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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6DE78C433EF for ; Thu, 9 Dec 2021 17:18:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242688AbhLIRWQ (ORCPT ); Thu, 9 Dec 2021 12:22:16 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47514 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242866AbhLIRWN (ORCPT ); Thu, 9 Dec 2021 12:22:13 -0500 Received: from sin.source.kernel.org (sin.source.kernel.org [IPv6:2604:1380:40e1:4800::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8B417C061746; Thu, 9 Dec 2021 09:18:39 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by sin.source.kernel.org (Postfix) with ESMTPS id 6330DCE276D; Thu, 9 Dec 2021 17:18:37 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 89E00C004DD; Thu, 9 Dec 2021 17:18:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1639070315; bh=6T+elCby+cclVVOmwvR5v3IA3Q/+iZs9ml9nncRr+Gk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=JRwkKn3KtZe/S+dT1F9q9QZ+EjZmQEDOUNwxorwKSPkxCmAVpl463VX5Z081g9ImN xOi92St25HtZa9ZraU4JPQwVTSlwWLq4WhM2c8kifrXR83v0dbk2333OJBCAp4vYl7 b36wAVBBm5EmVUSIEQtCFLVvxLZpYwEkLZL673isolCsn6wLErCl443KEKpqUOTDmC u73UHkAZFVO6CaOsWhC5OVcMZA3hwVEyNItSXS/+YOqgCozO+uDgBLJdmrYfo83HZo h/mUL0yjq+/i4lU/fBLa6U2jkkAMgzGbx+2ZXe0P0u4Dj9TmTll59+IM21VQ3FPu38 8oy3zzPy7InHg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 4CFB75C414D; Thu, 9 Dec 2021 09:18:35 -0800 (PST) Date: Thu, 9 Dec 2021 09:18:35 -0800 From: "Paul E. McKenney" To: David Woodhouse Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com, hushiyuan@huawei.com, luolongjun@huawei.com, hejingxian@huawei.com Subject: Re: [PATCH 02/11] rcu: Kill rnp->ofl_seq and use only rcu_state.ofl_lock for exclusion Message-ID: <20211209171835.GD641268@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20211209150938.3518-1-dwmw2@infradead.org> <20211209150938.3518-3-dwmw2@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211209150938.3518-3-dwmw2@infradead.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 09, 2021 at 03:09:29PM +0000, David Woodhouse wrote: > From: David Woodhouse > > If we allow architectures to bring APs online in parallel, then we end > up requiring rcu_cpu_starting() to be reentrant. But currently, the > manipulation of rnp->ofl_seq is not thread-safe. > > However, rnp->ofl_seq is also fairly much pointless anyway since both > rcu_cpu_starting() and rcu_report_dead() hold rcu_state.ofl_lock for > fairly much the whole time that rnp->ofl_seq is set to an odd number > to indicate that an operation is in progress. > > So drop rnp->ofl_seq completely, and use only rcu_state.ofl_lock. > > This has a couple of minor complexities: lockdep will complain when we > take rcu_state.ofl_lock, and currently accepts the 'excuse' of having > an odd value in rnp->ofl_seq. So switch it to an arch_spinlock_t to > avoid that false positive complaint. Since we're killing rnp->ofl_seq > of course that 'excuse' has to be changed too, so make it check for > arch_spin_is_locked(rcu_state.ofl_lock). > > There's no arch_spin_lock_irqsave() so we have to manually save and > restore local interrupts around the locking. > > At Paul's request, make rcu_gp_init not just wait but *exclude* any > CPU online/offline activity, which was fairly much true already by > virtue of it holding rcu_state.ofl_lock. Looks good! Could you please also make the first clause read something like this? "At Paul's request based on Neeraj's analysis, ..." I am going to pull this into -rcu for more intensive testing of this code for non-concurrent CPU-online operations (making the above change to the commit log). At some point, rcutorture needs to learn how to do concurrent CPU-online operations, but it would be good to bake the RCU-specific patches for a bit beforehand. Depending on timing, you might wish to send this patch with the rest of this group, so: Acked-by: Paul E. McKenney If testing goes well and if you don't get it there first, I expect to push this during the v5.18 merge window. Thanx, Paul > Signed-off-by: David Woodhouse > --- > kernel/rcu/tree.c | 64 +++++++++++++++++++++++------------------------ > kernel/rcu/tree.h | 4 +-- > 2 files changed, 32 insertions(+), 36 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index ef8d36f580fc..a1bb0b1229ed 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -91,7 +91,7 @@ static struct rcu_state rcu_state = { > .abbr = RCU_ABBR, > .exp_mutex = __MUTEX_INITIALIZER(rcu_state.exp_mutex), > .exp_wake_mutex = __MUTEX_INITIALIZER(rcu_state.exp_wake_mutex), > - .ofl_lock = __RAW_SPIN_LOCK_UNLOCKED(rcu_state.ofl_lock), > + .ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED, > }; > > /* Dump rcu_node combining tree at boot to verify correct setup. */ > @@ -1168,7 +1168,15 @@ bool rcu_lockdep_current_cpu_online(void) > preempt_disable_notrace(); > rdp = this_cpu_ptr(&rcu_data); > rnp = rdp->mynode; > - if (rdp->grpmask & rcu_rnp_online_cpus(rnp) || READ_ONCE(rnp->ofl_seq) & 0x1) > + /* > + * Strictly, we care here about the case where the current CPU is > + * in rcu_cpu_starting() and thus has an excuse for rdp->grpmask > + * not being up to date. So arch_spin_is_locked() might have a > + * false positive if it's held by some *other* CPU, but that's > + * OK because that just means a false *negative* on the warning. > + */ > + if (rdp->grpmask & rcu_rnp_online_cpus(rnp) || > + arch_spin_is_locked(&rcu_state.ofl_lock)) > ret = true; > preempt_enable_notrace(); > return ret; > @@ -1731,7 +1739,6 @@ static void rcu_strict_gp_boundary(void *unused) > */ > static noinline_for_stack bool rcu_gp_init(void) > { > - unsigned long firstseq; > unsigned long flags; > unsigned long oldmask; > unsigned long mask; > @@ -1774,22 +1781,17 @@ static noinline_for_stack bool rcu_gp_init(void) > * of RCU's Requirements documentation. > */ > WRITE_ONCE(rcu_state.gp_state, RCU_GP_ONOFF); > + /* Exclude CPU hotplug operations. */ > rcu_for_each_leaf_node(rnp) { > - // Wait for CPU-hotplug operations that might have > - // started before this grace period did. > - smp_mb(); // Pair with barriers used when updating ->ofl_seq to odd values. > - firstseq = READ_ONCE(rnp->ofl_seq); > - if (firstseq & 0x1) > - while (firstseq == READ_ONCE(rnp->ofl_seq)) > - schedule_timeout_idle(1); // Can't wake unless RCU is watching. > - smp_mb(); // Pair with barriers used when updating ->ofl_seq to even values. > - raw_spin_lock(&rcu_state.ofl_lock); > - raw_spin_lock_irq_rcu_node(rnp); > + local_irq_save(flags); > + arch_spin_lock(&rcu_state.ofl_lock); > + raw_spin_lock_rcu_node(rnp); > if (rnp->qsmaskinit == rnp->qsmaskinitnext && > !rnp->wait_blkd_tasks) { > /* Nothing to do on this leaf rcu_node structure. */ > - raw_spin_unlock_irq_rcu_node(rnp); > - raw_spin_unlock(&rcu_state.ofl_lock); > + raw_spin_unlock_rcu_node(rnp); > + arch_spin_unlock(&rcu_state.ofl_lock); > + local_irq_restore(flags); > continue; > } > > @@ -1824,8 +1826,9 @@ static noinline_for_stack bool rcu_gp_init(void) > rcu_cleanup_dead_rnp(rnp); > } > > - raw_spin_unlock_irq_rcu_node(rnp); > - raw_spin_unlock(&rcu_state.ofl_lock); > + raw_spin_unlock_rcu_node(rnp); > + arch_spin_unlock(&rcu_state.ofl_lock); > + local_irq_restore(flags); > } > rcu_gp_slow(gp_preinit_delay); /* Races with CPU hotplug. */ > > @@ -4233,7 +4236,7 @@ int rcutree_offline_cpu(unsigned int cpu) > */ > void rcu_cpu_starting(unsigned int cpu) > { > - unsigned long flags; > + unsigned long flags, seq_flags; > unsigned long mask; > struct rcu_data *rdp; > struct rcu_node *rnp; > @@ -4246,11 +4249,11 @@ void rcu_cpu_starting(unsigned int cpu) > > rnp = rdp->mynode; > mask = rdp->grpmask; > - WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1); > - WARN_ON_ONCE(!(rnp->ofl_seq & 0x1)); > + local_irq_save(seq_flags); > + arch_spin_lock(&rcu_state.ofl_lock); > rcu_dynticks_eqs_online(); > smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier(). > - raw_spin_lock_irqsave_rcu_node(rnp, flags); > + raw_spin_lock_rcu_node(rnp); > WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask); > newcpu = !(rnp->expmaskinitnext & mask); > rnp->expmaskinitnext |= mask; > @@ -4269,9 +4272,8 @@ void rcu_cpu_starting(unsigned int cpu) > } else { > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > } > - smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier(). > - WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1); > - WARN_ON_ONCE(rnp->ofl_seq & 0x1); > + arch_spin_unlock(&rcu_state.ofl_lock); > + local_irq_restore(seq_flags); > smp_mb(); /* Ensure RCU read-side usage follows above initialization. */ > } > > @@ -4285,7 +4287,7 @@ void rcu_cpu_starting(unsigned int cpu) > */ > void rcu_report_dead(unsigned int cpu) > { > - unsigned long flags; > + unsigned long flags, seq_flags; > unsigned long mask; > struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); > struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */ > @@ -4299,10 +4301,8 @@ void rcu_report_dead(unsigned int cpu) > > /* Remove outgoing CPU from mask in the leaf rcu_node structure. */ > mask = rdp->grpmask; > - WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1); > - WARN_ON_ONCE(!(rnp->ofl_seq & 0x1)); > - smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier(). > - raw_spin_lock(&rcu_state.ofl_lock); > + local_irq_save(seq_flags); > + arch_spin_lock(&rcu_state.ofl_lock); > raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */ > rdp->rcu_ofl_gp_seq = READ_ONCE(rcu_state.gp_seq); > rdp->rcu_ofl_gp_flags = READ_ONCE(rcu_state.gp_flags); > @@ -4313,10 +4313,8 @@ void rcu_report_dead(unsigned int cpu) > } > WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask); > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > - raw_spin_unlock(&rcu_state.ofl_lock); > - smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier(). > - WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1); > - WARN_ON_ONCE(rnp->ofl_seq & 0x1); > + arch_spin_unlock(&rcu_state.ofl_lock); > + local_irq_restore(seq_flags); > > rdp->cpu_started = false; > } > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > index 305cf6aeb408..aff4cc9303fb 100644 > --- a/kernel/rcu/tree.h > +++ b/kernel/rcu/tree.h > @@ -56,8 +56,6 @@ struct rcu_node { > /* Initialized from ->qsmaskinitnext at the */ > /* beginning of each grace period. */ > unsigned long qsmaskinitnext; > - unsigned long ofl_seq; /* CPU-hotplug operation sequence count. */ > - /* Online CPUs for next grace period. */ > unsigned long expmask; /* CPUs or groups that need to check in */ > /* to allow the current expedited GP */ > /* to complete. */ > @@ -358,7 +356,7 @@ struct rcu_state { > const char *name; /* Name of structure. */ > char abbr; /* Abbreviated name. */ > > - raw_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp; > + arch_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp; > /* Synchronize offline with */ > /* GP pre-initialization. */ > }; > -- > 2.31.1 >