Hi Hanoch,

Please find some comments below.

On 12/27/2015 10:39 AM, Hanoch Haim (hhaim) wrote:

Hi Bruce,

I'm Hanoch from Cisco Systems works for  the https://github.com/cisco-system-traffic-generator/trex-core traffic generator project.

While upgrading from DPDK 1.8 to 2.2 Ido found that the following commit creates a mbuf corruption and result in Tx hang



commit f20b50b946da9070d21e392e4dbc7d9f68bc983e

Author: Olivier Matz [off-list ref]

Date:   Mon Jun 8 16:57:22 2015 +0200



Looking at the change it is clear why there is an issue, wanted to get your input.



Init

-----

alloc const mbuf  ==> mbuf-a (ref=1)



Simple case that works

---------------------



thread 1 , tx: alloc-mbuf->attach(mbuf-a) (ref=2)  inc- non atomic

thread 1 , tx: alloc-mbuf->attach(mbuf-a) (ref32)  inc- atomic

do you mean "(ref=3)" ?

thread 1 , drv : free()                    (ref=2) dec- atomic

thread 1 , drv : free()                    (ref=3) dec - non atomic

do you mean "(ref=1)" ?

Simple case that does not work

---------------------



Both do that in parallel



thread 2 tx : alloc-mbuf->attach(mbuf-a)  (ref=2)  inc- non atomic

thread 1 tx : alloc-mbuf->attach(mbuf-a)  (ref=2)  inc- non atomic

It is not allowed to call a function from the mbuf API in parallel.

Example:

	core0			|	core1
--------------------------------|---------------------------------------
m = rte_pktmbuf_alloc(m);	|
enqueue(m);			|
				|m = dequeue();
do_something(m);		|do_something(m);


do_something() is not allowed because it accesses the same mbuf
structure.
do_something() can be any function of mbuf API: rte_pktmbuf_prepend(),
rte_pktmbuf_attach(), ...


This is allowed:

	core0			|	core1
--------------------------------|---------------------------------------
m = rte_pktmbuf_alloc(m);	|
m2 = rte_pktmbuf_attach(m);	|
enqueue(m2);			|
				|m2 = dequeue();
do_something(m);		|do_something(m2);



Regards,
Olivier