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 X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1D946C433E0 for ; Wed, 20 May 2020 17:43:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EC7F42075F for ; Wed, 20 May 2020 17:43:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="DnLHnQ5s" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726844AbgETRnC (ORCPT ); Wed, 20 May 2020 13:43:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58408 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726688AbgETRnB (ORCPT ); Wed, 20 May 2020 13:43:01 -0400 Received: from mail-qk1-x742.google.com (mail-qk1-x742.google.com [IPv6:2607:f8b0:4864:20::742]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3EC33C061A0E for ; Wed, 20 May 2020 10:43:01 -0700 (PDT) Received: by mail-qk1-x742.google.com with SMTP id f13so4478966qkh.2 for ; Wed, 20 May 2020 10:43:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=VBRfAgii8K8/gCukCvu8XCg+o66x9P2+cPM+CVImvio=; b=DnLHnQ5sjW0TrKjPiSFQuJoNHu1nh52R7i4YKRaAtQGIFAYQyxlX2Xr8mJrcX3jKww xuOXyXA3HijuVMJvaJw+QBdSvvEH/Kbpth1JAtrqv2DaK3zYrFXS5SN4dshFnhcvFDsj 8S0tIUvfnho9IxPjeojIhpUzb1HuBd0c/NWqU= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=VBRfAgii8K8/gCukCvu8XCg+o66x9P2+cPM+CVImvio=; b=GEvSUMqwfZZkeqpLnxohfnT/ihNyxArTwu9bA//FUrwJaWZeOGWITuunX1wpyVse9v Qsc/VBMnlkWSlOfb5c4E69cq2fOrwz8ovvvsiX+9IOiXbXmdLNu1PCkiaWfO4Kj06nHr 22dnA5jxhm5t1Hy9Qa2NSXuZueZDWN0LfgZiGhGAPzFBtjWIGz5DtdtHxFQJDltP4XEr t847OTp+RNixRDVXIGJysk6rWneFqbkWIaAOP0IDpobJP4z54xS6qnrCLN1HuihCL4/F CiPJsaT277qZGyvvxbSWHoMtmsCj/tMev/vS7uacwi0/r9T1ed/DySMPm0dGKbn/gIxv b0ug== X-Gm-Message-State: AOAM532MVDOBOr541v/g0gy4iSTcJxgTmptgqL3wc0s61mMbVRs1wZJ9 4Yf9r0dgxCDeHIem46WxiGirXA== X-Google-Smtp-Source: ABdhPJzX7+Xf26Khmb4D57YmI7QsAFvWHZ0SZ9/Q8msbMYR9tHpafxHNv4nq0+H1vkQ4JM6GKMoMbw== X-Received: by 2002:a37:688c:: with SMTP id d134mr6066461qkc.450.1589996580324; Wed, 20 May 2020 10:43:00 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id r18sm2863194qtn.1.2020.05.20.10.42.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 20 May 2020 10:42:59 -0700 (PDT) Date: Wed, 20 May 2020 13:42:59 -0400 From: Joel Fernandes To: Sebastian Andrzej Siewior Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, Ingo Molnar , Steven Rostedt , Will Deacon , Thomas Gleixner , "Paul E . McKenney" , Linus Torvalds , Lai Jiangshan , Josh Triplett , Mathieu Desnoyers , rcu@vger.kernel.org Subject: Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access Message-ID: <20200520174259.GA247557@google.com> References: <20200519201912.1564477-1-bigeasy@linutronix.de> <20200519201912.1564477-4-bigeasy@linutronix.de> <20200520102407.GF317569@hirez.programming.kicks-ass.net> <20200520120608.mwros5jurmidxxfv@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200520120608.mwros5jurmidxxfv@linutronix.de> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Sebastian, On Wed, May 20, 2020 at 02:06:08PM +0200, Sebastian Andrzej Siewior wrote: > On 2020-05-20 12:24:07 [+0200], Peter Zijlstra wrote: > > On Tue, May 19, 2020 at 10:19:07PM +0200, Sebastian Andrzej Siewior wrote: > > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > > index 0c71505f0e19c..8d2b5f75145d7 100644 > > > --- a/kernel/rcu/srcutree.c > > > +++ b/kernel/rcu/srcutree.c > > > @@ -25,6 +25,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > > > > #include "rcu.h" > > > #include "rcu_segcblist.h" > > > @@ -735,6 +736,7 @@ static void srcu_flip(struct srcu_struct *ssp) > > > smp_mb(); /* D */ /* Pairs with C. */ > > > } > > > > > > +static DEFINE_LOCAL_LOCK(sda_lock); > > > /* > > > * If SRCU is likely idle, return true, otherwise return false. > > > * > > > @@ -765,13 +767,13 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp) > > > unsigned long tlast; > > > > > > /* If the local srcu_data structure has callbacks, not idle. */ > > > - local_irq_save(flags); > > > + local_lock_irqsave(sda_lock, flags); > > > sdp = this_cpu_ptr(ssp->sda); > > > if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) { > > > - local_irq_restore(flags); > > > + local_unlock_irqrestore(sda_lock, flags); > > > return false; /* Callbacks already present, so not idle. */ > > > } > > > - local_irq_restore(flags); > > > + local_unlock_irqrestore(sda_lock, flags); > > > > Would it perhaps make sense to stick the local_lock in struct srcu_data ? > > In that case we would need something for pointer stability before the > lock is acquired. For pointer stability, can we just use get_local_ptr() and put_local_ptr() instead of adding an extra lock? This keeps the pointer stable while keeping the section preemptible on -rt. And we already have a lock in rcu_data, I prefer not to add another lock if possible. I wrote a diff below with get_local_ptr() (just build tested). Does this solve your issue? > I remember Paul looked at that patch a few years ago and he said that > that disabling interrupts here is important and matches the other part > instance where the interrupts are disabled. Looking at it now, it seems > that there is just pointer stability but I can't tell if > rcu_segcblist_pend_cbs() needs more than just this. Which 'other part' are you referring to? Your patch removed local_irq_save() from other places as well right? thanks, - Joel ---8<----------------------- diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 8ff71e5d0fe8b..5f49919205317 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -778,13 +778,17 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp) unsigned long tlast; /* If the local srcu_data structure has callbacks, not idle. */ - local_irq_save(flags); - sdp = this_cpu_ptr(ssp->sda); + sdp = get_local_ptr(ssp->sda); + spin_lock_irqsave_rcu_node(sdp, flags); + if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) { - local_irq_restore(flags); + spin_unlock_irqrestore_rcu_node(sdp, flags); + put_local_ptr(sdp); return false; /* Callbacks already present, so not idle. */ } - local_irq_restore(flags); + + spin_unlock_irqrestore_rcu_node(sdp, flags); + put_local_ptr(sdp); /* * No local callbacks, so probabalistically probe global state. @@ -864,9 +868,8 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp, } rhp->func = func; idx = srcu_read_lock(ssp); - local_irq_save(flags); - sdp = this_cpu_ptr(ssp->sda); - spin_lock_rcu_node(sdp); + sdp = get_local_ptr(ssp->sda); + spin_lock_irqsave_rcu_node(sdp, flags); rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp); rcu_segcblist_advance(&sdp->srcu_cblist, rcu_seq_current(&ssp->srcu_gp_seq)); @@ -886,6 +889,8 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp, else if (needexp) srcu_funnel_exp_start(ssp, sdp->mynode, s); srcu_read_unlock(ssp, idx); + + put_local_ptr(sdp); } /**