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 777DE2003CC; Wed, 13 Nov 2024 12:07:10 +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=1731499632; cv=none; b=rgoKMtRjckkAdOE66c0AhnXpk6iPO7i0oDVNR5dgTQAUL1omLxyaVeJxlwZuEdbrBmbvF//p8m6UR9PVRjXHNWS3sj9JrnP2ktX1vZf6Qx7unwPjRC2wX30OugtcId1ab3wsfPcBlTEJivgOZvEw18SOhLdeKon+i6VmZxkYLrw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731499632; c=relaxed/simple; bh=Q/IreS/W/Tve3q46cZOB9KqlOfPc7fe17oqubQX0Bsc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Ur3cDAV0QyvGMscvE2m9LP+b0FnTaH63sahCN85WsAgSfhWdAodSA9/aJOpb+GkBdPDg1IWeTMctwF8+Y4mOdpippUdk+c7sZ3JYtTA3lzBraxPmO0V78piKNo5/EYVy0YER7RS0eHj6KMZkqVcXxvzWhKZqlNDSDH+qxTAqtyw= 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=gjNfjjK/; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=QBPoP5ZA; 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="gjNfjjK/"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="QBPoP5ZA" Date: Wed, 13 Nov 2024 13:07:06 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1731499628; 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=0TCNt2Vac4NiFcRmr526QozckSKV5lxGdR7fjTzGh7Y=; b=gjNfjjK/xAvIgETR/0MlVrXkkBl2FM1/E4LEZR0TY4L/uINftKQpThsLtbKIkrhm5WZvdc 0f+MGRLDEaR4SuC9P/w0tM6UN8gvzwcEIr+bPfu3WjEDI+N++2lKqw9r23Cqn5vUwB2+L7 wTLKjylDhhxz9C8bTeXPB7JK4eQRIdOCzTWPyy3xMMEf5wczG4nLM420sQp4o0ZAmxRJs9 HwpHnxsRQ+eqerk9zV88ZzFE+6WAEDkvVqwPj2P4h4TYlkn/BqFIg+rUzPbj7WBaOELRPg /fDLuJ8kwi+8Bh5y7sGuTC+Kc5u7Sq1JCuQRmaGuFA2zfhvfOdSspnFPGAUxsg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1731499628; 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=0TCNt2Vac4NiFcRmr526QozckSKV5lxGdR7fjTzGh7Y=; b=QBPoP5ZA2qoGDEI/Uwe3mmqC51QeKC/sst18J2gjT0tNBE4kcvlRaHclzahRmPhnpwGdLC uj6utfYzYkJR4zCw== 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, syzbot+6ea37e2e6ffccf41a7e6@syzkaller.appspotmail.com Subject: Re: [PATCH v2 2/2] cgroup, kernfs: Move cgroup to the RCU interface for name lookups Message-ID: <20241113120706.rotCvUqt@linutronix.de> References: <20241112155713.269214-1-bigeasy@linutronix.de> <20241112155713.269214-3-bigeasy@linutronix.de> <20241113074331.B48iqBgp@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: <20241113074331.B48iqBgp@linutronix.de> On 2024-11-13 08:43:32 [+0100], To Tejun Heo wrote: > On 2024-11-12 08:59:16 [-1000], Tejun Heo wrote: > > Hello, > > Hi, > > > On Tue, Nov 12, 2024 at 04:52:39PM +0100, Sebastian Andrzej Siewior wrote: > > ... > > > /** > > > - * pr_cont_kernfs_name - pr_cont name of a kernfs_node > > > + * pr_cont_kernfs_name_rcu - pr_cont name of a kernfs_node > > > * @kn: kernfs_node of interest > > > * > > > - * This function can be called from any context. > > > + * This function can be called from any context. The root node must be with > > > + * KERNFS_ROOT_SAME_PARENT. > > > */ > > > -void pr_cont_kernfs_name(struct kernfs_node *kn) > > > +void pr_cont_kernfs_name_rcu(struct kernfs_node *kn) > > > > Having to split the interface all the way up isn't great. While there are > > also downsides, I wonder whether a better approach here is just making the > > backend function (kernfs_path_from_node()) automatically use RCU locking if > > the flag is set rather than propagating the difference by splitting the > > interface. The distinction doesn't mean anything to most users after all. > > Indeed. Now I see what the problems are. If we merge both into one, then I get this: | int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen) | { | struct kernfs_root *root; | bool rcu_lookup; | | if (!kn) | return strscpy(buf, "(null)", buflen); | | root = kernfs_root(kn); This is the tricky part. For KERNFS_ROOT_INVARIANT_PARENT I don't worry that the parent goes away and I need it to get a reference to the kernfs_root node. For the !KERNFS_ROOT_INVARIANT_PARENT I need the lock for kernfs_root() so I put the guard/ lock at the top. I think that is why you suggested the two functions (or this is what I understood). Looking at the remaining bits: | rcu_lookup = root->flags & KERNFS_ROOT_INVARIANT_PARENT; | if (rcu_lookup) { | guard(rcu)(); | return strscpy(buf, kn->parent ? rcu_dereference(kn->name) : "/", buflen); | } | guard(read_lock_irqsave)(&kernfs_rename_lock); | return strscpy(buf, kn->parent ? rcu_dereference(kn->name) : "/", buflen); | } This could collapse into the RCU version because read_lock_irqsave() implies RCU protection. And since ->name is always RCU assigned/ deallocated I don't really need the lock here, RCU would be enough. Except for the parent. The kn->parent does not matter here (it should be always be != NULL if assigned), the problematic part is kernfs_root() which checks the parent for the root node. To make this simple I could avoid kernfs_root lookup and just have: | int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen) | { | if (!kn) | return strscpy(buf, "(null)", buflen); | | guard(rcu)(); | return strscpy(buf, kn->parent ? rcu_dereference(kn->name) : "/", buflen); | } That is the easy part. kernfs_path_from_node() is different as it requires the parent pointer. In order to distinguish the RCU from the non-RCU version I need kernfs_root for the flag and depending on it, the lock so the parent does not go away. Would it work to add the pointer to kernfs_root into kernfs_node? This would shrink kernfs_elem_dir by a pointer but the union would remain the same size due to kernfs_elem_attr so the struct would grow. > > Thanks. Sebastian