From: Rainer Weikusat <rweiku...@mobileactivedefense.com>
Date: Thu, 03 Dec 2015 21:24:17 +0000
> David Miller <da...@davemloft.net> writes:
>> So with your patch, the "N * timeout" behavior, where N is the number
>> of queues reading threads, no longer occurs? Do they all now properly
>> get released at the appropriate timeout?
>
> As far as I can tell, yes. With the change, unix_dgram_recvmsg has a
> read loop looking like this:
>
> last = NULL; /* not really necessary */
> timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
>
> do {
> mutex_lock(&u->readlock);
>
> skip = sk_peek_offset(sk, flags);
> skb = __skb_try_recv_datagram(sk, flags, &peeked, &skip, &err,
> &last);
> if (skb)
> break;
>
> mutex_unlock(&u->readlock);
>
> if (err != -EAGAIN)
> break;
> } while (timeo &&
> !__skb_wait_for_more_packets(sk, &err, &timeo, last));
>
> u->readlock is only used to enforce serialized access while running code
> dealing with the peek offset. If there's currently nothing to receive,
> the mutex is dropped. Afterwards, non-blocking readers return with
> -EAGAIN and blocking readers go to sleep waiting for 'interesting
> events' via __skb_wait_for_more_packets without stuffing the mutex into
> a pocket and taking it with them: All non-blocking readers of a certain
> socket end up going to sleep via schedule_timeout call in the wait
> function, hence, each of them will be woken up once its timeout expires.
Great, thanks for the info. I think you should submit this patch
formally.
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