Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754651AbbLCVYd (ORCPT ); Thu, 3 Dec 2015 16:24:33 -0500 Received: from tiger.mobileactivedefense.com ([217.174.251.109]:32890 "EHLO tiger.mobileactivedefense.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754498AbbLCVYb (ORCPT ); Thu, 3 Dec 2015 16:24:31 -0500 From: Rainer Weikusat To: David Miller Cc: rweikusat@mobileactivedefense.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] af_unix: fix entry locking in unix_dgram_recvmsg In-Reply-To: <20151202.130244.140973474498435711.davem@davemloft.net> (David Miller's message of "Wed, 02 Dec 2015 13:02:44 -0500 (EST)") References: <87h9k41pyq.fsf@doppelsaurus.mobileactivedefense.com> <871tb6nlue.fsf@doppelsaurus.mobileactivedefense.com> <20151202.130244.140973474498435711.davem@davemloft.net> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.4 (gnu/linux) Date: Thu, 03 Dec 2015 21:24:17 +0000 Message-ID: <87zixrusxq.fsf@doppelsaurus.mobileactivedefense.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.4.3 (tiger.mobileactivedefense.com [217.174.251.109]); Thu, 03 Dec 2015 21:24:26 +0000 (GMT) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2161 Lines: 58 David Miller writes: > From: Rainer Weikusat >> Rainer Weikusat writes: >> >> [...] >> >>> Insofar I understand the comment in this code block correctly, [...] >>> /* 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. [...] > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/