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From: "Lombardo, Ed" <Ed.Lombardo@netscout.com>
To: Stephen Hemminger <stephen@networkplumber.org>
CC: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>, "users@dpdk.org"
<users@dpdk.org>
Subject: RE: mbuf refcnt issue
Thread-Topic: mbuf refcnt issue
Thread-Index: AdulnN5IfTxeCHcXQsa3xaXr7ECZhQAFBrWAAGffZNAAOS6kYAAzcomAAAGN4hAAGKknAAALaIUg
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Errors-To: users-bounces@dpdk.org
Hi,
I just finished modifying and testing our application to just do transmit o=
f packets received on an NIC interface and let the rte_eth_tx_burst () free=
the mbuf and all works fine for both traffic types. This proves to me tha=
t my implementation of processing the packets and queueing them to tx ring =
and transmit from the tx ring is not buggy, which I had carefully verified =
in gdb early on. I still believe there is a problem with our application w=
ith many threads that can do rte_pktmbuf_free() on the same mbuf. =20
I added these lines in my driver source file:
#define RTE_LITRTE_MBUF_DEBUG 1
#define RTE_LIBRTE_MEMPOOL_DEBUG 1
#define RTE_ENABLE_ASSERT 1
I don't see any asserts occur during my tx packet testing.
The dpdk header files show the Atomic ifdef checks
rte_build_config.h:#define RTE_MBUF_REFCNT_ATOMIC
rte_mbuf_core.h: * or non-atomic) is controlled by =
the RTE_MBUF_REFCNT_ATOMIC flag.
rte_mbuf.h:#ifdef RTE_MBUF_REFCNT_ATOMIC
rte_mbuf.h:#else /* ! RTE_MBUF_REFCNT_ATOMIC */
rte_mbuf.h:#endif /* RTE_MBUF_REFCNT_ATOMIC */
I verified in building our application with DPDK rte_mbuf.h header file tha=
t the atomic functions for mbuf refcnt read/writes are turned ON. I added =
junk characters, and the compiler spotted syntax errors.
So, I am back to the question as to why I get mbuf issue with multiple thre=
ads processing the same mbuf?
Any more suggestions.
Thanks,
Ed
-----Original Message-----
From: Stephen Hemminger <stephen@networkplumber.org>=20
Sent: Wednesday, April 9, 2025 12:24 PM
To: Lombardo, Ed <Ed.Lombardo@netscout.com>
Cc: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>; users@dpdk.org
Subject: Re: mbuf refcnt issue
External Email: This message originated outside of NETSCOUT. Do not click l=
inks or open attachments unless you recognize the sender and know the conte=
nt is safe.
On Wed, 9 Apr 2025 04:46:09 +0000
"Lombardo, Ed" <Ed.Lombardo@netscout.com> wrote:
> Hi Stephen,
> I am looking a the rte_mbuf.h file for rte_pktmbuf_free() and it is not c=
lear to me that it checks if the mbuf refcnt is 1 before decrementing it an=
d allowing the mbuf and segments (if any) to be returned to free pool.
>=20
> Could my application issue be I have tx threads that transmit packets and=
does rte_pktmbuf_free(), while one other thread will perform rte_pktmbuf_f=
ree() on the same mbuf? I ensured I bump the mbuf refcnt to 2 before other=
threads can process the same mbuf.
>=20
> Thanks,
> Ed
It doesn't need to check refcnt there. The check is done later (since mbuf =
can be multi segment).
rte_pktmbuf_free
-> rte_pktmbuf_free_seg
-> rte_pktmbuf_prefree_seg
=09
static __rte_always_inline struct rte_mbuf * rte_pktmbuf_prefree_seg(struct=
rte_mbuf *m) {
__rte_mbuf_sanity_check(m, 0);
if (likely(rte_mbuf_refcnt_read(m) =3D=3D 1)) {
normal fast path. breaks the chain.
} else if (__rte_mbuf_refcnt_update(m, -1) =3D=3D 0) {
refcnt > 1 logic
Note, the refcnt doesn't always go to zero when the mbuf is put back in the=
pool.
The refcnt for a freed mbuf (in the pool) doesn't matter, it is free, it is=
dead.
The refcnt is reset to 1 when mbuf is extracted from the pool.