RE: [PATCH] vdpa/sfc: resolve race between libvhost and dev_conf

["Srivastava, Vijay" <vijay.srivastava@amd.com>]

>-----Original Message-----
>From: David Marchand <david.marchand@redhat.com>
>Sent: Friday, July 8, 2022 1:43 PM
>To: abhimanyu.saini@xilinx.com; vsrivast@xilinx.com
>Cc: Maxime Coquelin <maxime.coquelin@redhat.com>; dev <dev@dpdk.org>;
>Xia, Chenbo <chenbo.xia@intel.com>; Andrew Rybchenko
><andrew.rybchenko@oktetlabs.ru>; Saini, Abhimanyu
><abhimanyu.saini@amd.com>
>Subject: Re: [PATCH] vdpa/sfc: resolve race between libvhost and dev_conf
>
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>
>Hello Abhimanyu, Vijay,
>
>On Thu, Jul 7, 2022 at 2:38 PM Maxime Coquelin
><maxime.coquelin@redhat.com> wrote:
>> On 7/6/22 11:24, abhimanyu.saini@xilinx.com wrote:
>> > From: Abhimanyu Saini <absaini@amd.com>
>> >
>> > libvhost calls dev_conf() before prosessing the
>> > VHOST_USER_SET_VRING_CALL message for the last VQ. So this message
>> > is processed after dev_conf() returns.
>> >
>> > However, the dev_conf() function spawns a thread to set
>> > rte_vhost_host_notifier_ctrl() before returning control to libvhost.
>> > This parallel thread in turn invokes get_notify_area().
>> > To get the notify_area, the vdpa driver needs to query the HW and
>> > for this query it needs an enabled VQ.
>> >
>> > But at the same time libvhost is processing the last
>> > VHOST_USER_SET_VRING_CALL, and to do that it disables the last VQ.
>> >
>> > Hence there is a race b/w the libvhost and the vdpa driver.
>> >
>> > To resolve this race condition, query the HW and cache notify_area
>> > inside dev_conf() instead of doing it the parallel thread.
>> >
>> > Signed-off-by: Abhimanyu Saini <absaini@amd.com>
>> > ---
>> >   drivers/vdpa/sfc/sfc_vdpa_ops.c | 36 ++++++++++++++++++------------------
>> >   drivers/vdpa/sfc/sfc_vdpa_ops.h |  1 +
>> >   2 files changed, 19 insertions(+), 18 deletions(-)
>> >
>>
>> During today's Release status meeting, Andrew mentioned that this
>> patch has been for a log time already in your internal tree.
>>
>> So it gives a bit of confidence in taking it in -rc4.
>
>- But it is neither reviewed, nor acked by the driver maintainer.
>
>Vijay, as this driver maintainer, your opinion matters.
>We are in rc4 stage and we merge only critical fixes now.
>There won't be much time to test this fix once merged (and I am not talking
>about fixing a regression).
>
>Are you confident with this fix? 
Yes. 

>is it required for the 22.07 release?
It is not a blocker issue, but it would be good to have in this release. 

>If we don't get an answer, the safer is to let those fixes slip to a next release.
>
>
>- Besides, I see there is a new fix for some sfc driver.
>https://patches.dpdk.org/project/dpdk/patch/20220708073702.29391-1-
>asaini@xilinx.com/
>The same questions will be asked.
>
>
>--
>David Marchand

Acked-by: Vijay Srivastava <vijays@amd.com>