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

[Marcelo Tosatti]

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.