loop inside uma_zfree critical section

[Monthadar Al Jaberi]

Hi,

I am not sure why I am having this problem, but looking
at the code I dont understand uma_core.c really good.
So I hope someone can shed a light on this:

I am running on an arm board and and running a kernel module
that behaves like a wlan interface. so I tx and rx packets.

For now tx is only only sending beacon like frames.
This is done through using ieee80211_beacon_alloc().

Then in a callout task to generate periodic beacons:

    m_dup(avp->beacon, M_DONTWAIT);
    mtx_lock(...);
    STAILQ_INSERT_TAIL(...);
    taskqueue_enqueue(...);
    mtx_unlock(...);
    callout_schedule(...);

On the RX side, the interrupt handler will read out buffer
then place it on a queue to be handled by wlan-glue code.
For now wlan-glue code just frees the mbuf it instead of
calling net80211 ieee80211_input() functions:

    m_copyback(...);
    /* Allocate new mbuf for next RX. */
    MGETHDR(..., M_DONTWAIT, MT_DATA);
    bzero((mtod(sc->Rx_m, void *)), MHLEN);
    sc->Rx_m->m_len = 0; /* NB: m_gethdr does not set */
    sc->Rx_m->m_data += 20; /* make headroom */

Then I use a lockmgr inside my kernel module that should
make sure that we either are on TX or RX path.

Here is an output of when it loops forever and a debug output.
I enabled some of the printfs in vm/uma_core.c:

KDB: debugger backends: ddb
KDB: current backend: ddb
Copyright (c) 1992-2011 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
	The Regents of the University of California. All rights reserved.
FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 10.0-CURRENT #931: Tue Dec 13 11:35:17 UTC 2011
...
beacon_callout.
becon m_dup: new 0xc0d99c00,  beacon: 0xc0eb0800.
queue beacon for TX.

LOCKMGR: TX path locked
sending...
Free TX mbuf: 0xc0d99c00
LOCKMGR: TX path unlocked

LOCKMGR: RX path locked
current RX mbuf: 0xc0d99900
queue packet for wlan-glue
allocating new Rx mbuf: 0xc0d99c00
LOCKMGR: RX path unlocked

wlan-glue task running...
wlan-glue code freeing mbuf: 0xc0d99900

beacon_callout.
becon m_dup: new 0xc0d99b00,  beacon: 0xc0eb0800.
queue beacon for TX.

LOCKMGR: TX path locked
sending...
Free TX mbuf: 0xc0d99b00
LOCKMGR: TX path unlocked

LOCKMGR: RX path locked
current RX mbuf: 0xc0d99c00
queue packet for wlan-glue
allocating new Rx mbuf: 0xc0d99b00                              [1]
LOCKMGR: RX path unlocked

wlan-glue task running...
uma_zfree: Swapping buckets.                                       [2]
wlan-glue code freeing mbuf: 0xc0d99c00

beacon_callout.
becon m_dup: new 0xc0d99b00,  beacon: 0xc0eb0800.     [3]
queue beacon for TX.

LOCKMGR: TX path locked
sending...
Free TX mbuf: 0xc0d99b00                                            [4]
LOCKMGR: TX path unlocked

LOCKMGR: RX path locked
current RX mbuf: 0xc0d99b00
queue packet for wlan-glue
allocating new Rx mbuf: 0xc0d99900
LOCKMGR: RX path unlocked

wlan-glue task running...                                                 [5]
uma_zfree: Swapping buckets.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
uma_zfree: Putting old bucket on the free list.
uma_zfree: Allocating new free bucket.
uma_zfree: Swapping buckets.
...

The problem seems to be at [2], somehow after swapping
buckets in uma_zfree m_dup returns a pointer to
an mbuf that is still being used by us, [1] and [3]
have same address.
Then we call m_freem twice on same mbuf, [4] and [5].
And a loop occurs inside uma_free.
I am using mbufs in a wrong way? Shouldnt mbufs be thread safe?
Problem seems to occur while swapping buckets.

Br,
-- 
Monthadar Al Jaberi