linux-kernel - Re: [PATCH v14 00/11] mux controller abstraction and iio/i2c muxes

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Date:   Mon, 24 Apr 2017 12:52:59 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Peter Rosin <peda@...ntia.se>
Cc:     Jonathan Cameron <jic23@...nel.org>, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Jonathan Corbet <corbet@....net>, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-iio@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Colin Ian King <colin.king@...onical.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        kernel@...gutronix.de
Subject: Re: [PATCH v14 00/11] mux controller abstraction and iio/i2c muxes

On Mon, 2017-04-24 at 10:36 +0200, Peter Rosin wrote:
> Hi!
> 
> The big change since v13 is that the mux state is now locked with a mutex
> instead of an rwsem. Other that that, it is mostly restructuring and doc
> changes. There are a few other "real" changes as well, but those changes
> feel kind of minor. I guess what I'm trying to say is that although the
> list of changes for v14 is longish, it's still basically the same as last
> time.

I have hooked this up to the video-multiplexer and now I trigger
the lockdep debug_check_no_locks_held error message when selecting the
mux input from userspace:

$ media-ctl --links "'imx6-mipi-csi2':1->'ipu1_csi0_mux':0[1]"
[   66.258368] 
[   66.259919] =====================================
[   66.265369] [ BUG: media-ctl/258 still has locks held! ]
[   66.270810] 4.11.0-rc8-20170424-1+ #1305 Not tainted
[   66.275863] -------------------------------------
[   66.282158] 1 lock held by media-ctl/258:
[   66.286464]  #0:  (&mux->lock){+.+.+.}, at: [<8074a6c0>] mux_control_select+0x24/0x50
[   66.294424] 
[   66.294424] stack backtrace:
[   66.298980] CPU: 0 PID: 258 Comm: media-ctl Not tainted 4.11.0-rc8+ #1305
[   66.306771] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
[   66.313334] Backtrace: 
[   66.315858] [<8010e5a4>] (dump_backtrace) from [<8010e8ac>] (show_stack+0x20/0x24)
[   66.323473]  r7:80e518d0 r6:80e518d0 r5:600d0013 r4:00000000
[   66.329190] [<8010e88c>] (show_stack) from [<80496cf4>] (dump_stack+0xb0/0xdc)
[   66.336470] [<80496c44>] (dump_stack) from [<8017e404>] (debug_check_no_locks_held+0xb8/0xbc)
[   66.345043]  r9:ae8566b8 r8:ad88dc84 r7:ad88df58 r6:ad88dc84 r5:ad88df58 r4:ae856400
[   66.352837] [<8017e34c>] (debug_check_no_locks_held) from [<8012b258>] (do_exit+0x79c/0xcc8)
[   66.361321] [<8012aabc>] (do_exit) from [<8012d25c>] (do_group_exit+0x4c/0xcc)
[   66.368581]  r7:000000f8
[   66.371161] [<8012d210>] (do_group_exit) from [<8012d2fc>] (__wake_up_parent+0x0/0x30)
[   66.379120]  r7:000000f8 r6:76f71798 r5:00000000 r4:00000001
[   66.384837] [<8012d2dc>] (SyS_exit_group) from [<80109380>] (ret_fast_syscall+0x0/0x1c)

That correctly warns that the media-ctl process caused the mux->lock to
be locked and still held when the process exited. Do we need a usage
counter based mechanism for muxes that are (indirectly) controlled from
userspace?

regards
Philipp