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SubjectRe: expand_stack fix [was Re: 2.4.9aa3]

On Mon, 3 Sep 2001, Andrea Arcangeli wrote:
>
> Linus please include the attached patch to the next kernel, expand_stack
> is totally broken at the moment, we cannot mess with the mm vma layout
> if we don't hold the mmap_sem in write mode.

I disagree with the diagnosis..

expand_stack() has _never_ messed with the vma layout, and never should.
As such, from a vma list integrity standpoint it is fine.

Do we mess with the contents? Yes. But I'd much rather see a much more
minimal approach to the problem, on the order of:
- make sure we only accept GROWSDOWN for anonymous areas (which don't
care about the offset)
- make the vm_start update atomic (possibly by just getting the pagetable
spinlock).

> I considered implementing a read->write semaphore upgrade primitive but
> it cannot be reliable

There is no such thing. Never has been. It's a fundamentally impossible
operation. We may, at some point, decide to have a "read_for_write()" and
then "upgrade()" operations on the semaphore, but those inherently imply
some level of single-threading (ie only one read-for-writer accepted at
one time, with many pure readers), which makes it useless for this
particular case anyway.

However, having a finer-granularity spinlock _inside_ the semaphore (see
above suggestion) is a perfectly valid approach.

Linus

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