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SubjectRe: 2.4.10pre VM changes: Potential race condition on swap code


On Fri, 14 Sep 2001, Marcelo Tosatti wrote:

>
>
> On Fri, 14 Sep 2001, Hugh Dickins wrote:
>
> > On Thu, 13 Sep 2001, Marcelo Tosatti wrote:
> > > > >
> > > > > CPU0 CPU1 CPU2
> > > > > do_swap_page() try_to_swap_out() swapin_readahead()
> > .....
> > > > > BOOM.
> > > > >
> > > > > Now, if we get additional references at valid_swaphandles() the above race
> > > > > is NOT possible: we're guaranteed that any get_swap_page() will not find
> > > >
> > > > Err I mean _will_ find the swap map entry used and not use it, then.
> > > >
> > > > > the swap map entry used. See?
> >
> > Yes, I see it now: had trouble with the line wrap!
> >
> > Sure, that's one of the scenarios we were talking about, and getting
> > additional references in valid_swaphandles will stop that particular
> > race.
> >
> > It won't stop the race with "bare" read_swap_cache_async (which can
> > happen with swapoff, or with vm_swap_full deletion if multithreaded),
>
> Could you please make a diagram of such a race ?
>
> > and won't stop the race when valid_swaphandles->swap_duplicate comes
> > all between try_to_swap_out's get_swap_page and add_to_swap_cache.
>
> Oh I see:
>
> CPU0 CPU1
>
> try_to_swap_out() swapin readahead
>
> get_swap_page()
> valid_swaphandles()
> swapduplicate()
> add_to_swap_cache()
> add_to_swap_cache()
>
> BOOM.
>
> Is that what you mean ?
>
> > The first of those is significantly less likely than swapin_readahead
> > instance. The second requires interrupt at the wrong moment: can
> > certainly happen, but again less likely.
> >
> > Adding back reference bumping in valid_swaphandles would reduce the
> > likelihood of malign read_swap_cache_async/try_to_swap_out races,
> > but please don't imagine it's the final fix.
>
> Right. Now I see that the diagram I just wrote (thanks for making me
> understand it :)) has been there forever. Ugh.

I mean the race has been there forever.

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