Re: Re: lockdep report in hibernate code

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Tuesday, 23 October 2007 23:56, Alan Stern wrote:
> On Tue, 23 Oct 2007, Rafael J. Wysocki wrote:
> 
> > > The problem here is that the buffer mutexes are not distinguishable.
> > 
> > I don't quite get the "which lock already depends on the new lock" part.
> > 
> > Well, I have always had problems with understanding what lockdep actually
> > traces ...
> 
> The basic idea is simple enough.  Lockdep looks for events which seem 
> to be problematic, such as lock A being acquired while lock B is held 
> if earlier on somebody acquired B while holding A.
> 
> The difficulty lies in the "_seem_ to be" part -- lockdep can't keep 
> track of each and every individual lock in the system.  Instead it 
> groups them into categories based on the structures they lie in.  So if 
> A and A' are both pm_mutex members but belonging to two different 
> structures, lockdep won't be able to tell them apart without help.  If 
> someone acquires A then B, and someone else acquires B then A', lockdep 
> will report a violation.

Yes, which is what I think is happening in this particular case.  More
precisely, we get pm_mutex while holding a buffer mutex, so lockdep is warning
when we get another buffer mutex afterwards.

Greetings (not sure what to do about that),
Rafael