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    SubjectRe: Regression - locking (all from 2.6.28)
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    On Friday 06 March 2009, Pavel Machek wrote:
    > Hi!
    >
    > On Fri 2009-03-06 11:19:46, Dave Hansen wrote:
    > > On Fri, 2009-03-06 at 18:00 +0000, Catalin Marinas wrote:
    > > > > I think you should be more worried about consistency rather than missing
    > > > > entries. Take these two lines of code:
    > > > >
    > > > > start_pfn = node->node_start_pfn;
    > > > > /* hotplug occurs here */
    > > > > end_pfn = start_pfn + node->node_spanned_pages;
    > > > >
    > > > > What if someone comes in and adds memory to the node, at the beginning
    > > > > of the node, after you have calculated start_pfn? Try to think of what
    > > > > value you'll get for end_pfn and whether it is consistent and was *ever*
    > > > > valid at all. Would that oops the kernel?
    > > >
    > > > I assume pfn_valid() should handle this and kmemleak wouldn't scan the
    > > > page, unless we need locks around pfn_valid as well but I haven't seen
    > > > any used in the kernel.
    > >
    > > You assume incorrectly. :(
    > >
    > > Take my above example, and assume that you have two nodes which are
    > > right next to each other. You might run over the end of one node and
    > > into the next one. Your pages will be pfn_valid() but you will be on
    > > the wrong node.
    > >
    > > Please take a look at those locks that I mentioned. Notice that they
    > > are lock the pgdat *span*, not the validity of pages inside the pgdat.
    > > Your code deals with what pages the pgdats *span* and thus needs that
    > > lock. Notice that my example also had to do with those two lines of
    > > code incorrectly guessing the pgdat's *span*.
    > >
    > > We recently went to some pain to make sure that the software suspend
    > > code (which walks pgdat ranges as well) worked with memory hotplug.
    > > There really isn't that much code around that actually cares at runtime
    > > about which physical areas a particular node or zone spans. Yours is a
    > > rarity and will require some caution.
    > >
    > > You could probably also use the memory hotplug mutex found here:
    > >
    > > https://lists.linux-foundation.org/pipermail/linux-pm/2008-November/018884.html
    > >
    > > But I'm not sure where those patches have gone. Hmmm. Pavel?
    >
    > I don't think they were applied. They probably should... Rafael was
    > about to look into that, but he lost the patch pointer.

    Yes. In fact, sending the patch again to me would be appreciated.

    Thanks,
    Rafael


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