Re: [PATCH] drivers/base: use a worker for sysfs unbind

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Dec 12, 2018 at 12:08:40PM +0100, Daniel Vetter wrote:
> On Mon, Dec 10, 2018 at 11:20:58AM +0100, Daniel Vetter wrote:
> > On Mon, Dec 10, 2018 at 11:18:32AM +0100, Daniel Vetter wrote:
> > > On Mon, Dec 10, 2018 at 11:06:34AM +0100, Greg Kroah-Hartman wrote:
> > > > On Mon, Dec 10, 2018 at 09:46:53AM +0100, Daniel Vetter wrote:
> > > > > Drivers might want to remove some sysfs files, which needs the same
> > > > > locks and ends up angering lockdep. Relevant snippet of the stack
> > > > > trace:
> > > > > 
> > > > >   kernfs_remove_by_name_ns+0x3b/0x80
> > > > >   bus_remove_driver+0x92/0xa0
> > > > >   acpi_video_unregister+0x24/0x40
> > > > >   i915_driver_unload+0x42/0x130 [i915]
> > > > >   i915_pci_remove+0x19/0x30 [i915]
> > > > >   pci_device_remove+0x36/0xb0
> > > > >   device_release_driver_internal+0x185/0x250
> > > > >   unbind_store+0xaf/0x180
> > > > >   kernfs_fop_write+0x104/0x190
> > > > > 
> > > > > I've stumbled over this because some new patches by Ram connect the
> > > > > snd-hda-intel unload (where we do use sysfs unbind) with the locking
> > > > > chains in the i915 unload code (but without creating a new loop),
> > > > > which upset our CI. But the bug is already there and can be easily
> > > > > reproduced by unbind i915 directly.
> > > > 
> > > > This is odd, why wouldn't any driver hit this issue?  And why now since
> > > > you say this is triggerable today?
> > > 
> > > The above backtrace is triggered by unbinding i915 on current upstream
> > > kernels. Note: Will crash later on rather badly in the
> > > fbdev/fbcon/vtconsole hell, but that's separate issue (which can be worked
> > > around by first unbinding fbcon manually through sysfs).
> > > 
> > > > I know scsi was doing some strange things like trying to remove the
> > > > device itself from a sysfs callback on the device, which requires it to
> > > > just call a different kobject function created just for that type of
> > > > thing.  Would that also make sense to do here instead of your workqueue?
> > > 
> > > Note how we blow up on unregistering sw device instances supported by i915
> > > in entirely different subsystems. I guess most drivers just have sysfs
> > > files for their own stuff, where this is done as you describe. The problem
> > > is that there's an awful lot of unrelated stuff hanging off i915.
> > > 
> > > Or maybe acpi_video is busted, and should be using a different function.
> > > You haven't said which one, and I have no idea which one it is ...
> > > 
> > > And in case the context wasn't clear: This is unbinding the i915 pci
> > > driver which triggers the above lockdep splat recursion.
> > 
> > btw another option for "fixing" this would be to annotate the mutex_lock
> > in kernfs_remove_by_name_ns as recursive. Which just shuts up lockdep (and
> > might hide some real bugs), but would get the job done since there's not
> > actually a deadlock here. Just lockdep being annoyed.
> 
> So what's the pick? I can do the typing, but I don't understand all the
> driver core interactions to know what we should be doing here best.

Sorry for the delay.

Look at sdev_store_delete() in drivers/scsi/scsi_sysfs.c and see if the
logic there makes sense to do here instead.

It still seems odd that removing a sysfs file by writing to a sysfs file
at the same level really invokes lockdep as I would have thought that
this path is well-tested by now.

thanks,

greg k-h