On Wed, 2010-03-17 at 13:52 -0400, Jeff Layton wrote:
> On Wed, 17 Mar 2010 13:26:59 -0400
> Trond Myklebust <Trond.Myklebust@xxxxxxxxxx> wrote:
>
> > On Wed, 2010-03-17 at 12:49 -0400, Christoph Hellwig wrote:
> > > On Thu, Mar 11, 2010 at 09:26:22AM -0500, Trond Myklebust wrote:
> > > > @@ -112,12 +112,10 @@ void nfs_unlock_request(struct nfs_page *req)
> > > > */
> > > > int nfs_set_page_tag_locked(struct nfs_page *req)
> > > > {
> > > > - struct nfs_inode *nfsi = NFS_I(req->wb_context->path.dentry->d_inode);
> > > > -
> > > > if (!nfs_lock_request_dontget(req))
> > > > return 0;
> > > > if (req->wb_page != NULL)
> > > > - radix_tree_tag_set(&nfsi->nfs_page_tree, req->wb_index, NFS_PAGE_TAG_LOCKED);
> > > > + radix_tree_tag_set(&NFS_I(req->wb_context->path.dentry->d_inode)->nfs_page_tree, req->wb_index, NFS_PAGE_TAG_LOCKED);
> > > > return 1;
> > > > }
> > > >
> > >
> > > This hunk is not only entirely unrealted to the fix, but also osbfucates
> > > the code.
> > >
> > > > @@ -126,10 +124,10 @@ int nfs_set_page_tag_locked(struct nfs_page *req)
> > > > */
> > > > void nfs_clear_page_tag_locked(struct nfs_page *req)
> > > > {
> > > > - struct inode *inode = req->wb_context->path.dentry->d_inode;
> > > > - struct nfs_inode *nfsi = NFS_I(inode);
> > > > -
> > > > if (req->wb_page != NULL) {
> > > > + struct inode *inode = req->wb_context->path.dentry->d_inode;
> > > > + struct nfs_inode *nfsi = NFS_I(inode);
> > > > +
> > > > spin_lock(&inode->i_lock);
> > > > radix_tree_tag_clear(&nfsi->nfs_page_tree, req->wb_index, NFS_PAGE_TAG_LOCKED);
> > > > nfs_unlock_request(req);
> > >
> > > Another one unrelated to the fix.
> > >
> >
> > No. They are needed because we don't want to dereference req->wb_context
> > after it (and req->wb_page) have been cleared. Without these 2 hunks,
> > the resulting kernel Oopses.
> >
>
> It seems like that just tightens up the race window without actually
> closing it.
>
> Just because wb_page was non-NULL when you checked it gives no
> guarantee that wb_context won't be NULL when you go to dereference it,
> right?
>
The above 2 functions are the ones that lock and unlock the request.
Once locked by means of the call to nfs_lock_request_dontget(req), no
other threads can change the contents of wb_page and wb_context.
IOW: yes, there is definitely such a guarantee...
Trond
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