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From: David Miller <davem@davemloft.net>
Subject: Re: Multicast packet loss
Date: Sun, 08 Mar 2009 19:49:22 -0700 (PDT)
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From: Eric Dumazet <dada1@cosmosbay.com>
Date: Sun, 08 Mar 2009 17:46:13 +0100

> +	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep)) {
> +		if (in_softirq()) {
> +			if (!softirq_del(&sk->sk_del, sock_readable_defer))
> +				goto unlock;
> +			return;
> +		}

This is interesting.

I think you should make softirq_del() more flexible.  Make it the
socket's job to make sure it doesn't try to defer different
functions, and put the onus on locking there as well.

The cmpxchg() and all of this checking is just wasted work.

I'd really like to get rid of that callback lock too, then we'd
really be in business. :-)