loop inside uma_zfree critical section

[Monthadar Al Jaberi]

On Tue, Dec 13, 2011 at 3:35 PM, John Baldwin <jhb at freebsd.org> wrote:
> On Tuesday, December 13, 2011 7:46:34 am Monthadar Al Jaberi wrote:
>> 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.
>
> Uh, you can't use a lockmgr lock in interrupt handlers or in
> if_transmit/if_start routines.  You should most likely just be using a plain
> mutex instead.  Also, new code shouldn't use lockmgr in general.  If you
> need a sleepable lock, use sx instead.  It has a more straightforward API.

Ok, I will change the interrupt handler to do something like this:

    disaple_interrupt();
    taskqueue_enqueue(...); /* on new rx task queue */

Then on the new rx proc:

    sx_slock(...);
    m_copyback(...);
    enable_interrupt();
    /* 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 */
    sx_sunlock(...);

I lock TX/RX paths to make sure my code is threading safe.
Also because while programming my deivce (SPI communicatioin)
there will be a tsleep() waiting for the DMA interrupt and
thus we could be prempted by e.g. a beacon_callout etc...

>
>> 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.
>
> Hmm, the uma uses its own locking, so it should be safe, yes.  However, you
> are correct about [1] and [3].  The thing is, after [1] the mbuf shouldn't
> be in any buckets at all (it only gets put back into the bucket during a
> free).  Are you sure the mbuf wasn't double free'd previously?

>From my log I can only see the mbuf being used once before
by a beacon_callout and then it was freed by m_freem().
So I cant see that it was freed twice before that.

How can I go by debbuging this?

>
> --
> John Baldwin



-- 
Monthadar Al Jaberi