From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
George Dunlap <George.Dunlap@eu.citrix.com>,
Ian Jackson <iwj@xenproject.org>, Wei Liu <wl@xen.org>,
Stefano Stabellini <sstabellini@kernel.org>,
Tamas K Lengyel <lengyelt@ainfosec.com>,
Petre Ovidiu PIRCALABU <ppircalabu@bitdefender.com>,
Alexandru Isaila <aisaila@bitdefender.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held
Date: Thu, 3 Dec 2020 11:09:13 +0100 [thread overview]
Message-ID: <7a768bcd-80c1-d193-8796-7fb6720fa22a@suse.com> (raw)
In-Reply-To: <17c90493-b438-fbc1-ca10-3bc4d89c4e5e@xen.org>
On 02.12.2020 22:10, Julien Grall wrote:
> On 23/11/2020 13:30, Jan Beulich wrote:
>> While there don't look to be any problems with this right now, the lock
>> order implications from holding the lock can be very difficult to follow
>> (and may be easy to violate unknowingly). The present callbacks don't
>> (and no such callback should) have any need for the lock to be held.
>>
>> However, vm_event_disable() frees the structures used by respective
>> callbacks and isn't otherwise synchronized with invocations of these
>> callbacks, so maintain a count of in-progress calls, for evtchn_close()
>> to wait to drop to zero before freeing the port (and dropping the lock).
>
> AFAICT, this callback is not the only place where the synchronization is
> missing in the VM event code.
>
> For instance, vm_event_put_request() can also race against
> vm_event_disable().
>
> So shouldn't we handle this issue properly in VM event?
I suppose that's a question to the VM event folks rather than me?
>> ---
>> Should we make this accounting optional, to be requested through a new
>> parameter to alloc_unbound_xen_event_channel(), or derived from other
>> than the default callback being requested?
>
> Aside the VM event, do you see any value for the other caller?
No (albeit I'm not entirely certain about vpl011_notification()'s
needs), hence the consideration. It's unnecessary overhead in
those cases.
>> @@ -781,9 +786,15 @@ int evtchn_send(struct domain *ld, unsig
>> rport = lchn->u.interdomain.remote_port;
>> rchn = evtchn_from_port(rd, rport);
>> if ( consumer_is_xen(rchn) )
>> + {
>> + /* Don't keep holding the lock for the call below. */
>> + atomic_inc(&rchn->u.interdomain.active_calls);
>> + evtchn_read_unlock(lchn);
>> xen_notification_fn(rchn)(rd->vcpu[rchn->notify_vcpu_id], rport);
>> - else
>> - evtchn_port_set_pending(rd, rchn->notify_vcpu_id, rchn);
>
> atomic_dec() doesn't contain any memory barrier, so we will want one
> between xen_notification_fn() and atomic_dec() to avoid re-ordering.
Oh, indeed. But smp_mb() is too heavy handed here - x86 doesn't
really need any barrier, yet would gain a full MFENCE that way.
Actually - looks like I forgot we gained smp_mb__before_atomic()
a little over half a year ago.
Jan
next prev parent reply other threads:[~2020-12-03 10:09 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-23 13:26 [PATCH v3 0/5] evtchn: (not so) recent XSAs follow-on Jan Beulich
2020-11-23 13:28 ` [PATCH v3 1/5] evtchn: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq() Jan Beulich
2020-12-02 19:03 ` Julien Grall
2020-12-03 9:46 ` Jan Beulich
2020-12-09 9:53 ` Julien Grall
2020-12-09 14:24 ` Jan Beulich
2020-11-23 13:28 ` [PATCH v3 2/5] evtchn: avoid access tearing for ->virq_to_evtchn[] accesses Jan Beulich
2020-12-02 21:14 ` Julien Grall
2020-11-23 13:28 ` [PATCH v3 3/5] evtchn: convert vIRQ lock to an r/w one Jan Beulich
2020-12-09 11:16 ` Julien Grall
2020-11-23 13:29 ` [PATCH v3 4/5] evtchn: convert domain event " Jan Beulich
2020-12-09 11:54 ` Julien Grall
2020-12-11 10:32 ` Jan Beulich
2020-12-11 10:57 ` Julien Grall
2020-12-14 9:40 ` Jan Beulich
2020-12-21 17:45 ` Julien Grall
2020-12-22 9:46 ` Jan Beulich
2020-12-23 11:22 ` Julien Grall
2020-12-23 12:57 ` Jan Beulich
2020-12-23 13:19 ` Julien Grall
2020-12-23 13:36 ` Jan Beulich
2020-11-23 13:30 ` [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held Jan Beulich
2020-11-30 10:39 ` Isaila Alexandru
2020-12-02 21:10 ` Julien Grall
2020-12-03 10:09 ` Jan Beulich [this message]
2020-12-03 14:40 ` Tamas K Lengyel
2020-12-04 11:28 ` Julien Grall
2020-12-04 11:48 ` Jan Beulich
2020-12-04 11:51 ` Julien Grall
2020-12-04 12:01 ` Jan Beulich
2020-12-04 15:09 ` Julien Grall
2020-12-07 8:02 ` Jan Beulich
2020-12-07 17:22 ` Julien Grall
2020-12-04 15:21 ` Tamas K Lengyel
2020-12-04 15:29 ` Julien Grall
2020-12-04 19:15 ` Tamas K Lengyel
2020-12-04 19:22 ` Julien Grall
2020-12-04 21:23 ` Tamas K Lengyel
2020-12-07 15:28 ` Jan Beulich
2020-12-07 17:30 ` Julien Grall
2020-12-07 17:35 ` Tamas K Lengyel
2020-12-23 13:12 ` Jan Beulich
2020-12-23 13:33 ` Julien Grall
2020-12-23 13:41 ` Jan Beulich
2020-12-23 14:44 ` Julien Grall
2020-12-23 14:56 ` Jan Beulich
2020-12-23 15:08 ` Julien Grall
2020-12-23 15:15 ` Tamas K Lengyel
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