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=-17.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 90F75C2D0E4 for ; Sat, 21 Nov 2020 00:13:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3353323A65 for ; Sat, 21 Nov 2020 00:13:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="spE2Dzdu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728334AbgKUANi (ORCPT ); Fri, 20 Nov 2020 19:13:38 -0500 Received: from mail.kernel.org ([198.145.29.99]:57864 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728124AbgKUANh (ORCPT ); Fri, 20 Nov 2020 19:13:37 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-104-11.bvtn.or.frontiernet.net [50.39.104.11]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 7E57122470; Sat, 21 Nov 2020 00:13:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605917616; bh=5zB1AfWDQYwfDmz40UJYUCjQwB9bwxL+MKW9vHuE2G4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=spE2DzduUV//465dUX/iVEtr1ncMWxR0ZUty1K39hRjqZD7LO1qFkxS/X/ZWfRp4h iULl/zC2Fxh934+zVvzWGyrI5LU3YYiDa/YqLcVdJBiYowrP5z1TkQPpAFPuhqTW7v IsxyVmhzSD/RbyllaTvIL02/rk9BebZfrFVe5/Jk= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 3A5F33522A6E; Fri, 20 Nov 2020 16:13:36 -0800 (PST) Date: Fri, 20 Nov 2020 16:13:36 -0800 From: "Paul E. McKenney" To: Neeraj Upadhyay Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces for Tiny SRCU grace periods Message-ID: <20201121001336.GN1437@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20201117004017.GA7444@paulmck-ThinkPad-P72> <20201117004052.14758-4-paulmck@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote: > Hi Paul, > > On 11/17/2020 6:10 AM, paulmck@kernel.org wrote: > > From: "Paul E. McKenney" > > > > There is a need for a polling interface for SRCU grace > > periods, so this commit supplies get_state_synchronize_srcu(), > > start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this > > purpose. The first can be used if future grace periods are inevitable > > (perhaps due to a later call_srcu() invocation), the second if future > > grace periods might not otherwise happen, and the third to check if a > > grace period has elapsed since the corresponding call to either of the > > first two. > > > > As with get_state_synchronize_rcu() and cond_synchronize_rcu(), > > the return value from either get_state_synchronize_srcu() or > > start_poll_synchronize_srcu() must be passed in to a later call to > > poll_state_synchronize_srcu(). > > > > Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/ > > Reported-by: Kent Overstreet > > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ] > > Signed-off-by: Paul E. McKenney > > --- > > include/linux/rcupdate.h | 2 ++ > > include/linux/srcu.h | 3 +++ > > include/linux/srcutiny.h | 1 + > > kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++-- > > 4 files changed, 56 insertions(+), 2 deletions(-) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index de08264..e09c0d8 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -33,6 +33,8 @@ > > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b)) > > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b)) > > #define ulong2long(a) (*(long *)(&(a))) > > +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b))) > > +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b))) > > /* Exported common interfaces */ > > void call_rcu(struct rcu_head *head, rcu_callback_t func); > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h > > index e432cc9..a0895bb 100644 > > --- a/include/linux/srcu.h > > +++ b/include/linux/srcu.h > > @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp); > > int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp); > > void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp); > > void synchronize_srcu(struct srcu_struct *ssp); > > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp); > > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp); > > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie); > > #ifdef CONFIG_DEBUG_LOCK_ALLOC > > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h > > index fed4a2d..e9bd6fb 100644 > > --- a/include/linux/srcutiny.h > > +++ b/include/linux/srcutiny.h > > @@ -16,6 +16,7 @@ > > struct srcu_struct { > > short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */ > > unsigned short srcu_idx; /* Current reader array element in bit 0x2. */ > > + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */ > > u8 srcu_gp_running; /* GP workqueue running? */ > > u8 srcu_gp_waiting; /* GP waiting for readers? */ > > struct swait_queue_head srcu_wq; > > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c > > index 3bac1db..b405811 100644 > > --- a/kernel/rcu/srcutiny.c > > +++ b/kernel/rcu/srcutiny.c > > @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp) > > ssp->srcu_gp_running = false; > > ssp->srcu_gp_waiting = false; > > ssp->srcu_idx = 0; > > + ssp->srcu_idx_max = 0; > > INIT_WORK(&ssp->srcu_work, srcu_drive_gp); > > INIT_LIST_HEAD(&ssp->srcu_work.entry); > > return 0; > > @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp) > > struct srcu_struct *ssp; > > ssp = container_of(wp, struct srcu_struct, srcu_work); > > - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head)) > > + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) > > return; /* Already running or nothing to do. */ > > /* Remove recently arrived callbacks and wait for readers. */ > > @@ -147,14 +148,19 @@ void srcu_drive_gp(struct work_struct *wp) > > * straighten that out. > > */ > > WRITE_ONCE(ssp->srcu_gp_running, false); > > - if (READ_ONCE(ssp->srcu_cb_head)) > > + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) > > Should this be USHORT_CMP_LT ? I believe that you are correct. As is, it works but does needless grace periods. > > schedule_work(&ssp->srcu_work); > > } > > EXPORT_SYMBOL_GPL(srcu_drive_gp); > > static void srcu_gp_start_if_needed(struct srcu_struct *ssp) > > { > > + unsigned short cookie; > > + > > if (!READ_ONCE(ssp->srcu_gp_running)) { > > + cookie = get_state_synchronize_srcu(ssp); > > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie)) > > + WRITE_ONCE(ssp->srcu_idx_max, cookie); > > I was thinking of a case which might break with this. > > Consider a scenario, where GP is in progress and kworker is right > before below point, after executing callbacks: > > void srcu_drive_gp(struct work_struct *wp) { > We updated ->srcu_idx up here, correct? So it has bottom bit zero. > while (lh) { > > } > >>> CURRENT EXECUTION POINT Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be due to an interrupt. > WRITE_ONCE(ssp->srcu_gp_running, false); > > if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) > schedule_work(&ssp->srcu_work); > } > > Now, at this instance, srcu_gp_start_if_needed() runs and samples > srcu_gp_running and returns, without updating srcu_idx_max > > static void srcu_gp_start_if_needed(struct srcu_struct *ssp) > { > if (!READ_ONCE(ssp->srcu_gp_running)) returns true > > } This could happen in an interrupt handler, so with you thus far. > kworker running srcu_drive_gp() resumes and returns without queueing a new > schedule_work(&ssp->srcu_work); for new GP? > > Prior to this patch, call_srcu() enqueues a cb before entering > srcu_gp_start_if_needed(), and srcu_drive_gp() observes this > queuing, and schedule a work for the new GP, for this scenario. > > > WRITE_ONCE(ssp->srcu_gp_running, false); > - if (READ_ONCE(ssp->srcu_cb_head)) > + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) > schedule_work(&ssp->srcu_work); > > So, should the "cookie" calculation and "srcu_idx_max" setting be moved > outside of ssp->srcu_gp_running check and maybe return the same cookie to > caller and use that as the returned cookie from > start_poll_synchronize_srcu() ? > > srcu_gp_start_if_needed() > cookie = get_state_synchronize_srcu(ssp); > if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie)) > WRITE_ONCE(ssp->srcu_idx_max, cookie); > if (!READ_ONCE(ssp->srcu_gp_running)) { > > } I believe that you are quite correct, thank you! But rcutorture does have a call_srcu() (really a ->call, but same if SRCU) in a timer handler. The race window is quite narrow, so testing it might be a challenge... This is what I end up with: static void srcu_gp_start_if_needed(struct srcu_struct *ssp) { unsigned short cookie; cookie = get_state_synchronize_srcu(ssp); if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie)) WRITE_ONCE(ssp->srcu_idx_max, cookie); if (!READ_ONCE(ssp->srcu_gp_running)) { if (likely(srcu_init_done)) schedule_work(&ssp->srcu_work); else if (list_empty(&ssp->srcu_work.entry)) list_add(&ssp->srcu_work.entry, &srcu_boot_list); } } Does that look plausible? Thanx, Paul