Re: [PATCH 22/30] fbdev/smscufx: Detect registered fb_info from refcount

Javier Martinez Canillas
2023-06-12T10:40:24Z

Thomas Zimmermann <tzimmermann@suse.de> writes:

Hello Thomas,

> Hi Javier
>
> Am 08.06.23 um 00:22 schrieb Javier Martinez Canillas:
>> Thomas Zimmermann <tzimmermann@suse.de> writes:
>>
>>> Detect registered instances of fb_info by reading the reference
>>> counter from struct fb_info.read. Avoids looking at the dev field
>>> and prepares fbdev for making struct fb_info.dev optional.
>>>
>>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>>> Cc: Steve Glendinning <steve.glendinning@shawell.net>
>>> ---
>>> drivers/video/fbdev/smscufx.c | 4 ++--
>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/video/fbdev/smscufx.c b/drivers/video/fbdev/smscufx.c
>>> index 17cec62cc65d..adb2b1fe8383 100644
>>> --- a/drivers/video/fbdev/smscufx.c
>>> +++ b/drivers/video/fbdev/smscufx.c
>>> @@ -1496,7 +1496,7 @@ static int ufx_setup_modes(struct ufx_data *dev, struct fb_info *info,
>>> u8 *edid;
>>> int i, result = 0, tries = 3;
>>>
>>> - if (info->dev) /* only use mutex if info has been registered */
>>> + if (refcount_read(&info->count)) /* only use mutex if info has been registered */
>>
>> The struct fb_info .count refcount is protected by the registration_lock
>> mutex in register_framebuffer(). I think this operation isn't thread safe ?
>
> It's an atomic read.
>
> https://elixir.bootlin.com/linux/latest/source/include/linux/refcount.h#L147
>

Yes, is an atomic read but by reading [0] my understanding is that does
not provide memory ordering guarantees. Maybe I misunderstood though...

[0]: https://www.kernel.org/doc/html/latest/core-api/refcount-vs-atomic.html

> And that function is only being called from the USB probe callback
> before registering the framebuffer. It's not clear to me how the value
> could be anything but zero. I'd best leave it as is with the ref counter.
>

Yes, I'm OK with that and as mentioned I don't think that's less safe
than the previous info->dev check with regard to race conditions.

--
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat