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SubjectRe: [PATCH net 3/3] net: skb_queue_purge(): lock/unlock the queue only once
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On Sun, 1 Oct 2017 17:59:09 -0700, Stephen Hemminger wrote:

> On Sun, 01 Oct 2017 22:19:20 -0000 Michael Witten wrote:
>
>> + spin_lock_irqsave(&q->lock, flags);
>> + skb = q->next;
>> + __skb_queue_head_init(q);
>> + spin_unlock_irqrestore(&q->lock, flags);
>
> Other code manipulating lists uses splice operation and
> a sk_buff_head temporary on the stack. That would be easier
> to understand.
>
> struct sk_buf_head head;
>
> __skb_queue_head_init(&head);
> spin_lock_irqsave(&q->lock, flags);
> skb_queue_splice_init(q, &head);
> spin_unlock_irqrestore(&q->lock, flags);
>
>
>> + while (skb != head) {
>> + next = skb->next;
>> kfree_skb(skb);
>> + skb = next;
>> + }
>
> It would be cleaner if you could use
> skb_queue_walk_safe rather than open coding the loop.
>
> skb_queue_walk_safe(&head, skb, tmp)
> kfree_skb(skb);

I appreciate abstraction as much as anybody, but I do not believe
that such abstractions would actually be an improvement here.

* Splice-initing seems more like an idiom than an abstraction;
at first blush, it wouldn't be clear to me what the intention
is.

* Such abstractions are fairly unnecessary.

* The function as written is already so short as to be
easily digested.

* More to the point, this function is not some generic,
higher-level algorithm that just happens to employ the
socket buffer interface; rather, it is a function that
implements part of that very interface, and may thus
twiddle the intimate bits of these data structures
without being accused of abusing a leaky abstraction.

* Such abstractions add overhead, if only conceptually. In this
case, a temporary socket buffer queue allocates *3* unnecessary
struct members, including a whole `spinlock_t' member:

prev
qlen
lock

It's possible that the compiler will be smart enough to leave
those out, but I have my suspicions that it won't, not only
given that the interface contract requires that the temporary
socket buffer queue be properly initialized before use, but
also because splicing into the temporary will manipulate its
`qlen'. Yet, why worry whether optimization happens? The whole
issue can simply be avoided by exploiting the intimate details
that are already philosophically available to us.

Similarly, the function `skb_queue_walk_safe' is nice, but it
loses value both because a temporary queue loses value (as just
described), and because it ignores the fact that legitimate
access to the internals of these data structures allows for
setting up the requested loop in advance; that is to say, the
two parts of the function that we are now debating can be woven
together more tightly than `skb_queue_walk_safe' allows.

For these reasons, I stand by the way that the patch currently
implements this function; it does exactly what is desired, no more
or less.

Sincerely,
Michael Witten

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