Re: [PATCH] media: add GFP flag to media_*() that could get called in atomic context

[Sakari Ailus]

Hi Mauro,

On Mon, Mar 14, 2016 at 08:46:33AM -0300, Mauro Carvalho Chehab wrote:
> Em Mon, 14 Mar 2016 12:52:54 +0200
> Sakari Ailus <sakari.ailus@xxxxxx> escreveu:
> 
> > Hi Mauro,
> > 
> > On Mon, Mar 14, 2016 at 07:13:58AM -0300, Mauro Carvalho Chehab wrote:
> > > Em Mon, 14 Mar 2016 09:22:37 +0200
> > > Sakari Ailus <sakari.ailus@xxxxxx> escreveu:
> > >   
> > > > Hi Shuah,
> > > > 
> > > > On Sat, Mar 12, 2016 at 06:48:09PM -0700, Shuah Khan wrote:  
> > > > > Add GFP flags to media_create_pad_link(), media_create_intf_link(),
> > > > > media_devnode_create(), and media_add_link() that could get called
> > > > > in atomic context to allow callers to pass in the right flags for
> > > > > memory allocation.
> > > > > 
> > > > > tree-wide driver changes for media_*() GFP flags change:
> > > > > Change drivers to add gfpflags to interffaces, media_create_pad_link(),
> > > > > media_create_intf_link() and media_devnode_create().
> > > > > 
> > > > > Signed-off-by: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
> > > > > Suggested-by: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxx>    
> > > > 
> > > > What's the use case for calling the above functions in an atomic context?
> > > >   
> > > 
> > > ALSA code seems to do a lot of stuff at atomic context. That's what
> > > happens on my test machine when au0828 gets probed before
> > > snd-usb-audio:
> > > 	http://pastebin.com/LEX5LD5K
> > > 
> > > It seems that ALSA USB probe routine (usb_audio_probe) happens in
> > > atomic context.  
> > 
> > usb_audio_probe() grabs a mutex (register_mutex) on its own. It certainly
> > cannot be called in atomic context.
> > 
> > In the above log, what I did notice, though, was that because *we* grab
> > mdev->lock spinlock in media_device_register_entity(), we may not sleep
> > which is what the notify() callback implementation in au0828 driver does
> > (for memory allocation).
> 
> True. After looking into the code, the problem is that the notify
> callbacks are called with the spinlock hold. I don't see any reason
> to do that.

Notify callbacks, perhaps not, but the list is still protected by the
spinlock. It perhaps is not likely that another process would change it but
I don't think we can rely on that.

> 
> > Could we instead replace mdev->lock by a mutex?
> 
> We changed the code to use a spinlock for a reason: this fixed some
> troubles in the past with the code locking (can't remember the problem,
> but this was documented at the kernel logs and at the ML). Yet, the code
> under the spinlock never sleeps, so this is fine.

struct media_device.lock was added by this patch:

commit 53e269c102fbaf77e7dc526b1606ad4a48e57200
Author: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
Date:   Wed Dec 9 08:40:00 2009 -0300

    [media] media: Entities, pads and links

    As video hardware pipelines become increasingly complex and
    configurable, the current hardware description through v4l2 subdevices
    reaches its limits. In addition to enumerating and configuring
    subdevices, video camera drivers need a way to discover and modify at
    runtime how those subdevices are connected. This is done through new
    elements called entities, pads and links.

...

    Signed-off-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
    Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxx>
    Acked-by: Hans Verkuil <hverkuil@xxxxxxxxx>
    Signed-off-by: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>

I think it was always a spinlock, for the reason you stated above as well:
it did not need to sleep. But if there is a need to sleep, I think we should
consider changing that.

> 
> Yet, in the future, we'll need to do a review of all the locking schema,
> in order to better support dynamic graph changes.

Agreed. I think more fine grained locking should be considered. The media
graph mutex will become a bottleneck at some point, especially if we make
the media devices system wide at some point.

> 
> > If there is no pressing need to implement atomic memory allocation I simply
> > wouldn't do it, especially in device initialisation where an allocation
> > failure will lead to probe failure as well.
> 
> The fix for this issue should be simple. See the enclosed code. Btw.
> it probably makes sense to add some code here to avoid starving the
> stack, as a notify callback could try to create an entity, with,
> in turn, would call the notify callback again.
> 
> I'll run some tests here to double check if it fixes the issue.
> 
> ---
> 
> [media] media-device: Don't call notify callbacks holding a spinlock
> 
> The notify routines may sleep. So, they can't be called in spinlock
> context. Also, they may want to call other media routines that might
> be spinning the spinlock, like creating a link.
> 
> This fixes the following bug:
> 
> [ 1839.510587] BUG: sleeping function called from invalid context at mm/slub.c:1289
> [ 1839.510881] in_atomic(): 1, irqs_disabled(): 0, pid: 3479, name: modprobe
> [ 1839.511157] 4 locks held by modprobe/3479:
> [ 1839.511415]  #0:  (&dev->mutex){......}, at: [<ffffffff81ce8933>] __driver_attach+0xa3/0x160
> [ 1839.512381]  #1:  (&dev->mutex){......}, at: [<ffffffff81ce8941>] __driver_attach+0xb1/0x160
> [ 1839.512388]  #2:  (register_mutex#5){+.+.+.}, at: [<ffffffffa10596c7>] usb_audio_probe+0x257/0x1c90 [snd_usb_audio]
> [ 1839.512401]  #3:  (&(&mdev->lock)->rlock){+.+.+.}, at: [<ffffffffa0e6051b>] media_device_register_entity+0x1cb/0x700 [media]
> [ 1839.512412] CPU: 2 PID: 3479 Comm: modprobe Not tainted 4.5.0-rc3+ #49
> [ 1839.512415] Hardware name:                  /NUC5i7RYB, BIOS RYBDWi35.86A.0350.2015.0812.1722 08/12/2015
> [ 1839.512417]  0000000000000000 ffff8803b3f6f288 ffffffff81933901 ffff8803c4bae000
> [ 1839.512422]  ffff8803c4bae5c8 ffff8803b3f6f2b0 ffffffff811c6af5 ffff8803c4bae000
> [ 1839.512427]  ffffffff8285d7f6 0000000000000509 ffff8803b3f6f2f0 ffffffff811c6ce5
> [ 1839.512432] Call Trace:
> [ 1839.512436]  [<ffffffff81933901>] dump_stack+0x85/0xc4
> [ 1839.512440]  [<ffffffff811c6af5>] ___might_sleep+0x245/0x3a0
> [ 1839.512443]  [<ffffffff811c6ce5>] __might_sleep+0x95/0x1a0
> [ 1839.512446]  [<ffffffff8155aade>] kmem_cache_alloc_trace+0x20e/0x300
> [ 1839.512451]  [<ffffffffa0e66e3d>] ? media_add_link+0x4d/0x140 [media]
> [ 1839.512455]  [<ffffffffa0e66e3d>] media_add_link+0x4d/0x140 [media]
> [ 1839.512459]  [<ffffffffa0e69931>] media_create_pad_link+0xa1/0x600 [media]
> [ 1839.512463]  [<ffffffffa0fe11b3>] au0828_media_graph_notify+0x173/0x360 [au0828]
> [ 1839.512467]  [<ffffffffa0e68a6a>] ? media_gobj_create+0x1ba/0x480 [media]
> [ 1839.512471]  [<ffffffffa0e606fb>] media_device_register_entity+0x3ab/0x700 [media]
> 
> (untested)
> 
> Signed-off-by: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxx>
> 
> diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> index 6ba6e8f982fc..fc3c199e5500 100644
> --- a/drivers/media/media-device.c
> +++ b/drivers/media/media-device.c
> @@ -587,14 +587,15 @@ int __must_check media_device_register_entity(struct media_device *mdev,
>  		media_gobj_create(mdev, MEDIA_GRAPH_PAD,
>  			       &entity->pads[i].graph_obj);
>  
> +	spin_unlock(&mdev->lock);
> +
>  	/* invoke entity_notify callbacks */
>  	list_for_each_entry_safe(notify, next, &mdev->entity_notify, list) {
>  		(notify)->notify(entity, notify->notify_data);
>  	}
>  
> -	spin_unlock(&mdev->lock);
> -
>  	mutex_lock(&mdev->graph_mutex);
> +
>  	if (mdev->entity_internal_idx_max
>  	    >= mdev->pm_count_walk.ent_enum.idx_max) {
>  		struct media_entity_graph new = { .top = 0 };
> 

-- 
Regards,

Sakari Ailus
e-mail: sakari.ailus@xxxxxx	XMPP: sailus@xxxxxxxxxxxxxx