linux-kernel - RE: [EXT] Re: [4.20 PATCH] Revert "mwifiex: restructure rx_reorder_tbl

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Date:   Tue, 4 Jun 2019 03:03:59 +0000
From:   Ganapathi Bhat <gbhat@...vell.com>
To:     Brian Norris <briannorris@...omium.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Amitkumar Karwar" <amitkarwar@...il.com>,
        Nishant Sarmukadam <nishants@...vell.com>,
        Xinming Hu <huxinming820@...il.com>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [EXT] Re: [4.20 PATCH] Revert "mwifiex: restructure
 rx_reorder_tbl_lock usage"

Hi Brian,

> >    netif_rx_ni+0xe8/0x120
> >    mwifiex_recv_packet+0xfc/0x10c [mwifiex]
> >    mwifiex_process_rx_packet+0x1d4/0x238 [mwifiex]
> >    mwifiex_11n_dispatch_pkt+0x190/0x1ac [mwifiex]
> >    mwifiex_11n_rx_reorder_pkt+0x28c/0x354 [mwifiex]
> 
> TL;DR: the problem was right here ^^^
> where you started running mwifiex_11n_dispatch_pkt() (via
> mwifiex_11n_scan_and_dispatch()) while holding a spinlock.
> 
> When you do that, you eventually call netif_rx_ni(), which specifically defers
> to softirq contexts. Then, if you happen to have your flush timer expiring just
> before that, you end up in mwifiex_flush_data(), which also needs that
> spinlock.

Understood; Thanks for this detail;

> 
> There are a few possible ways to handle this:
> (a) prevent processing softirqs in that context; e.g., with
>     local_bh_disable(). This seems somewhat of a hack.
>     (Side note: I think most of the locks in this driver really could be
>     spin_lock_bh(), not spin_lock_irqsave() -- we don't really care
>     about hardirq context for 99% of these locks.)
> (b) restructure so that packet processing (e.g., netif_rx_ni()) is done
>     outside of the spinlock.
> 
> It's actually not that hard to do (b). You can just queue your skb's up in a
> temporary sk_buff_head list and process them all at once after you've
> finished processing the reorder table. I have a local patch to do this, and I
> might send it your way if I can give it a bit more testing.


OK; That will be good; We will run a complete test after the patch; (OR we can work on this, share for review);

Regards,
Ganapathi