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

    * David S. Miller <davem@davemloft.net> wrote:

    > From: Herbert Xu <herbert@gondor.apana.org.au>
    > Date: Tue, 31 Jan 2006 21:27:58 +1100
    >
    > > 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.
    >
    > Herbert's analysis is correct. This unique locking strategy is used
    > by tcp_close() because at this point it knows that every single
    > reference to this socket in the system is gone once it takes the
    > socket lock with BH disabled.
    >
    > And that known invariant is why this is correct, and the locking
    > validator has no way to figure this out.

    ok, thanks for the analysis! I'll fix this with an explicit hint to the
    validator.

    Ingo
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