Re: mbufs getting reused despite nonzero refcnt

[Stephen Hemminger <stephen@networkplumber.org>]

On Sun, 10 Nov 2024 11:23:29 -0500
Alan Beadle <ab.beadle@gmail.com> wrote:

> Hi everyone,
> 
> I am using DPDK to send two-way traffic between a pair of machines. My
> application has local readers, remote acknowledgments, as well as
> automatic retries when a packet is lost. For these reasons I am using
> rte_mbuf_refcnt_update() to prevent the NIC from freeing the packet
> and recycling the mbuf before my local readers are done and the remote
> reader has acknowledged the message. I was advised to do this in an
> earlier thread on this mailing list.
> 
> However, this does not seem to be working. After running my app for
> awhile and exchanging about 1000 messages in this way, my queue of
> unacknowledged mbufs is getting corrupted. The mbufs attached to my
> queue seem to contain data for newer messages than what is supposed to
> be in them, and in some cases contains a totally different type of
> packet (an acknack for instance). Obviously this results in retries of
> those messages failing to send the correct data and my application
> gets stuck.
> 
> I have ensured that the refcount is not reaching 0. Each new mbuf
> immediately has the refcnt incremented by 1. I was concerned that
> retries might need the refcnt bumped again, but if I bump the refcount
> every time I resend a specific mbuf to the NIC, the refcounts just
> keep getting higher. So it looks like re-bumping it on a resend is not
> necessary.
> 
> I have ruled out other possible explanations. The mbufs are being
> reused by rte_pktmbuf_alloc. I even tried playing with the EAL
> settings related to the number of mbuf descriptors and saw my changes
> directly correlate with how long it takes this problem to occur. How
> do I really prevent the driver from reusing packets that I still might
> need to resend?
> 
> Thanks in advance,
> -Alan

Which driver, could be a driver bug.

Also, you should be able to trace mbuf functions, either with rte_trace
or by other facility.