > > > 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.
Isn't it already possible to have two hypercalls doing continuations at 
the same time? (Assuming this was your concern)
-boris

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