From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id E81F190010B for ; Mon, 16 May 2011 16:35:37 -0400 (EDT) Received: from d03relay04.boulder.ibm.com (d03relay04.boulder.ibm.com [9.17.195.106]) by e34.co.us.ibm.com (8.14.4/8.13.1) with ESMTP id p4GKN04f015438 for ; Mon, 16 May 2011 14:23:00 -0600 Received: from d03av02.boulder.ibm.com (d03av02.boulder.ibm.com [9.17.195.168]) by d03relay04.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p4GKaIeS077294 for ; Mon, 16 May 2011 14:36:20 -0600 Received: from d03av02.boulder.ibm.com (loopback [127.0.0.1]) by d03av02.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p4GEYVT9020200 for ; Mon, 16 May 2011 08:34:33 -0600 Subject: Re: [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access From: John Stultz In-Reply-To: References: <1305241371-25276-1-git-send-email-john.stultz@linaro.org> <1305241371-25276-2-git-send-email-john.stultz@linaro.org> <4DCD1256.4070808@jp.fujitsu.com> <1305311276.2680.34.camel@work-vm> Content-Type: text/plain; charset="UTF-8" Date: Mon, 16 May 2011 13:34:54 -0700 Message-ID: <1305578094.2915.53.camel@work-vm> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: KOSAKI Motohiro Cc: LKML , Ted Ts'o , David Rientjes , Dave Hansen , Andrew Morton , linux-mm@kvack.org On Sat, 2011-05-14 at 20:12 +0900, KOSAKI Motohiro wrote: > >> Can you please explain why we should use seqlock? That said, > >> we didn't use seqlock for /proc items. because, plenty seqlock > >> write may makes readers busy wait. Then, if we don't have another > >> protection, we give the local DoS attack way to attackers. > > > > So you're saying that heavy write contention can cause reader > > starvation? > > Yes. > > >> task->comm is used for very fundamentally. then, I doubt we can > >> assume write is enough rare. Why can't we use normal spinlock? > > > > I think writes are likely to be fairly rare. Tasks can only name > > themselves or sibling threads, so I'm not sure I see the risk here. > > reader starvation may cause another task's starvation if reader have > an another lock. So the risk is a thread rewriting its own comm over and over could starve some other critical task trying to read the comm. Ok. It makes it a little more costly, but fair enough. thanks -john -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org