media: add GFP flag to media_*() that could get called in atomic context
diff mbox

Message ID 20160314084633.521d3e35@recife.lan
State New
Headers show

Commit Message

Mauro Carvalho Chehab March 14, 2016, 11:46 a.m. UTC
Em Mon, 14 Mar 2016 12:52:54 +0200
Sakari Ailus <sakari.ailus@iki.fi> 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@iki.fi> 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@osg.samsung.com>
> > > > Suggested-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>    
> > > 
> > > 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.

> 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.

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

> 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@osg.samsung.com>


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Comments

Sakari Ailus March 14, 2016, 12:09 p.m. UTC | #1
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@iki.fi> 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@iki.fi> 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@osg.samsung.com>
> > > > > Suggested-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>    
> > > > 
> > > > 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@ideasonboard.com>
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@ideasonboard.com>
    Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
    Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
    Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

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@osg.samsung.com>
> 
> 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 };
>
Mauro Carvalho Chehab March 15, 2016, 3:55 p.m. UTC | #2
Em Mon, 14 Mar 2016 14:09:09 +0200
Sakari Ailus <sakari.ailus@iki.fi> escreveu:

> 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@iki.fi> 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@iki.fi> 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@osg.samsung.com>
> > > > > > Suggested-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>      
> > > > > 
> > > > > 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.

I can see only 2 risks protected by the lock:

1) mdev gets freed while an entity is being created. This is a problem
   with the current memory protection schema we're using. I guess the
   only way to fix it is to use kref for mdev/entities/interfaces/links/pads.
   This change doesn't make it better or worse.
   Also, I don't think we have such risk with the current devices.

2) a notifier may be inserted or removed by another driver, while the
   loop is running.

To avoid (2), I see 3 alternatives:

a) keep the loop as proposed on this patch. As the list is navigated using 
list_for_each_entry_safe(), I guess[1] it should be safe to remove/add
new notify callbacks there while the loop is running by some other process. 

[1] It *is* safe if the change were done inside the loop - but I'm not
100% sure that it is safe if some other CPU touches the notify list.

b) Unlock/relock the spinlock every time:

	/* previous code that locks mdev->lock spinlock */

 	/* invoke entity_notify callbacks */
 	list_for_each_entry_safe(notify, next, &mdev->entity_notify, list) {
		spin_unlock(&mdev->lock);
 		(notify)->notify(entity, notify->notify_data);
		spin_lock(&mdev->lock);
 	}
 
	spin_unlock(&mdev->lock);

c) use a separate lock for the notify list -this seems to be an overkill.

d) Protect it with the graph traversal mutex. That sounds the worse idea,
   IMHO, as we'll be abusing the lock.

> 
> >   
> > > 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@ideasonboard.com>
> 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@ideasonboard.com>
>     Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
>     Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
>     Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
> 
> 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.

True, but there were some places that were using the graph_mutex
instead of the spinlock. 

> 
> > 
> > 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.

Yes. I guess we should protect the memory allocated stuff with a kref,
and try to use RCS on most places, but we need more discussions and
more tests to implement a solution that would be reliable even if the
callers don't behave well.

> >   
> > > 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@osg.samsung.com>
> > 
> > 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 };
> >   
>
Sakari Ailus March 16, 2016, 8:28 a.m. UTC | #3
Hi Mauro,

On Tue, Mar 15, 2016 at 12:55:35PM -0300, Mauro Carvalho Chehab wrote:
> Em Mon, 14 Mar 2016 14:09:09 +0200
> Sakari Ailus <sakari.ailus@iki.fi> escreveu:
> 
> > 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@iki.fi> 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@iki.fi> 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@osg.samsung.com>
> > > > > > > Suggested-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>      
> > > > > > 
> > > > > > 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.
> 
> I can see only 2 risks protected by the lock:
> 
> 1) mdev gets freed while an entity is being created. This is a problem
>    with the current memory protection schema we're using. I guess the
>    only way to fix it is to use kref for mdev/entities/interfaces/links/pads.
>    This change doesn't make it better or worse.
>    Also, I don't think we have such risk with the current devices.
> 
> 2) a notifier may be inserted or removed by another driver, while the
>    loop is running.
> 
> To avoid (2), I see 3 alternatives:
> 
> a) keep the loop as proposed on this patch. As the list is navigated using 
> list_for_each_entry_safe(), I guess[1] it should be safe to remove/add
> new notify callbacks there while the loop is running by some other process. 

list_for_each_entry_safe() does not protect against concurrent access, only
against adding and removing list entries by the same user. List access
serialisation is still needed, whether you use _safe() functions or not.

> 
> [1] It *is* safe if the change were done inside the loop - but I'm not
> 100% sure that it is safe if some other CPU touches the notify list.

Indeed.

> 
> b) Unlock/relock the spinlock every time:
> 
> 	/* previous code that locks mdev->lock spinlock */
> 
>  	/* invoke entity_notify callbacks */
>  	list_for_each_entry_safe(notify, next, &mdev->entity_notify, list) {
> 		spin_unlock(&mdev->lock);
>  		(notify)->notify(entity, notify->notify_data);
> 		spin_lock(&mdev->lock);
>  	}
>  
> 	spin_unlock(&mdev->lock);
> 
> c) use a separate lock for the notify list -this seems to be an overkill.
> 
> d) Protect it with the graph traversal mutex. That sounds the worse idea,
>    IMHO, as we'll be abusing the lock.

I'd simply replace the spinlock with a mutex here. As we want to get rid of
the graph mutex anyway in the long run, let's not mix the two as they're
well separated now. As long as the mutex users do not sleep (i.e. the
notify() callback) the mutex is about as fast to use as the spinlock.

> 
> > 
> > >   
> > > > 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@ideasonboard.com>
> > 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@ideasonboard.com>
> >     Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
> >     Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
> >     Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
> > 
> > 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.
> 
> True, but there were some places that were using the graph_mutex
> instead of the spinlock. 

Hmm. If it's used to serialise access to the same data, it's most likely a
bug.

> 
> > 
> > > 
> > > 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.
> 
> Yes. I guess we should protect the memory allocated stuff with a kref,
> and try to use RCS on most places, but we need more discussions and
> more tests to implement a solution that would be reliable even if the
> callers don't behave well.

Yes.
Mauro Carvalho Chehab March 16, 2016, 12:02 p.m. UTC | #4
Em Wed, 16 Mar 2016 10:28:35 +0200
Sakari Ailus <sakari.ailus@iki.fi> escreveu:

> Hi Mauro,
> 
> On Tue, Mar 15, 2016 at 12:55:35PM -0300, Mauro Carvalho Chehab wrote:
> > Em Mon, 14 Mar 2016 14:09:09 +0200
> > Sakari Ailus <sakari.ailus@iki.fi> escreveu:
> >   
> > > Hi Mauro,
> > > 

...

> > > 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.  
> > 
> > I can see only 2 risks protected by the lock:
> > 
> > 1) mdev gets freed while an entity is being created. This is a problem
> >    with the current memory protection schema we're using. I guess the
> >    only way to fix it is to use kref for mdev/entities/interfaces/links/pads.
> >    This change doesn't make it better or worse.
> >    Also, I don't think we have such risk with the current devices.
> > 
> > 2) a notifier may be inserted or removed by another driver, while the
> >    loop is running.
> > 
> > To avoid (2), I see 3 alternatives:
> > 
> > a) keep the loop as proposed on this patch. As the list is navigated using 
> > list_for_each_entry_safe(), I guess[1] it should be safe to remove/add
> > new notify callbacks there while the loop is running by some other process.   
> 
> list_for_each_entry_safe() does not protect against concurrent access, only
> against adding and removing list entries by the same user. List access
> serialisation is still needed, whether you use _safe() functions or not.
> 
> > 
> > [1] It *is* safe if the change were done inside the loop - but I'm not
> > 100% sure that it is safe if some other CPU touches the notify list.  
> 
> Indeed.
> 
> > 
> > b) Unlock/relock the spinlock every time:
> > 
> > 	/* previous code that locks mdev->lock spinlock */
> > 
> >  	/* invoke entity_notify callbacks */
> >  	list_for_each_entry_safe(notify, next, &mdev->entity_notify, list) {
> > 		spin_unlock(&mdev->lock);
> >  		(notify)->notify(entity, notify->notify_data);
> > 		spin_lock(&mdev->lock);
> >  	}
> >  
> > 	spin_unlock(&mdev->lock);
> > 
> > c) use a separate lock for the notify list -this seems to be an overkill.
> > 
> > d) Protect it with the graph traversal mutex. That sounds the worse idea,
> >    IMHO, as we'll be abusing the lock.  
> 
> I'd simply replace the spinlock with a mutex here. As we want to get rid of
> the graph mutex anyway in the long run, let's not mix the two as they're
> well separated now. As long as the mutex users do not sleep (i.e. the
> notify() callback) the mutex is about as fast to use as the spinlock.

It could work. I added such patch on an experimental branch, where
I'm addressing a few troubles with au0828 unbind logic:
	https://git.linuxtv.org/mchehab/experimental.git/log/?h=au0828-unbind-fixes

The patch itself is at:
	https://git.linuxtv.org/mchehab/experimental.git/commit/?h=au0828-unbind-fixes&id=dba4d41bdfa6bb8dc51cb0f692102919b2b7c8b4

At least for au0828, it seems to work fine.

Regards,
Mauro
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Patch
diff mbox

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 };