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

[Hugh Dickins]

On Sat, 15 Sep 2001, Hugh Dickins wrote:
> On Fri, 14 Sep 2001, Marcelo Tosatti wrote:
> > 
> > I would prefer to make get_swap_page() not lock the swap lock anymore,
> > making it necessary to its callers to do the locking themselves. So:
> > ...
> This does gloss over the distinction between the swap_list_lock()
> and the swap_device_lock(si).  The latter is the more crucial here,
> but difficult to use in this way.  Though if you were to throw it
> away and convert to swap_list_lock() throughout, I wonder if we'd
> lose much (only gain on systems with just one swap area).  But I
> wasn't daring to combine them myself.

I think I get your idea now.  swap_device_lock stays where it was.
You want to move swap_list_lock out of get_swap_page, use it to
bracket swap_duplicate with add_to_swap_cache and get_swap_page
with add_to_swap_cache, easily done without changing interfaces,
since swap_duplicate doesn't currently use it at all.

That does add further locking in read_swap_cache_async, but if BKL
goes, I think that's okay; and it's a neat way to keep the familiar
primitives with little change.  I'll give it a try.

Hugh