From: George Dunlap <George.Dunlap@eu.citrix.com>
To: "Wu, Feng" <feng.wu@intel.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
"keir@xen.org" <keir@xen.org>,
"andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
"dario.faggioli@citrix.com" <dario.faggioli@citrix.com>,
"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
Jan Beulich <JBeulich@suse.com>
Subject: Re: [PATCH v2 4/4] VMX: fixup PI descritpor when cpu is offline
Date: Wed, 22 Jun 2016 19:33:01 +0100 [thread overview]
Message-ID: <CAFLBxZYQQxku7Wq0oVA=KGw1heYqXvMHdbduYuKwdwKsddK-5g@mail.gmail.com> (raw)
In-Reply-To: <E959C4978C3B6342920538CF579893F019700A6A@SHSMSX103.ccr.corp.intel.com>
On Tue, May 31, 2016 at 11:31 AM, Wu, Feng <feng.wu@intel.com> wrote:
>
>
>> -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@suse.com]
>> Sent: Friday, May 27, 2016 10:57 PM
>> To: Wu, Feng <feng.wu@intel.com>
>> Cc: andrew.cooper3@citrix.com; dario.faggioli@citrix.com;
>> george.dunlap@eu.citrix.com; Tian, Kevin <kevin.tian@intel.com>; xen-
>> devel@lists.xen.org; konrad.wilk@oracle.com; keir@xen.org
>> Subject: Re: [PATCH v2 4/4] VMX: fixup PI descritpor when cpu is offline
>>
>> >>> On 26.05.16 at 15:39, <feng.wu@intel.com> wrote:
>> > @@ -102,9 +103,10 @@ void vmx_pi_per_cpu_init(unsigned int cpu)
>> > {
>> > INIT_LIST_HEAD(&per_cpu(vmx_pi_blocking, cpu).list);
>> > spin_lock_init(&per_cpu(vmx_pi_blocking, cpu).lock);
>> > + per_cpu(vmx_pi_blocking, cpu).down = 0;
>>
>> This seems pointless - per-CPU data starts out all zero (and there
>> are various places already which rely on that).
>>
>> > @@ -122,10 +124,25 @@ static void vmx_vcpu_block(struct vcpu *v)
>> > * new vCPU to the list.
>> > */
>> > spin_unlock_irqrestore(&v->arch.hvm_vmx.pi_hotplug_lock, flags);
>> > - return;
>> > + return 1;
>> > }
>> >
>> > spin_lock(pi_blocking_list_lock);
>> > + if ( unlikely(per_cpu(vmx_pi_blocking, v->processor).down) )
>>
>> Is this something that can actually happen? vmx_pi_desc_fixup()
>> runs in stop-machine context, i.e. no CPU can actively be here (or
>> anywhere near the arch_vcpu_block() call sites).
>
> This is related to scheduler, maybe Dario can give some input about
> this. Dario?
Yeah, I was going to say just looking through this -- there's no way
that vmx_cpu_dead() will be called while there are vcpus *still
running* on that pcpu. vcpus will be migrated to other pcpus before
that happens. All of your changes around vmx_vcpu_block() are
unnecessary.
>> > + new_lock = &per_cpu(vmx_pi_blocking, cpu).lock;
>>
>> DYM new_cpu here? In fact with ...
>>
>> > + spin_lock(new_lock);
>>
>> ... this I can't see how you would have successfully tested this
>> new code path, as I can't see how this would end in other than
>> a deadlock (as you hold this very lock already).
Indeed, it's not very nice to send us complicated code that obviously
can't even run.
-George
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next prev parent reply other threads:[~2016-06-22 18:33 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-26 13:39 [PATCH v2 0/4] VMX: Properly handle pi descriptor and per-cpu blocking list Feng Wu
2016-05-26 13:39 ` [PATCH v2 1/4] VMX: Properly handle pi when all the assigned devices are removed Feng Wu
2016-05-27 13:43 ` Jan Beulich
2016-05-31 10:22 ` Wu, Feng
2016-05-31 11:52 ` Jan Beulich
2016-06-03 5:12 ` Wu, Feng
2016-06-03 9:45 ` Jan Beulich
2016-05-26 13:39 ` [PATCH v2 2/4] VMX: Cleanup PI per-cpu blocking list when vcpu is destroyed Feng Wu
2016-05-27 13:49 ` Jan Beulich
2016-05-31 10:22 ` Wu, Feng
2016-05-31 11:54 ` Jan Beulich
2016-05-26 13:39 ` [PATCH v2 3/4] VMX: Assign the right value to 'NDST' field in a concern case Feng Wu
2016-05-27 14:00 ` Jan Beulich
2016-05-31 10:27 ` Wu, Feng
2016-05-31 11:58 ` Jan Beulich
2016-06-03 5:23 ` Wu, Feng
2016-06-03 9:57 ` Jan Beulich
2016-06-22 18:00 ` George Dunlap
2016-06-24 9:08 ` Wu, Feng
2016-05-26 13:39 ` [PATCH v2 4/4] VMX: fixup PI descritpor when cpu is offline Feng Wu
2016-05-27 14:56 ` Jan Beulich
2016-05-31 10:31 ` Wu, Feng
2016-06-22 18:33 ` George Dunlap [this message]
2016-06-24 6:34 ` Wu, Feng
2016-05-26 17:20 ` [PATCH v2 0/4] VMX: Properly handle pi descriptor and per-cpu blocking list Dario Faggioli
2016-05-31 10:19 ` Wu, Feng
2016-06-22 21:33 ` Dario Faggioli
2016-06-24 6:33 ` Wu, Feng
2016-06-24 10:29 ` Dario Faggioli
2016-06-24 13:42 ` Wu, Feng
2016-06-24 13:49 ` George Dunlap
2016-06-24 14:36 ` Dario Faggioli
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