linux-kernel - Re: [PATCH v2] usb: typec: fusb302: Fix debugfs mutex initialisation

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Date:   Fri, 22 Mar 2019 10:38:17 +0200
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Guenter Roeck <linux@...ck-us.net>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] usb: typec: fusb302: Fix debugfs mutex initialisation

On Thu, Mar 21, 2019 at 04:02:27PM +0000, Marc Zyngier wrote:
> On Thu, 21 Mar 2019 15:24:18 +0200
> Heikki Krogerus <heikki.krogerus@...ux.intel.com> wrote:
> 
> > Hi,
> > 
> > On Wed, Mar 20, 2019 at 06:34:33PM +0200, Heikki Krogerus wrote:
> > > > > After applying this there was no more "fusb302" debugfs directory, and
> > > > > attempt to unload the fusb302 module dead locked. Also, attempt to
> > > > > reboot caused this to happen on my GDPWin board after applying the
> > > > > patch:
> > > > > 
> > > > >         BUG: Dentry 0000000012f2a05d{i=149,n=i2c-fusb302}  still in use (1) [unmount of sysfs sysfs]
> > > > >         WARNING: CPU: 3 PID: 1639 at fs/dcache.c:1529 umount_check.cold.55+0x2e/0x3a
> > > > >         Modules linked in: intel_xhci_usb_role_switch roles pi3usb30532 typec i915 intel_gtt intel_cht_int33fe [last unloaded: tcpm]
> > > > >         CPU: 3 PID: 1639 Comm: umount Not tainted 5.1.0-rc1-heikki+ #916
> > > > >         Hardware name: Default string Default string/Default string, BIOS 5.11 05/25/2017
> > > > >         RIP: 0010:umount_check.cold.55+0x2e/0x3a
> > > > >         ...
> > > > > 
> > > > > Note. Your patch has also a conflict with patches from Hans, I
> > > > > think with this one: https://patchwork.kernel.org/patch/10847275/
> > > > > I can take care of that, but you can also rebase the next version on
> > > > > top of my typec-next branch to solve that problem:
> > > > > https://github.com/krohei/linux/commits/typec-next  
> > > > 
> > > > OK, this is very weird. I can't reproduce any of the issues you're
> > > > reporting:
> > > > 
> > > > - the patch applies cleanly on top of typec-next
> > > > - removing the fusb302 module works
> > > > - I see the debugfs file whenever fsusb302 is inserted
> > > > 
> > > > Maybe you were trying this on another branch?  
> > > 
> > > No, the branch is correct. Actually, I tested this on top of mainline
> > > and linux-next. I saw that happen on both.
> > > 
> > > On these Intel Cherrytrail based boards like my GDBWin, fusb302 is one
> > > of the functions of a weir MFD device (the driver for that device is
> > > drivers/platform/x86/intel_cht_int33fe.c). It's entirely possible that
> > > we are doing something wrong in that driver, and your patch just makes
> > > the problem visible.
> > > 
> > > I'll continue debugging.  
> > 
> > I figured out what's the problem. It seems that the driver does not
> > probe successfully, which is why I don't see that "fusb302" debugfs
> > directory.
> > 
> > The reason is that if tcpm_register_port() returns with -EPROBE_DEFER,
> > we end up with that rootdir already pointing to something, even though
> > the entry is destroyed in that case. So next time the driver is
> > probed, that "fusb302" directory does get created as rootdir has a
> > value, and debugfs_create_file() fails.
> > 
> > I think the correct fix is to just initialize the mutex earlier.
> > Something like this should work:
> > 
> > diff --git a/drivers/usb/typec/tcpm/fusb302.c b/drivers/usb/typec/tcpm/fusb302.c
> > index 261b82900fec..8e43ea27f26d 100644
> > --- a/drivers/usb/typec/tcpm/fusb302.c
> > +++ b/drivers/usb/typec/tcpm/fusb302.c
> > @@ -211,7 +211,6 @@ static struct dentry *rootdir;
> >  
> >  static void fusb302_debugfs_init(struct fusb302_chip *chip)
> >  {
> > -       mutex_init(&chip->logbuffer_lock);
> >         if (!rootdir)
> >                 rootdir = debugfs_create_dir("fusb302", NULL);
> >  
> > @@ -1667,6 +1666,7 @@ static int fusb302_probe(struct i2c_client *client,
> >         chip->tcpc_config = fusb302_tcpc_config;
> >         chip->tcpc_dev.config = &chip->tcpc_config;
> >         mutex_init(&chip->lock);
> > +       mutex_init(&chip->logbuffer_lock);
> >  
> >         chip->tcpc_dev.fwnode =
> >                 device_get_named_child_node(dev, "connector");
> 
> Looks good to me, although you probably want to make that conditional
> on CONFIG_DEBUG_FS being set.

Just move that logbuffer_lock member outside of the ifdef
CONFIG_DEBUG_FS condition.

For the record, I don't see any use for those ifdef checks. Those
logbuffer members in struct fusb302_chip could be kept in their own
structure, for example struct fusb302_log, that we allocate
separately and only if debugfs_initialized() returns true.


thanks,

-- 
heikki