netdev - Re: IPv4/IPv6 sysctl unregistration deadlock

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Message-ID: <20090225084321.GA1101@gondor.apana.org.au>
Date:	Wed, 25 Feb 2009 16:43:21 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: IPv4/IPv6 sysctl unregistration deadlock

On Wed, Feb 25, 2009 at 08:18:47AM +0100, Patrick McHardy wrote:
>
> Unfortunately its more complicated than I thought because of
> device renames, where the sysctl pointer is reused after
> unregistration and the rename/unregistration/re-registration
> should be atomic. Deferring unregistration means we can't perform
> the new registration immediately unless we allow multiple
> registrations for a single device to be active simulaneously,
> which introduces a whole new set of problems.

Good point.

> Simply ignoring the request during unregistration doesn't seem
> so bad after all, the main problem is that it intoduces a different
> race on renames where a write to the "forwarding" file returns
> success, but the change doesn't take effect. We could return
> -ENOENT, but that seems a bit strange after open() returned success.
> Maybe -EBUSY, although I would prefer to make this transparent
> to userspace.

I'd like to avoid that for the rename case just because shell
scripts know how to deal with echo foo > /nonexist/file but not
necessarily a failed echo on write/close.

> I think I'm stuck. Will rethink it after some coffee :)

Yes we need more coffee :)

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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