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

Dmitry Torokhov
2015-12-15T19:08:07Z

On Tue, Dec 15, 2015 at 11:00 AM, Gustavo Padovan <gustavo@padovan.org> wrote:
> 2015-12-15 Daniel Vetter <daniel@ffwll.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@chromium.org>
>> > Signed-off-by: Dmitry Torokhov <dtor@chromium.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