From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with SMTP id C846C6B01EE for ; Thu, 1 Apr 2010 11:39:41 -0400 (EDT) Date: Thu, 1 Apr 2010 17:37:56 +0200 From: Oleg Nesterov Subject: Re: [PATCH -mm] proc: don't take ->siglock for /proc/pid/oom_adj Message-ID: <20100401153756.GD14603@redhat.com> References: <20100329112111.GA16971@redhat.com> <20100330163909.GA16884@redhat.com> <20100330174337.GA21663@redhat.com> <20100331185950.GB11635@redhat.com> <20100331230032.GB4025@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: David Rientjes Cc: Andrew Morton , anfei , KOSAKI Motohiro , nishimura@mxp.nes.nec.co.jp, KAMEZAWA Hiroyuki , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On 04/01, David Rientjes wrote: > > On Thu, 1 Apr 2010, Oleg Nesterov wrote: > > > > That doesn't work for depraceted_mode (sic), you'd need to test for > > > OOM_ADJUST_MIN and OOM_ADJUST_MAX in that case. > > > > Yes, probably "if (depraceted_mode)" should do more checks, I didn't try > > to verify that MIN/MAX are correctly converted. I showed this code to explain > > what I mean. > > > > Ok, please cc me on the patch, it will be good to get rid of the duplicate > code and remove oom_adj from struct signal_struct. OK, great, will do tomorrow. > Do we need ->siglock? Why can't we just do > > struct sighand_struct *sighand; > struct signal_struct *sig; > > rcu_read_lock(); > sighand = rcu_dereference(task->sighand); > if (!sighand) { > rcu_read_unlock(); > return; > } > sig = task->signal; > > ... load/store to sig ... > > rcu_read_unlock(); No. Before signals-make-task_struct-signal-immutable-refcountable.patch (actually, series of patches), this can't work. ->signal is not protected by rcu, and ->sighand != NULL doesn't mean ->signal != NULL. (yes, thread_group_cputime() is wrong too, but currently it is never called lockless). After signals-make-task_struct-signal-immutable-refcountable.patch, we do not need any checks at all, it is always safe to use ->signal. But. Unless we kill signal->oom_adj, we have another reason for ->siglock, we can't update both oom_adj and oom_score_adj atomically, and if we race with another thread they can be inconsistent wrt each other. Yes, oom_adj is not actually used, except we report it back to user-space, but still. So, I am going to send 2 patches. The first one factors out the code in base.c and kills signal->oom_adj, the next one removes ->siglock. Oleg. -- 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/ . Don't email: email@kvack.org