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

[Hugh Dickins]

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),
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.

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.

Hugh