Messages in this thread | | | From | Rainer Weikusat <> | Subject | Re: [RFC PATCH] af_unix: fix entry locking in unix_dgram_recvmsg | Date | Thu, 03 Dec 2015 21:24:17 +0000 |
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David Miller <davem@davemloft.net> writes: > From: Rainer Weikusat <rweikusat@mobileactivedefense.com> >> Rainer Weikusat <rw@doppelsaurus.mobileactivedefense.com> writes: >> >> [...] >> >>> Insofar I understand the comment in this code block correctly,
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>>> /* recvmsg() in non blocking mode is supposed to return -EAGAIN >>> * sk_rcvtimeo is not honored by mutex_lock_interruptible() >>> >>> setting a receive timeout for an AF_UNIX datagram socket also doesn't >>> work as intended because of this: In case of n readers with the same >>> timeout, the nth reader will end up blocking n times the timeout.
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> 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.
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