On Tue, 2006-07-04 at 14:05 +0100, David Howells wrote:
> Arjan van de Ven <[email protected]> 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)
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