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


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.

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