Hi Daniel, On Tue, Feb 08, 2022 at 03:03:28PM +0100, Daniel Vetter wrote: > On Fri, Feb 04, 2022 at 09:15:40PM +0100, Sam Ravnborg wrote: > > Hi Daniel, > > > > On Mon, Jan 31, 2022 at 10:05:50PM +0100, Daniel Vetter wrote: > > > Accessing the one in fbmem.c without taking the right locks is a bad > > > idea. Instead maintain our own private copy, which is fully protected > > > by console_lock() (like everything else in fbcon.c). That copy is > > > serialized through fbcon_fb_registered/unregistered() calls. > > > > I fail to see why we can make a private copy of registered_fb > > just like that - are they not somehow shared between fbcon and fbmem. > > So when fbmem updates it, then fbcon will use the entry or such? > > > > I guess I am just ignorant of how registered_fb is used - but please > > explain. > > The private copy is protected under console_lock, and hence safe to access > from fbcon.c code. > > The main registered_fb array is protected by a different mutex, so we > could indeed end up with hilarious corruption because the value is > inconsistent while we try to access it (e.g. we check for !NULL, but later > on gcc decides to reload the value and now it's suddenly become NULL and > we blow up). > > The two are synchronized by fbmem.c calling fbcon_register/unregister, so > aside from the different locks if there's no race going on, they will > always be identical. IT was this part that I missed, and it is already spelled out in the commit message. > > Other option would be to roll out get_fb_info() to fbcon.c, but since > fbcon.c is fully protected by console_lock that would add complexity in > the code flow that we don't really need. And we'd have to wire fb_info > through all call chains, since the right way to use get_fb_info is to look > it up once and then only drop it when your callback has finished. > > Since the current code just assume it's all protected by console_lock and > we never drop that during a callback this would mean major surgery and > essentially refactoring all of fbcon.c to only access the fbcon stuff > through fb_info, i.e. to get rid of _all_ the global arrays we have more > or less. I'm not volunteering for that (despite that really this would be > the right thing to do if we'd have infinite engineering time). > > Ack with that explainer added to the commit message? I consider the current commit message fine - it helps when you actually read it. Acked-by: Sam Ravnborg <sam@xxxxxxxxxxxx>
