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 8AF99CA0EEB for ; Tue, 12 Sep 2023 14:23:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235011AbjILOXR (ORCPT ); Tue, 12 Sep 2023 10:23:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43232 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235796AbjILOXQ (ORCPT ); Tue, 12 Sep 2023 10:23:16 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CEC9810D; Tue, 12 Sep 2023 07:23:12 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=/H+bFDVMZGBYCVEP7wj9urnwFfRSQ9S3k0Idab6o22M=; b=FVBXqHi4R1OgjgueOAXk3SoMxu yJsijKjYeue08jJnaQ7TSXQhceSpFtRcZR/hmKb6+HClpksbboY1aO4FhUQm8+9NeGVX//ETl1kUM rep3tDv674hRzvI1l/hPmOqzJFoT2ucT7qnhyxq9LZq+qYmZ2mIyxdJgpR2yf1toVfsGUm/hFPOs7 p9IdlUDHFw/3qtM1UOdx6ATcL0phlP6XIVvVcVNcQhik/meARVeUaGNSfGi6Oq9w3Wj1j991KwiAy 9Ghw0CfwmhayBX63nGFeY/o/tc5YJAUHzz8nttDzS5x5ccgNO1UrhaYunkKZv9XcbVz3jrVYoFdVN W3sAHEmg==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1qg4I8-0083sg-Ox; Tue, 12 Sep 2023 14:23:01 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 68BEE3005B2; Tue, 12 Sep 2023 16:23:00 +0200 (CEST) Date: Tue, 12 Sep 2023 16:23:00 +0200 From: Peter Zijlstra To: Matthew Wilcox Cc: Dave Chinner , Waiman Long , Ingo Molnar , Will Deacon , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Chandan Babu R , "Darrick J . Wong" , linux-xfs@vger.kernel.org Subject: Re: [PATCH 1/5] locking: Add rwsem_is_write_locked() Message-ID: <20230912142300.GC22127@noisy.programming.kicks-ass.net> References: <20230908104434.GB24372@noisy.programming.kicks-ass.net> <70d89bf4-708b-f131-f90e-5250b6804d48@redhat.com> <20230912090342.GC35261@noisy.programming.kicks-ass.net> <20230912135213.GA22127@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Tue, Sep 12, 2023 at 02:58:32PM +0100, Matthew Wilcox wrote: > On Tue, Sep 12, 2023 at 03:52:13PM +0200, Peter Zijlstra wrote: > > On Tue, Sep 12, 2023 at 01:28:13PM +0100, Matthew Wilcox wrote: > > > On Tue, Sep 12, 2023 at 11:03:42AM +0200, Peter Zijlstra wrote: > > > > If not, then sure we can do this; it's not like I managed to get rid of > > > > muteX_is_locked() -- and I actually tried at some point :/ > > > > > > > > And just now I grepped for it, and look what I find: > > > > > > > > drivers/hid/hid-nintendo.c: if (unlikely(mutex_is_locked(&ctlr->output_mutex))) { > > > > drivers/nvdimm/btt.c: if (mutex_is_locked(&arena->err_lock) > > > > > > > > And there's more :-( > > > > > > Are these actually abuse? I looked at these two, and they both seem to > > > be asking "Does somebody else currently have this mutex?" rather than > > > "Do I have this mutex?". > > > > It's effectively a random number generator in that capacity. Someone > > might have it or might have had it when you looked and no longer have > > it, or might have it now but not when you asked. > > Well, no. > > if (mutex_is_locked(&arena->err_lock) > || arena->freelist[lane].has_err) { > nd_region_release_lane(btt->nd_region, lane); > > ret = arena_clear_freelist_error(arena, lane); > > So that's "Is somebody currently processing an error, or have they > already finished setting an error". Sure, it's somewhat racy, but > it looks like a performance optimisation, not something that needs > 100% accuracy. We're arguing past one another I think. Yes mutex_is_locked() is a random number generator when asked for something you don't own. But it might not be a bug because the code is ok with races. It is still fully dodgy IMO, such usage is pretty close to UB.