From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (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 E4F4D189BA9; Wed, 13 Nov 2024 07:42:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731483736; cv=none; b=SM2ai21rmKyupQg4Qpeseictprkp0mpoS44OW+y5ekw36Kl44zYTIb8ctnYrskvW5ZczW075TWoRL/keedJJT6sLYlVT0V2yfeIpminDTVp/SmgjLA9gpuUZn0Xd5cbWyOptDJko5q/S3uV1ExeUWd46B+EtZSRYWh4bgGpnc9M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731483736; c=relaxed/simple; bh=ma4wONWvouC+vxfRc+J6/pDb0lSowyLWhkuynaKssU0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=NfLqQBOXDmg266sh7e+tGQj6cV9lqUqm76pze+HV7z7I/SPMw/y5GLqddzpZe2sYRo6HN2nP6NM24oIGD8ztLjKcqr3iLYFJ7g8cpi4PAjP31NovDrErgDMOYx7xD6ikI4CT8KiZlTn5h33Q2imotH2Crrwe0MAdxkA68wZIa6M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=MrTdHZOT; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=RMWd9kZr; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="MrTdHZOT"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="RMWd9kZr" Date: Wed, 13 Nov 2024 08:42:10 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1731483732; 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=KTj/9ZUYWeBiclfz3d3AfplQIm3LCpEopprUsZwwNQw=; b=MrTdHZOTSZOnHXmnBBOSmHCsTe3xoi1gLgWQUEZqkhy+1I87eLHAaKBc7z/Y62YAcJNEHT YHY4v9TGStmSQpGfWchhIrqGhUQhm7sGZZZ1kkCVa1vxKtn7t/wmOvfb5ZvnAeLGYAvN9N +5EXaElqnHVEaWCtiY0YbUzDzVge2EBLkUg1S4c9a9CUUvFbYywezF937iybCEfznd2xqI MIpZuDtwySp6x3+VBWf4o2b9MkXgzgnNCfAH0E59+dxYyhb8jmPqELyr+MMdz3XZVl/Cja QnpVQjXP7UFfMGLELBDU8tKKrCyqsGvrmvRNnE/hjYgwU2CB9XiysU6jzapI3w== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1731483732; 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=KTj/9ZUYWeBiclfz3d3AfplQIm3LCpEopprUsZwwNQw=; b=RMWd9kZr3AFNjFtioeK3GhaySGej6FJAALQXJ6ZgOH9+au5/TC+O+YQrQbTVBZCjPCkfGD H6fD1oD9xHIngzAg== From: Sebastian Andrzej Siewior To: Tejun Heo Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Michal =?utf-8?Q?Koutn=C3=BD?= , "Paul E. McKenney" , Boqun Feng , Greg Kroah-Hartman , Hillf Danton , Johannes Weiner , Marco Elver , Zefan Li , tglx@linutronix.de Subject: Re: [PATCH v2 1/2] kernfs: Make it possible to use RCU for kernfs_node::name lookup. Message-ID: <20241113074210.rlH2mJiD@linutronix.de> References: <20241112155713.269214-1-bigeasy@linutronix.de> <20241112155713.269214-2-bigeasy@linutronix.de> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On 2024-11-12 08:52:11 [-1000], Tejun Heo wrote: > Hello, Hi, > On Tue, Nov 12, 2024 at 04:52:38PM +0100, Sebastian Andrzej Siewior wrote: > ... > > KERNFS_ROOT_SAME_PARENT is added to signal that the parent never > > Maybe KERNFS_ROOT_INVARIANT_PARENT captures it better? Sure. > ... > > @@ -195,13 +191,47 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to, > > */ > > int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen) > > { > > + struct kernfs_root *root; > > > > + guard(read_lock_irqsave)(&kernfs_rename_lock); > > + if (kn) { > > + root = kernfs_root(kn); > > + if (WARN_ON_ONCE(root->flags & KERNFS_ROOT_SAME_PARENT)) > > + kn = NULL; > > Hmm... does kn need to be set to NULL here? actually no, because read_lock() implies RCU protection. > > + } > > + > > + if (!kn) > > + return strscpy(buf, "(null)", buflen); > > + > > + return strscpy(buf, kn->parent ? kn->name : "/", buflen); > ... > > +int kernfs_name_rcu(struct kernfs_node *kn, char *buf, size_t buflen) > > +{ > > + struct kernfs_root *root; > > + > > + if (kn) { > > + root = kernfs_root(kn); > > + if (WARN_ON_ONCE(!(root->flags & KERNFS_ROOT_SAME_PARENT))) > > + kn = NULL; > > Ah, I suppose it's to keep things symmetric. That's fine. > > > + } > > + if (!kn) > > + return strscpy(buf, "(null)", buflen); > > + > > + guard(rcu)(); > > Also, why are guards in different locations? Even when !SAME_PARENT, kn's > can't jump across roots, so guard there can also be in the same location as > this one? I tried to limit the scope but it can be symmetrical. > ... > > @@ -200,7 +205,10 @@ struct kernfs_node { > > * parent directly. > > */ > > struct kernfs_node *parent; > > - const char *name; > > + union { > > + const char __rcu *name_rcu; > > + const char *name; > > + }; > > Wouldn't it be simpler if ->name is always __rcu and !SAME_PARENT just > requires further protection on the read side? Let me try that again. > Thanks. > Sebastian