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SubjectRe: [patch 0/5] seqlock consolidation


On Thu, 15 Mar 2012, Al Viro wrote:

> On Thu, Mar 15, 2012 at 08:17:12PM +0100, Thomas Gleixner wrote:
> > > + hits += replace_path(&fs->root, old_root, new_root);
> > > + hits += replace_path(&fs->pwd, old_root, new_root);
> >
> > Wouldn't it be simpler to just do:
> >
> > + count += replace_path(&fs->root, old_root, new_root);
> > + count += replace_path(&fs->pwd, old_root, new_root);
> >
> > > + write_seqcount_end(&fs->seq);
> > > + while (hits--) {
> > > count++;
> >
> > Instead of that loop ?
>
> This loop also contains path_get_longterm() and we need to do it before
> dropping fs->lock. We are holding a reference to new_root, all right,
> but once it's place into ->fs->{root,pwd} of another task and ->fs->lock
> is dropped, there's nothing to stop that task of doing chdir() and dropping
> its reference. Which could outweight the single reference we are holding
> pretty soon...

Oops, sorry I missed that path_get_longterm() in the loop due to the
+/- forrest around it.



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