Re: [PATCH v2] net-fq: Add WARN_ON check for null flow.

[Cong Wang <xiyou.wangcong@gmail.com>]

On Thu, Jun 7, 2018 at 4:48 PM,  <greearb@candelatech.com> wrote:
> From: Ben Greear <greearb@candelatech.com>
>
> While testing an ath10k firmware that often crashed under load,
> I was seeing kernel crashes as well.  One of them appeared to
> be a dereference of a NULL flow object in fq_tin_dequeue.
>
> I have since fixed the firmware flaw, but I think it would be
> worth adding the WARN_ON in case the problem appears again.
>
> BUG: unable to handle kernel NULL pointer dereference at 000000000000003c
> IP: ieee80211_tx_dequeue+0xfb/0xb10 [mac80211]

Instead of adding WARN_ON(), you need to think about
the locking there, it is suspicious:

fq is from struct ieee80211_local:

struct fq *fq = &local->fq;

tin is from struct txq_info:

struct fq_tin *tin = &txqi->tin;

I don't know if fq and tin are supposed to be 1:1, if not there is
a bug in the locking, because ->new_flows and ->old_flows are
both inside tin instead of fq, but they are protected by fq->lock....