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

[Dmitry Torokhov]

On Tue, Dec 15, 2015 at 2:01 AM, Maarten Lankhorst
<maarten.lankhorst at linux.intel.com> wrote:
> Op 15-12-15 om 02:29 schreef Dmitry Torokhov:
>> 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 at chromium.org>
>> Signed-off-by: Dmitry Torokhov <dtor at chromium.org>
>>
> NACK! There's no way to do this race free.

Can you please explain the race because as far as I can see there is not one.

> The driver should hold a refcount until fence is signaled.

If we are no longer interested in fence why do we need to wait for the
fence to be signaled?

Thanks.

-- 
Dmitry