From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DEC51314A9F; Fri, 21 Nov 2025 06:26:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763706367; cv=none; b=mFHOZ4gnXORN3sUHvhTZtukftDZy/4qlozG1CV502vbIlmMaXQ7h/77nCKVZ7jAtWJo7NTuCkpMuYn5vmejNWJ8aopyQfMRHqK90B1BIieQQbx4VYE0/3nyvgZtC5w/eZ9PMCpCImM9G4IHREAjJiVNfU5rIsjLHavyMi/s+Mbk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763706367; c=relaxed/simple; bh=c0uBimvPR+f8Bi9TPxSgkEGvTjeNcSS7HB65AFlQYFc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kOmagR/m/UvnAuxN3+c/xAfLNdmHmOMaGb9/cRIKM28vT36MRG7h/DTDi5zUZUHI+ySt7qcxAGzIPtZ+RppCwxzJfZH/nWLXg8fxRVSp/lSGlHX5vvOtsPTCbYHwMUt9kAuD3nsKykkYFI9DAAAi1mowPy+WqDwCO1TX6vlJ9uE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ovdBEuMk; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ovdBEuMk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E8DD7C4CEF1; Fri, 21 Nov 2025 06:26:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1763706365; bh=c0uBimvPR+f8Bi9TPxSgkEGvTjeNcSS7HB65AFlQYFc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ovdBEuMkM1FFSUa415kt5bWubtoSgk5K4y3MJQfnStBav9QU2QNXCOL+gHSzdmhHE 1rmjQ4oc3nA4u5dQN8WnlDHKa9kbegxq6FPPr2o9YKlQ/7/jPfowazMAzRBEQAz8z0 9Izq6PnqLgvOXbd2bMpDCV2DXaVXFTZgkgFtL803lD4nKaFFxvbtbCmPi0NQ6hYohS dGFj+hPbCf2imJRb4aDXTj/Ad6iemUt2qfHW/9UhrX2EhYdpD4B8nHL1XqTuRZts7Z 4FcvrN8HPdMoifbOBEMVMbtL2z3GRGMW01gJps8WF084RwC76Ym6kiLxVO6I9z/pQr xTkdkP4et3uUg== Date: Thu, 20 Nov 2025 23:26:00 -0700 From: Nathan Chancellor To: Peter Zijlstra Cc: Tim Chen , Shrikanth Hegde , linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org, Chen Yu , Vincent Guittot , K Prateek Nayak , Srikar Dronamraju , Mohini Narkhede , x86@kernel.org Subject: Re: [tip: sched/core] sched/fair: Skip sched_balance_running cmpxchg when balance is not due Message-ID: <20251121062600.GA256626@ax162> References: <6fed119b723c71552943bfe5798c93851b30a361.1762800251.git.tim.c.chen@linux.intel.com> <176312274812.498.6548506845675120622.tip-bot2@tip-bot2> <20251118095432.GN3245006@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251118095432.GN3245006@noisy.programming.kicks-ass.net> On Tue, Nov 18, 2025 at 10:54:32AM +0100, Peter Zijlstra wrote: > On Mon, Nov 17, 2025 at 10:55:07AM -0800, Tim Chen wrote: > > > > if (!need_unlock && (sd->flags & SD_SERIALIZE)) { > > > - if (!atomic_try_cmpxchg_acquire(&sched_balance_running, 0, 1)) > > > > The second argument of atomic_try_cmpxchg_acquire is "int *old" while that of atomic_cmpxchg_acquire > > is "int old". So the above check would result in NULL pointer access. Probably have > > to do something like the following to use atomic_try_cmpxchg_acquire() > > > > int zero = 0; > > if (!atomic_try_cmpxchg_acquire(&sched_balance_running, &zero, 1)) > > > > Otherwise we should do atomic_cmpxchg_acquire() as below > > Yes, and I'm all mightily miffed all the compilers accept 0 (which is > int) for an 'int *' argument without so much as a warning :/ The C11 standard says in 6.3.2.3p3 An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. which seems to indicate to me that int *foo = 0; and #define NULL (void *)0 int *foo = NULL; have to be treated the same way :/ I think that is a big part of the motivation to bring nullptr into C in C23: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3042.htm > Nathan, you looked into this a bit yesterday, afaict there is: > > -Wzero-as-null-pointer-constant > > which is supposed to issue a warn here, but I can't get clang-22 to > object :/ (GCC doesn't take that warning for C mode, only C++, perhaps > that's the problem?). Right, it appears to be the same case for clang, notice the comment in diagnoseZeroToNullptrConversion(): https://github.com/llvm/llvm-project/commit/d7ba86b6bf54740dd4007e65a927151cb9f510b4 That warning should probably be updated to work for C23 but that does not really help us now because nullptr is not available in older standards (and I think the support for C23 is only solid in really recent compilers IIUC). > Help? Maybe we could have something like -Wnon-literal-null-conversion-strict in clang that would behave like -Wnon-literal-null-conversion but warn even in the literal zero conversion case (i.e., require a 'void *' cast)... That does not really help GCC though since it does not warn on any case of implicit conversion to NULL: https://godbolt.org/z/M5WE5covz Cheers, Nathan