From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4297EC35656 for ; Fri, 21 Feb 2020 14:24:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 19306208C4 for ; Fri, 21 Feb 2020 14:24:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="HR2+2PNt" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728922AbgBUOYB (ORCPT ); Fri, 21 Feb 2020 09:24:01 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:56141 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728392AbgBUOYB (ORCPT ); Fri, 21 Feb 2020 09:24:01 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1582295040; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=c8mWqxlRjMqzWv3h3wuv1G8u9oII3zP+jyI9+XnLcJA=; b=HR2+2PNt7ChpqxRVT0QC6isaDDjOcejMK69/XW7Y7Soo4lkjqgNIX+DehWrJ6orfJm3rUt WZnCE8Yb358XKbwBCSo/z9818lph/gNENv5tRs/2OirLRZMvwGkPdYXl1ryf2i60FwvHMB LK99pTUUsX+aAHFwTPSdEuX0VtULCrg= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-295-K3W4URiHOyKQlUHyroESXw-1; Fri, 21 Feb 2020 09:23:58 -0500 X-MC-Unique: K3W4URiHOyKQlUHyroESXw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 086AF8010EB; Fri, 21 Feb 2020 14:23:57 +0000 (UTC) Received: from warthog.procyon.org.uk (ovpn-122-163.rdu2.redhat.com [10.10.122.163]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6B95A60BE0; Fri, 21 Feb 2020 14:23:55 +0000 (UTC) Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: References: <158204549488.3299825.3783690177353088425.stgit@warthog.procyon.org.uk> <158204561120.3299825.5242636508455859327.stgit@warthog.procyon.org.uk> To: Jann Horn Cc: dhowells@redhat.com, Al Viro , raven@themaw.net, Miklos Szeredi , Christian Brauner , Linux API , linux-fsdevel , kernel list Subject: Re: [PATCH 15/19] vfs: Add superblock notifications [ver #16] MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <1897787.1582295034.1@warthog.procyon.org.uk> Date: Fri, 21 Feb 2020 14:23:54 +0000 Message-ID: <1897788.1582295034@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Jann Horn wrote: > > + if (!s->s_watchers) { > > READ_ONCE() ? I'm not sure it matters. It can only be set once, and the next time we read it we're inside the lock. And at this point, I don't actually dereference it, and if it's non-NULL, it's not going to change. > > + ret = add_watch_to_object(watch, s->s_watchers); > > + if (ret == 0) { > > + spin_lock(&sb_lock); > > + s->s_count++; > > + spin_unlock(&sb_lock); > > Where is the corresponding decrement of s->s_count? I'm guessing that > it should be in the ->release_watch() handler, except that there isn't > one... Um. Good question. I think this should do the job: static void sb_release_watch(struct watch *watch) { put_super(watch->private); } And this then has to be set later: init_watch_list(wlist, sb_release_watch); > > + } else { > > + ret = -EBADSLT; > > + if (READ_ONCE(s->s_watchers)) { > > (Nit: I don't get why you do a lockless check here before taking the > lock - it'd be more straightforward to take the lock first, and it's > not like you want to optimize for the case where someone calls > sys_watch_sb() with invalid arguments...) Fair enough. I'll remove it. > > +#ifdef CONFIG_SB_NOTIFICATIONS > > + if (unlikely(s->s_watchers)) { > > READ_ONCE() ? Shouldn't matter. It's only read once and then a decision is made on it immediately thereafter. And if it's non-NULL, the value cannot change thereafter. David