Re: [RFC] Alternative raceless page free

Christian Ehrhardt
2002-09-05T15:59:58Z

On Thu, Sep 05, 2002 at 05:21:31PM +0200, Daniel Phillips wrote:
> On Thursday 05 September 2002 14:34, Christian Ehrhardt wrote:
> > @@ -455,7 +458,7 @@
> > } else {
> > /* failed to drop the buffers so stop here */
> > UnlockPage(page);
> > - page_cache_release(page);
> > + put_page(page);
> >
> > spin_lock(&pagemap_lru_lock);
> > continue;
> >
> > looks a bit suspicious. put_page is not allowed if the page is still
> > on the lru and there is no other reference to it. As we don't hold any
> > locks between UnlockPage and put_page there is no formal guarantee that
> > the above condition is met. I don't have another path that could race
> > with this one though and chances are that there actually is none.
>
> The corresponding get_page is just above, you must have overlooked it.

See it. The scenario im thinking about is:

CPU1 CPU2
pick page from lru
Lock it
get_page
try_to_release_buffers fails
UnlockPage
/* Window here */
Pick page form lru
page_cache_get
Lock it
actually free the buffers
UnlockPage
page_cache_release
doesn't remove the page from
the lru because CPU 1 holds
a reference
put_page dropps last reference
but doenn't remove the page from
the lru because it is put_page
and not page_cache release.
---> Page gets freed while it is still on the lru. If the lru cache
holds a reference that's a non issue.

CPU1 is the path in shrink_cache.
I admit that I don't have a real path that does what CPU2 does in
this scenario but doing what CPU2 does should be perfectly legal.

> Besides that, we have a promise that the page still has buffers, worth

Not after UnlockPage.

> another count, and the page will never be freed here. That's fragile
> though, and this particular piece of code can no doubt be considerably
> simplified, while improving robustness and efficiency at the same time.
> But that goes beyond the scope of this patch.

Well yes ;-) There's funny things going on like accessing a page
after page_cache_release...

regards Christian

--
THAT'S ALL FOLKS!