Messages in this thread |  | | | Date | Tue, 14 Aug 2012 11:33:20 +0300 | | From | "Michael S. Tsirkin" <> | | Subject | Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages |
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On Tue, Aug 14, 2012 at 09:29:49AM +0930, Rusty Russell wrote: > On Mon, 13 Aug 2012 11:41:23 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote: > > On Fri, Aug 10, 2012 at 02:55:15PM -0300, Rafael Aquini wrote: > > > +/* > > > + * Populate balloon_mapping->a_ops->freepage method to help compaction on > > > + * re-inserting an isolated page into the balloon page list. > > > + */ > > > +void virtballoon_putbackpage(struct page *page) > > > +{ > > > + spin_lock(&pages_lock); > > > + list_add(&page->lru, &vb_ptr->pages); > > > + spin_unlock(&pages_lock); > > > > Could the following race trigger: > > migration happens while module unloading is in progress, > > module goes away between here and when the function > > returns, then code for this function gets overwritten? > > If yes we need locking external to module to prevent this. > > Maybe add a spinlock to struct address_space? > > The balloon module cannot be unloaded until it has leaked all its pages, > so I think this is safe: > > static void remove_common(struct virtio_balloon *vb) > { > /* There might be pages left in the balloon: free them. */ > while (vb->num_pages) > leak_balloon(vb, vb->num_pages); > > Cheers, > Rusty.
I know I meant something else. Let me lay this out:
CPU1 executes: void virtballoon_putbackpage(struct page *page) { spin_lock(&pages_lock); list_add(&page->lru, &vb_ptr->pages); spin_unlock(&pages_lock);
at this point CPU2 unloads module: leak_balloon ...... next CPU2 loads another module so code memory gets overwritten
now CPU1 executes the next instruction:
} which would normally return to function's caller, but it has been overwritten by CPU2 so we get corruption.
No?
-- MST
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