RE: [EXTERNAL] [PATCH] net/netvsc: Fix on race condition of multiple commands

[Long Li <longli@microsoft.com>]

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Hi Madhukar,

I suggest caching the result of hn_rndis_query_hwcaps(), as suggested by Stephen. This can be done inside PMD. Do you want me to submit a patch?

For querying link status, the lock should be implemented inside hn_rndis_exec1(). The drawback is that this function can potentially wait for up to 60 seconds on response from host, maybe not suitable for spinlock in production use. But I think it's better to have application retry on BUSY (with some delay logic), as the netvsc is designed in this way since introduced.

Thanks,
Long



From: madhukar mythri <madhukar.mythri@gmail.com>
Sent: Saturday, December 20, 2025 9:37 PM
To: Stephen Hemminger <stephen@networkplumber.org>
Cc: Long Li <longli@microsoft.com>; dev@dpdk.org; Madhuker Mythri <madhuker.mythri@oracle.com>
Subject: Re: [EXTERNAL] [PATCH] net/netvsc: Fix on race condition of multiple commands

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Hi Li and Stephen,

We have a common DPDK application for all the PMD's, in which we are seeing issue for this Netvsc PMD only.
I mean, for KVM hypervisor with Intel or Mellanox NICs we did not see such sync issues. Also, with failsafe PMD on hyper-v did not seen such sync issues.

So, i thought this would be better to fix at PMD level using spinlock.

@Stephen Hemminger<mailto:stephen@networkplumber.org> , yes we can store the device info get details after probe and reuse it later.
For Link-status get with multiple threads we can go with retry mechanism.

However, w.r.t all other PMD's this device info get and Link-status get has issues in multi threaded application.

Regards,
Madhuker.

On Sat, 20 Dec, 2025, 23:55 Stephen Hemminger, <stephen@networkplumber.org<mailto:stephen@networkplumber.org>> wrote:
On Fri, 19 Dec 2025 17:35:33 +0000
Long Li <longli@microsoft.com<mailto:longli@microsoft.com>> wrote:

> > When multiple processes issue command requests(like: device info get and
> > link-status) at same-time, then we could see the command request failures,
> > due to race-condition of common function execution.
>
> Hi Madhuker,
>
> I'm not sure if we should use a lock in the driver for this. It's not clear in DPDK documents but in general the calls to query device status are not thread safe.
>
> Is it possible that the application uses a lock to sync calling to this?
>

I do not know of any restrictions about threads calling query operations.

For info_get() the transaction is in rndis_get_offload().
There are couple of ways to handle this better. One would to do
the query during probe and remember the result. The hypervisor is
not going to change supported offload. The other and simpler way
would be to just have hardcoded offload values. The code for query
got compute offloads is inherited for BSD and unless someone was trying
to run on Windows 2012 or earlier version of Hyper-V it would never change.

Link status is a little more complex. Does the hyper-visor ever report
that the software path is down? And reading through the hn_rdis_exec code
it looks like if multiple operations are in process the second one
should return -EBUSY. Application could retry in that case.

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