Re: [PATCH] fbcon: fix deadlock in fbcon_generic_blank()

Hi,

On 18/09/13 01:29, John Tapsell wrote:

Do not lock fb_info when calling sending the FB_EVENT_CONBLANK
event.

In fbmem.c, the semantics are that we acquire the lock_fb_info first,
and then console_lock.  However when fbcon.c fbcon_generic_blank() is
called, the console lock could already be held.  Locking fb_info can
thus cause a deadlock.

So has this happened for you? Or is it just theoretical?

fbmem.c sends the FB_EVENT_BLANK without locking lock_fb_info first, so
this change introduces similar behaviour.

I don't think this is true. FB_EVENT_BLANK is sent in fb_blank(). That
one is called when FBIOBLANK ioctl is called, and it does lock_fb_info().

I'm not familiar with the console code, but removing a lock makes me
feel rather uneasy... But looking at the code, I can also see that
console_lock could already be held, so something here definitely looks
broken.

The only place using FB_EVENT_CONBLANK seems to be backlight, and if I'm
not mistaken, it has its own lock, and doesn't depend on the fb_info
being locked.

 Tomi