Date:	Mon, 14 Oct 2013 20:39:57 +0200
From:	Jan Kara <jack@...e.cz>
To:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	Hans de Goede <hdegoede@...hat.com>,
	Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: Warning when calling radix_tree_insert on 3.12-rc4

On Mon 14-10-13 11:17:21, Sarah Sharp wrote:
> On Mon, Oct 14, 2013 at 02:30:34PM +0200, Jan Kara wrote:
> >   Hello Sarah,
> > 
> > On Fri 11-10-13 15:13:15, Sarah Sharp wrote:
> > > I'm testing out some changes to the xHCI USB host controller driver
> > > (which uses a radix tree when a UAS device is attached to the host), and
> > > I noticed the following warning:
> > > 
> > > Oct 11 14:42:08 xanatos kernel: [18165.819014] usb 2-2: new SuperSpeed USB device number 2 using xhci_hcd
> > > Oct 11 14:42:08 xanatos kernel: [18165.836264] usb 2-2: New USB device found, idVendor=174c, idProduct=55aa
> > > Oct 11 14:42:08 xanatos kernel: [18165.836271] usb 2-2: New USB device strings: Mfr=2, Product=3, SerialNumber=1
> > > Oct 11 14:42:08 xanatos kernel: [18165.836275] usb 2-2: Product: Plugable USB3-SATA-UASP1
> > > Oct 11 14:42:08 xanatos kernel: [18165.836279] usb 2-2: Manufacturer: ASM1053E
> > > Oct 11 14:42:08 xanatos kernel: [18165.836291] usb 2-2: SerialNumber: 123456789045
> > > Oct 11 14:42:08 xanatos kernel: [18165.847661] BUG: using smp_processor_id() in preemptible [00000000] code: modprobe/8759
> > > Oct 11 14:42:08 xanatos kernel: [18165.847667] caller is radix_tree_node_alloc+0x5c/0xa0
> >   Well, this warning seems to come from the __get_cpu_var() function. I don't
> > see how my commit could have caused what you observe. It seems you are
> > calling radix_tree_insert() for a radix tree which has atomic gfp mask set
> > and you don't do radix_tree_preload() / radix_tree_preload_end() around
> > that? That was always problematic and could lead to the above warning.
> 
> I see.  Ok, we'll need to fix that.  The code went into the kernel years
> ago, but wasn't really tested until now.
> 
> Do we only need to call radix_tree_preload() and
> radix_tree_preload_end() only around the radix_tree_insert()?  Or will
> we need it around radix_tree_delete() as well?
  Only around insert. The reason is radix_tree_insert() may need to
allocate new radix tree nodes. radix_tree_delete() doesn't need to allocate
anything.

> > I'm not sure in which contexts xhci_update_stream_ring() can be called. But
> > if you are guaranteed non-atomic context, then using radix_tree_preload()
> > with a more relaxed gfp mask is good (lowers pressure on atomic allocations).
> >
> > If the context depends on the caller, things are more complex. Generally,
> > you can use radix_tree_maybe_preload() but you have to set gfp mask
> > argument according to the context. The function then figures out whether
> > it's worth it to do a preload or not (but it always does preempt_disable()
> > which will silence the warning).
> 
> There are a couple ways xhci_update_stream_ring() could be called:
> 
>  - xhci_alloc_stream_info can be called while the bandwidth mutex is
>    held, so that must be in process context
> 
>  - xhci_ring_free is called when the command to disable a slot
>    completes, in interrupt context.
> 
>  - xhci_ring_expansion during URB submission, which can happen in
>    interrupt context.
> 
> So it looks like the context depends on the caller, and we'll have to
> call radix_tree_maybe_preload().
  Yes, you need radix_tree_maybe_preload() in that case. Usually we handle
similar situations (e.g. in block/blk-ioc.c) by passing gfp mask from the
caller which knows the context.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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