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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8B3D7C433EF for ; Wed, 2 Feb 2022 19:57:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234314AbiBBT5a (ORCPT ); Wed, 2 Feb 2022 14:57:30 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33304 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229481AbiBBT53 (ORCPT ); Wed, 2 Feb 2022 14:57:29 -0500 Received: from mail-lj1-x22b.google.com (mail-lj1-x22b.google.com [IPv6:2a00:1450:4864:20::22b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 11717C061714 for ; Wed, 2 Feb 2022 11:57:29 -0800 (PST) Received: by mail-lj1-x22b.google.com with SMTP id q22so665068ljh.7 for ; Wed, 02 Feb 2022 11:57:28 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=3CIuft7MFInnh1xqsvdSm7aJQ1m6iJ5S3C53p3CCUHc=; b=T+H7wnmd7kfHJC8X6vmZGalF+103bfBAMVkRiADef73+EKFKl9I+0ZlOTqDGdHCbH8 Q1oEbfCllEYRsp1prWpBrTlb+wDNxqeKUCEiyHGRfzjNO5kIqgYkyrw4wwd8RflWRmTf Dkk3ZXyyLRzNW7qPpijDbsy8MZ3iQPAwtclg2Ct3ARAa0hPaa+nPn4DYtTVDW5uKy4P9 UJ0Px0+NhrQOFwDsqscBS8Tx2i0jbB/YL7rF42aizdgvyxaUYlDt/jxwHDk7l3Sl+pwb zUwh9fKqC3w8wLR2xhFRNELmJ5MgyIReJLSwXta04u6CnM18+RljvoLA+vzD1Bn4h5AJ dN0Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=3CIuft7MFInnh1xqsvdSm7aJQ1m6iJ5S3C53p3CCUHc=; b=tA1L8IoA1gEJffyiN+0uAQMwppUmwyRuIJRWzE8Fa26RJq7wrB5TNDOP9wfNhdxmAS 6CoqqZu6mqqSYAamANU+1e4vuy5iRBW2LeH/x0Cek0D7MXjmm3f8ExaOxHaCg641TBAA ErBDlVblkPw070Rld/XhzBU0rnbZNySQLhAjOgVGoTq5tOFJJDjMH1W9NpnHaIivkq6R nhpdrpQzssMamC81iiL/jOVLGJFU4t0VX1ERyPCw8hueJgy3x3HN77wUklVLc94Om/4x 1YHbDZRbLqbEoOrIq1U4L+IRcnnZP9rDWb0+LjyxbHrJ6Zqy+LDz4sUPyahMbyzraRTk EpaA== X-Gm-Message-State: AOAM531JldYLnWvQiX/GQ1uWXHhwNWhxj2SDdeohdSFjA0Z/y2j8iaGJ 2jkqc3SSY+1F9XdHCAHD3Ch0lP4C/WI= X-Google-Smtp-Source: ABdhPJy6YqLj2e3pYrZsKt8LhaB5KzLWfvsSR0JWSNWS0CYNQ1l6b/Lka0LlJ0u8hGKE8GE3PaWlDw== X-Received: by 2002:a2e:97cf:: with SMTP id m15mr20067068ljj.316.1643831847205; Wed, 02 Feb 2022 11:57:27 -0800 (PST) Received: from grain.localdomain ([5.18.251.97]) by smtp.gmail.com with ESMTPSA id a2sm3967086lfl.246.2022.02.02.11.57.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 02 Feb 2022 11:57:26 -0800 (PST) Received: by grain.localdomain (Postfix, from userid 1000) id 567C95A0020; Wed, 2 Feb 2022 22:57:25 +0300 (MSK) Date: Wed, 2 Feb 2022 22:57:25 +0300 From: Cyrill Gorcunov To: Jason Andryuk Cc: open list , "Eric W . Biederman" Subject: Re: [PATCH] kcmp: Comment get_file_raw_ptr() RCU usage Message-ID: References: <20220202151734.10418-1-jandryuk@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 02, 2022 at 02:48:48PM -0500, Jason Andryuk wrote: > On Wed, Feb 2, 2022 at 12:44 PM Cyrill Gorcunov wrote: > > > > On Wed, Feb 02, 2022 at 10:17:34AM -0500, Jason Andryuk wrote: > > > This usage of RCU appears wrong since the pointer is passed outside the > > > RCU region. However, it is not dereferenced, so it is "okay". Leave a > > > comment for the next reader. > > > > > > Without a reference, these comparisons are racy, but even with their use > > > inside an RCU region, the result could go stale. > > > > > > Signed-off-by: Jason Andryuk > > > --- > > > I was looking for examples of task_lookup_fd_rcu()/files_lookup_fd_rcu() > > > and found this. It differed from the example given in > > > Documentation/filesystems/files.rst, so I was initially confused. A > > > comment seemed appropriate to avoid confusion. > > > > > > kernel/kcmp.c | 3 +++ > > > 1 file changed, 3 insertions(+) > > > > > > diff --git a/kernel/kcmp.c b/kernel/kcmp.c > > > index 5353edfad8e1..4fb23f242e0f 100644 > > > --- a/kernel/kcmp.c > > > +++ b/kernel/kcmp.c > > > @@ -63,6 +63,9 @@ get_file_raw_ptr(struct task_struct *task, unsigned int idx) > > > { > > > struct file *file; > > > > > > + /* This RCU locking is only present to silence warnings. The pointer > > > + * value is only used for comparison and not dereferenced, so it is > > > + * acceptable. */ > > > rcu_read_lock(); > > > file = task_lookup_fd_rcu(task, idx); > > > rcu_read_unlock(); > > > > They are not wrong, this is just such a bit weird semantics where > > we fetch the pointers and strictly speaking map them into numbers > > set to compare. But I agree that such tricks might confuse. How about > > > > /* > > * Fetching file pointers inside RCU read-lock section > > * and reuse them as plain numbers is done in a sake > > * of speed. But make sure never dereference them after. > > */ > > I would tweak it a little to "Fetch file pointers inside RCU read-lock > section, but skip additional locking for speed. The pointer values > will be used as integers, and must not be dereferenced." Up to you. Initially I use _raw_ptr suffix in function name trying to point out that the pointer obtained should not be considered in any way except a natural number. So I'm fine with any comment which helps readability. > > One other idea I had was to switch the return value to "void *". That > way it isn't a struct file, and it isn't readily dereference-able. > But I wasn't sure if that would be overkill. What do you think? Lets better stick with a comment I think. Thanks!