linux-kernel - Re: [PATCH] android: fix warning when releasing active sync point

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Date:	Tue, 15 Dec 2015 11:08:01 -0800
From:	Dmitry Torokhov <dtor@...omium.org>
To:	Gustavo Padovan <gustavo@...ovan.org>,
	Dmitry Torokhov <dtor@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	devel@...verdev.osuosl.org,
	Andrew Bresticker <abrestic@...omium.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	dri-devel@...ts.freedesktop.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Riley Andrews <riandrews@...roid.com>,
	linux-media@...r.kernel.org
Subject: Re: [PATCH] android: fix warning when releasing active sync point

On Tue, Dec 15, 2015 at 11:00 AM, Gustavo Padovan <gustavo@...ovan.org> wrote:
> 2015-12-15 Daniel Vetter <daniel@...ll.ch>:
>
>> On Mon, Dec 14, 2015 at 05:29:55PM -0800, Dmitry Torokhov wrote:
>> > Userspace can close the sync device while there are still active fence
>> > points, in which case kernel produces the following warning:
>> >
>> > [   43.853176] ------------[ cut here ]------------
>> > [   43.857834] WARNING: CPU: 0 PID: 892 at /mnt/host/source/src/third_party/kernel/v3.18/drivers/staging/android/sync.c:439 android_fence_release+0x88/0x104()
>> > [   43.871741] CPU: 0 PID: 892 Comm: Binder_5 Tainted: G     U 3.18.0-07661-g0550ce9 #1
>> > [   43.880176] Hardware name: Google Tegra210 Smaug Rev 1+ (DT)
>> > [   43.885834] Call trace:
>> > [   43.888294] [<ffffffc000207464>] dump_backtrace+0x0/0x10c
>> > [   43.893697] [<ffffffc000207580>] show_stack+0x10/0x1c
>> > [   43.898756] [<ffffffc000ab1258>] dump_stack+0x74/0xb8
>> > [   43.903814] [<ffffffc00021d414>] warn_slowpath_common+0x84/0xb0
>> > [   43.909736] [<ffffffc00021d530>] warn_slowpath_null+0x14/0x20
>> > [   43.915482] [<ffffffc00088aefc>] android_fence_release+0x84/0x104
>> > [   43.921582] [<ffffffc000671cc4>] fence_release+0x104/0x134
>> > [   43.927066] [<ffffffc00088b0cc>] sync_fence_free+0x74/0x9c
>> > [   43.932552] [<ffffffc00088b128>] sync_fence_release+0x34/0x48
>> > [   43.938304] [<ffffffc000317bbc>] __fput+0x100/0x1b8
>> > [   43.943185] [<ffffffc000317cc8>] ____fput+0x8/0x14
>> > [   43.947982] [<ffffffc000237f38>] task_work_run+0xb0/0xe4
>> > [   43.953297] [<ffffffc000207074>] do_notify_resume+0x44/0x5c
>> > [   43.958867] ---[ end trace 5a2aa4027cc5d171 ]---
>> >
>> > Let's fix it by introducing a new optional callback (disable_signaling)
>> > to fence operations so that drivers can do proper clean ups when we
>> > remove last callback for given fence.
>> >
>> > Reviewed-by: Andrew Bresticker <abrestic@...omium.org>
>> > Signed-off-by: Dmitry Torokhov <dtor@...omium.org>
>> > ---
>> >  drivers/dma-buf/fence.c        | 6 +++++-
>> >  drivers/staging/android/sync.c | 8 ++++++++
>> >  include/linux/fence.h          | 2 ++
>> >  3 files changed, 15 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/dma-buf/fence.c b/drivers/dma-buf/fence.c
>> > index 7b05dbe..0ed73ad 100644
>> > --- a/drivers/dma-buf/fence.c
>> > +++ b/drivers/dma-buf/fence.c
>> > @@ -304,8 +304,12 @@ fence_remove_callback(struct fence *fence, struct fence_cb *cb)
>> >     spin_lock_irqsave(fence->lock, flags);
>> >
>> >     ret = !list_empty(&cb->node);
>> > -   if (ret)
>> > +   if (ret) {
>> >             list_del_init(&cb->node);
>> > +           if (list_empty(&fence->cb_list))
>> > +                   if (fence->ops->disable_signaling)
>> > +                           fence->ops->disable_signaling(fence);
>>
>> What exactly is the bug here? A fence with no callbacks registered any
>> more shouldn't have any problem. Why exactly does this blow up?
>
> The WARN_ON is probably this one:
> https://android.googlesource.com/kernel/common/+/android-3.18/drivers/staging/android/sync.c#433
>
> I've been wondering in the last few days if this warning is really
> necessary. If the user is closing a sync_timeline that has unsignalled
> fences it should probably be aware of that already. Then I think it is
> okay to remove the the sync_pt from the active_list at the release-time.
> In fact I've already prepared a patch doing that. Thoughts?
>

Maybe, but you need to make sure that you only affecting your fences.

My main objection is that still leaves fence_remove_callback() being
not mirror image of fence_add_callback().

-- 
Dmitry
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