From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 3B66B185091; Wed, 26 Jun 2024 14:28:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719412131; cv=none; b=ZmiEZID+vl8hSbpCBkBaCFnrLA3vwu7mGKuo8OAVp8+1H0xab8c9XvmPa556bz6JIr+E4ygJMiHeVNiElszTeKb5RSm4EWe5vZeUBeWbMvbE19g6ficIu/A6ri1WDhIgp+m1tR8rLy4OzC/rqasbVmfgQmYxc2ky0Nt8OaDd94k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719412131; c=relaxed/simple; bh=ZWHIPidE5USRmxsE4CrRem4oMewJdmGkXzT3eWY/mYM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kEkhahxEVpMURpBftWU+sHdBBc5ikiL73kBYE0Quu2oT8BNZJnzrA01FjO7iiffmSjPZ5DTzUh9TOc/svZe29Zm7JXBr4ispaHBxiEm8nlwCotyB1BFwGUI/xBwMVxgACHh6eSQVxDsw3XswSAbTUxzH9EPjDu/Z+weNjxPKVJo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Fb6FG/TY; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Fb6FG/TY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5B157C116B1; Wed, 26 Jun 2024 14:28:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1719412130; bh=ZWHIPidE5USRmxsE4CrRem4oMewJdmGkXzT3eWY/mYM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Fb6FG/TYxVDDlbbui5Np51JXNUtkbQS/mkh2aiI/1Uz76L+is3MtCw4IkHfcCCYgD kBz8cw5urxKboeVEflAyv+RA4g3sP2yOgsDTPxWbgtj5r3CutK7Oznl6zaFw7d3KGx 2i9HnGsihjO45hWvb1v+bHuUuT62Q37mOBhfI8MHoNgjZ4dazH4ixuI5ZZh2ddVDNO hQ+b+ZjQ7q8lpbDy3//FyN+38UJy2xZWc7sIrRFOTx9I/Hk5mh4x6RRZt+WC6yKylR iDysWLW/OiH+UWv2Msl/P8Iy0t5futTeA+XE0DxiZsJz4EKQDhCjydzdH3oVfj7TmZ hGMCbeC7jVRUA== Date: Wed, 26 Jun 2024 16:28:48 +0200 From: Frederic Weisbecker To: Neeraj upadhyay Cc: "Paul E. McKenney" , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org Subject: Re: [PATCH rcu 3/6] rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot Message-ID: References: <81f4e616-8125-4934-a8e2-fd5beae90995@paulmck-laptop> <20240604222652.2370998-3-paulmck@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Le Wed, Jun 12, 2024 at 02:14:14PM +0530, Neeraj upadhyay a écrit : > On Wed, Jun 5, 2024 at 3:58 AM Paul E. McKenney wrote: > > > > From: Frederic Weisbecker > > > > When the grace period kthread checks the extended quiescent state > > counter of a CPU, full ordering is necessary to ensure that either: > > > > * If the GP kthread observes the remote target in an extended quiescent > > state, then that target must observe all accesses prior to the current > > grace period, including the current grace period sequence number, once > > it exits that extended quiescent state. > > > > or: > > > > * If the GP kthread observes the remote target NOT in an extended > > quiescent state, then the target further entering in an extended > > quiescent state must observe all accesses prior to the current > > grace period, including the current grace period sequence number, once > > it enters that extended quiescent state. > > > > This ordering is enforced through a full memory barrier placed right > > before taking the first EQS snapshot. However this is superfluous > > because the snapshot is taken while holding the target's rnp lock which > > provides the necessary ordering through its chain of > > smp_mb__after_unlock_lock(). > > > > Remove the needless explicit barrier before the snapshot and put a > > comment about the implicit barrier newly relied upon here. > > > > Signed-off-by: Frederic Weisbecker > > Signed-off-by: Paul E. McKenney > > --- > > kernel/rcu/tree_exp.h | 8 +++++++- > > 1 file changed, 7 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index 8a1d9c8bd9f74..bec24ea6777e8 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -357,7 +357,13 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp) > > !(rnp->qsmaskinitnext & mask)) { > > mask_ofl_test |= mask; > > } else { > > - snap = rcu_dynticks_snap(cpu); > > + /* > > + * Full ordering against accesses prior current GP and > > + * also against current GP sequence number is enforced > > + * by current rnp locking with chained > > + * smp_mb__after_unlock_lock(). > > Again, worth mentioning the chaining sites sync_exp_reset_tree() and > this function? How about this? /* * Full ordering against accesses prior current GP and also against * current GP sequence number is enforced by rcu_seq_start() implicit * barrier, relayed by kworkers locking and even further by * smp_mb__after_unlock_lock() barriers chained all the way throughout * the rnp locking tree since sync_exp_reset_tree() and up to the current * leaf rnp locking. */ Thanks. > > > Thanks > Neeraj > > > + */ > > + snap = ct_dynticks_cpu_acquire(cpu); > > if (rcu_dynticks_in_eqs(snap)) > > mask_ofl_test |= mask; > > else > > -- > > 2.40.1 > > > >