From mboxrd@z Thu Jan  1 00:00:00 1970
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v8 10/19] x86/VPMU: Interface for setting
 PMU mode and flags
Date: Mon, 28 Jul 2014 13:13:57 -0400
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To: Jan Beulich <JBeulich@suse.com>
Cc: kevin.tian@intel.com, keir@xen.org, suravee.suthikulpanit@amd.com, andrew.cooper3@citrix.com, tim@xen.org, dietmar.hahn@ts.fujitsu.com, xen-devel@lists.xen.org, jun.nakajima@intel.com
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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)

-boris