Re: Regression - locking (all from 2.6.28)

[Rafael J. Wysocki]

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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