[dpdk-dev] [PATCH] vhost_user: protect active rings from async ring changes

Maxime Coquelin maxime.coquelin at redhat.com
Thu Dec 7 11:02:16 CET 2017

On 12/07/2017 10:33 AM, Tan, Jianfeng wrote:
>> -----Original Message-----
>> From: Victor Kaplansky [mailto:vkaplans at redhat.com]
>> Sent: Wednesday, December 6, 2017 9:56 PM
>> To: dev at dpdk.org; yliu at fridaylinux.org; Bie, Tiwei; Tan, Jianfeng;
>> vkaplans at redhat.com
>> Cc: stable at dpdk.org; jfreiman at redhat.com; Maxime Coquelin
>> Subject: [PATCH] vhost_user: protect active rings from async ring changes
>> When performing live migration or memory hot-plugging,
>> the changes to the device and vrings made by message handler
>> done independently from vring usage by PMD threads.
>> This causes for example segfauls during live-migration
> segfauls ->segfaults?
>> with MQ enable, but in general virtually any request
>> sent by qemu changing the state of device can cause
>> problems.
>> These patches fixes all above issues by adding a spinlock
>> to every vring and requiring message handler to start operation
>> only after ensuring that all PMD threads related to the divece
> Another typo: divece.
>> are out of critical section accessing the vring data.
>> Each vring has its own lock in order to not create contention
>> between PMD threads of different vrings and to prevent
>> performance degradation by scaling queue pair number.
> Also wonder how much overhead it brings.
> Instead of locking each vring, can we just, waiting a while (10us for example) after call destroy_device() callback so that every PMD thread has enough time to skip out the criterial area?

No, because we are not destroying the device when it is needed.
Actually, once destroy_device() is called, it is likely that the
application has taken care the ring aren't being processed anymore
before returning from the callback (This is at least the case with Vhost 

The reason we need the lock is to protect PMD threads from the handling
of some vhost-user protocol requests.
For example SET_MEM_TABLE in the case of memory hotplug, or SET_LOG_BASE
in case of multiqueue, which is sent for every queue pair and results in
unmapping/remapping the logging area.


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