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 4099625A631; Sat, 25 Jan 2025 00:01:55 +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=1737763316; cv=none; b=HqgUpCUQmy/CkBl9FaJC39N2FtHkzixUWIYFityqeCVGNzeFc+fwvWuJNC6H9MXMOsnrqtYl0I91URdC2PUp2L1aqHHNhKWfdA2atXx2rCQGbO9H2McLFpiVk6JYNBY11O+Dm6L4PlMwkmBqBD8IazWHebMhC2ZEmH2DBH4sPeQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737763316; c=relaxed/simple; bh=WzQYtYKYljIjHkV/4VwzFXzd16Fsn+C2g/r687UfU0U=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Z8hJWO9gp2VzvufuhCxneXBEOJ+QPlnaeeZtV03/GEpdEHrSvjxb75PPPzysZXwCFmUyydDkKgpZBihD+PUjnNP8iUyKkj/p//z1zLSR2VW0ArqJUE4qt7V9VmpM+axaFPf1pBo8gfJphuBiiBDOneQ7N8AjJp7JrfhnioLRlVU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Y1YFWExZ; 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="Y1YFWExZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AF663C4CED2; Sat, 25 Jan 2025 00:01:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1737763315; bh=WzQYtYKYljIjHkV/4VwzFXzd16Fsn+C2g/r687UfU0U=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Y1YFWExZzlEsuHQ7JHQXvX9cC/Ae78Q351rf2dgHPsbHigPE1oQ+i+FneBM3P/bLX ujyGvRR/YuE/rkQwdpoCxVjSF1xSdG4j7mGyQq2DMu+0/mA2An9/7fKeGUDN/iPx9w 5xOIAzgZBrydWLkzzbyNXmJvRYxkN0zHShLYi42xbjkfWYFjgtXi35Argh+8+vLhR7 +mO3Z646STrjcuTyATKcr7yIFgNVbBjKqTrvU6K/eZJqhq6dd2UH28TjkhtcAKKiBp MVZ3i9UU/OC+hTQV6ukJ64GRRDqrAM/dXegEMIkTAAarxqaLWFGdMG7R1q43udCB6Y 1Xj9+0nN8OI7g== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 51726CE13C5; Fri, 24 Jan 2025 16:01:55 -0800 (PST) Date: Fri, 24 Jan 2025 16:01:55 -0800 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org Subject: Re: [PATCH RFC v2 rcu] Fix get_state_synchronize_rcu_full() GP-start detection Message-ID: Reply-To: paulmck@kernel.org References: 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=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Sat, Jan 25, 2025 at 12:03:58AM +0100, Frederic Weisbecker wrote: > Le Fri, Dec 13, 2024 at 11:49:49AM -0800, Paul E. McKenney a écrit : > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > > index 2f9c9272cd486..d2a91f705a4ab 100644 > > --- a/kernel/rcu/rcu.h > > +++ b/kernel/rcu/rcu.h > > @@ -162,7 +162,7 @@ static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s) > > { > > unsigned long cur_s = READ_ONCE(*sp); > > > > - return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (2 * RCU_SEQ_STATE_MASK + 1)); > > + return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (3 * RCU_SEQ_STATE_MASK + 1)); > > This might need a comment. Good point! Would you like to propose one? > The way I understand it is that rcu_state.gp_seq might be seen started while > root_rnp->gp_seq is not. So rcu_seq_snap() on the started rcu_state.gp_seq > may return maximum 2 full GPs ahead of root_rnp->gp_seq. And therefore it takes below > 2 GPs to safely deduce we wrapped around. Exactly! > Should it be ULONG_CMP_LT(cur_s, s - (2 * (RCU_SEQ_STATE_MASK + 1))) ? Quite possibly. I freely admit that I allowed a bit of slop because time was of the essence (holidays and all that) and also it does not hurt much to lose a couple of counts out of a 2^32 cycle, to say nothing of the common-case 2^64 cycle. It would not hurt to be exact, but it would be necessary to convince ourselves that we were not off by one in the wrong direction. I would be happy to see a patch, as long as it was sufficiently convincing. > Or am I missing something? Not that I can see. So the answer is probably "yes". ;-) Thanx, Paul