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    SubjectRe: hung task in mac80211
    Date
    On Wednesday, September 6, 2017 1:57:47 PM CEST Matteo Croce wrote:
    > Hi,
    >
    > I have an hung task on vanilla 4.13 kernel which I haven't on 4.12.
    > The problem is present both on my AP and on my notebook,
    > so it seems it affects AP and STA mode as well.
    > The generated messages are:
    >
    > INFO: task kworker/u16:6:120 blocked for more than 120 seconds.
    > Not tainted 4.13.0 #57
    > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    > kworker/u16:6 D 0 120 2 0x00000000
    > Workqueue: phy0 ieee80211_ba_session_work [mac80211]
    > Call Trace:
    > ? __schedule+0x174/0x5b0
    > ? schedule+0x31/0x80
    > ? schedule_preempt_disabled+0x9/0x10
    > ? __mutex_lock.isra.2+0x163/0x480
    > ? select_task_rq_fair+0xb9f/0xc60
    > ? __ieee80211_start_rx_ba_session+0x135/0x4d0 [mac80211]
    > ? __ieee80211_start_rx_ba_session+0x135/0x4d0 [mac80211]
    > ? try_to_wake_up+0x1f1/0x340
    > ? update_curr+0x88/0xd0
    > ? ieee80211_ba_session_work+0x148/0x230 [mac80211]
    >
    > I did a bisect and the offending commit is:
    >
    > commit 699cb58c8a52ff39bf659bff7971893ebe111bf2
    > Author: Johannes Berg <johannes.berg@intel.com>
    > Date: Tue May 30 16:34:46 2017 +0200
    >
    > mac80211: manage RX BA session offload without SKB queue

    I looked at this briefly:

    ieee80211_ba_session_work acquires:
    mutex_lock(&sta->ampdu_mlme.mtx) @
    <http://elixir.free-electrons.com/linux/v4.13/source/net/mac80211/ht.c#L321>

    But it now also calls
    __ieee80211_start_rx_ba_session() @
    http://elixir.free-electrons.com/linux/v4.13/source/net/mac80211/ht.c#L336

    which also wants to hold mutex_lock(&sta->ampdu_mlme.mtx) in:
    http://elixir.free-electrons.com/linux/v4.13/source/net/mac80211/agg-rx.c#L314

    I guess this is where it deadlocks?

    Regards,
    Christian

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