Re: Possible deadlock when suspending framebuffer

[Francis Moreau <francis.moro@gmail.com>]

On Wed, Jun 15, 2011 at 7:58 AM, Bruno Prémont
<bonbons@linux-vserver.org> wrote:
> Hi,
>
> On Wed, 15 Jun 2011 09:09:24 Wanlong Gao <wanlong.gao@gmail.com> wrote:
>> <snip>
>> Hi Francis:
>> can you test this patch?
>
> Do you have a deadlock trace which you are trying to fix?
>
> It's either the caller of unregister_framebuffer() which must be
> changed to not call unregister_framebuffer with info's lock held or
> the code reacting on the notification that must not try to acquire the
> lock again.
>
> The interesting par is if console semaphore has some relation to this
> deadlock as the order for taking both varies... It could be
> lock_fb_info(); console_lock()  versus console_lock(); lock_fb_info()
>
> Bruno
>
>
>> Thanks
>>
>> From fe026c42af4cbdce053460a428a445e99071586a Mon Sep 17 00:00:00 2001
>> From: Wanlong Gao <wanlong.gao@gmail.com>
>> Date: Wed, 15 Jun 2011 09:03:41 +0800
>> Subject: [PATCH] test
>>
>>
>>
>> Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
>> ---
>>  drivers/video/fbmem.c |    3 ---
>>  1 files changed, 0 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
>> index 5aac00e..6e6cef3 100644
>> --- a/drivers/video/fbmem.c
>> +++ b/drivers/video/fbmem.c
>> @@ -1642,11 +1642,8 @@ static int do_unregister_framebuffer(struct
>> fb_info *fb_info)
>>       if (i < 0 || i >= FB_MAX || registered_fb[i] != fb_info)
>>               return -EINVAL;
>>
>> -     if (!lock_fb_info(fb_info))
>> -             return -ENODEV;
>>       event.info = fb_info;
>>       ret = fb_notifier_call_chain(FB_EVENT_FB_UNBIND, &event);
>> -     unlock_fb_info(fb_info);
>
> Not a good idea to stop taking fb_lock here.
> Pretty all calls of fb_notifier_call_chain are protected by info's
> lock, except the one for FB_EVENT_FB_UNREGISTERED a few lines further.
>
> IMHO it wou make sense to add the lock around that last one so all
> notifier chain calls are handled the same.
>
>>       if (ret)
>>               return -EINVAL;
>
>

Well, sorry for the dumb question but the fb/fbcon code is pretty hard
to follow for me.

Why does store_fbstate() and any fb driver's suspsend methods acquire
the console lock at all ?

Thanks
-- 
Francis