Re: [lock validator] inet6_destroy_sock(): soft-safe -> soft-unsafe lock dependency

[Herbert Xu <herbert@gondor.apana.org.au>]

On Sat, Jan 28, 2006 at 04:22:04PM +0100, Ingo Molnar wrote:
> 
>  ===========================================================
>  [ BUG: soft-safe -> soft-unsafe lock dependency detected! ]
>  -----------------------------------------------------------
>  sshd/2853 [HC0[0]:SC0[1]:HE1:SE0] is trying to acquire:
>   (&newsk->sk_dst_lock){+-}, at: [<c048f385>] inet6_destroy_sock+0x25/0x100
>  
>  and this task is already holding:
>   (&((sk)->sk_lock.slock)){+-}, at: [<c0464832>] tcp_close+0x142/0x650
>  which would create a new lock dependency:
>    (&((sk)->sk_lock.slock)){+-} -> (&newsk->sk_dst_lock){+-}

OK, I believe this is a false positive.  However, Dave should
double-check this.

tcp_close is only called from process context.  The rule for sk_dst_lock
is that it must also only be obtained in process context.  On the other
hand, it is true that sk_lock can be obtained in softirq context.

In this particular case, sk_dst_lock is obtained by tcp_close with
softirqs disabled.  This is not a problem in itself since we're not
trying to get sk_dst_lock from a real softirq context (as opposed to
process context with softirq disabled).

I believe this warning comes about because the validator creates a
dependency between sk_lock and sk_dst_lock.  It then infers from this
dependency that in softirq contexts where sk_lock is obtained the code
may also attempt to obtain sk_dst_lock.

This inference is where the validator errs.  sk_dst_lock is never
(or should never be, and as far as I can see none of the traces show
it to do so) obtained in a real softirq context.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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