Re: R/W semaphore changes

[Arjan van de Ven <arjan@infradead.org>]

On Tue, 2006-07-04 at 14:05 +0100, David Howells wrote:
> Arjan van de Ven <arjan@infradead.org> wrote:
> 
> > > Please, please, please don't.  R/W semaphores are _not_ permitted to nest.
> > 
> > yet they do in places, there where there is a natural hierarchy..
> 
> Where?  I believe the mm used to but no longer does.
> 
> They still aren't allowed to.  Consider:
> 
> 	CPU 1			CPU 2
> 	=======================	=======================
> 	-->down_read(&A);
> 	<--down_read(&A);
> 				-->down_write(&A);
> 				   --- SLEEPING ---
> 	-->down_read(&A);
> 	   --- DEADLOCKED ---

you mean recursion, while nesting != recursion!
(for examples of nesting see the general lockdep documentation)
With nesting we mean

down_read(&A);
down_read(&B);

where A and B are a similar lock but not the same exact instance (for
example two inode locks of different inodes)