Re: [PATCH] Fix races in 2.4.2-ac22 SysV shared memory

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Fri, Mar 23, 2001 at 11:58:50AM -0800, Linus Torvalds wrote:

> Ehh.. Sleeping with the spin-lock held? Sounds like a truly bad idea.

Uggh --- the shmem code already does, see:

shmem_truncate->shmem_truncate_part->shmem_free_swp->
lookup_swap_cache->find_lock_page

It looks messy: lookup_swap_cache seems to be abusing the page lock
gratuitously, but there are probably callers of it which rely on the
assumption that it performs an implicit wait_on_page().

Rik, do you think it is really necessary to take the page lock and
release it inside lookup_swap_cache?  I may be overlooking something,
but I can't see the benefit of it --- we can still race against
page_launder, so the page may still get locked behind our backs after
we get the reference from lookup_swap_cache (page_launder explicitly
avoids taking the pagecache hash spinlock which might avoid this
particular race).

--Stephen
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