From patchwork Tue Jun 4 22:26:49 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. McKenney" X-Patchwork-Id: 13685930 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 A6F6414B092; Tue, 4 Jun 2024 22:26:54 +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=1717540014; cv=none; b=nzVjG/877BLyfo6Y6U7w3KVgGFR/Dl9OOwirqxEtZDjMJRfXxRCwmsqanLontfIYJFJm6PyV9YtQu2THlBFxfN8uhUnXB2UgC+O9tEpVhrCtWbFtRdGNeDZYRejq1WjGj4kTm+iUOjpj1aUUs2C57AonrV047EzI0Pz5/fHdZCg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1717540014; c=relaxed/simple; bh=TioFm82Snd8j9pH8jLZE3PHQ5qk2yV0osr+xyGd7g+c=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=atJgD3z4OlRkCA1NYNDOGafuHOtIjFTj3QCKMF9aOwmFf5LhTk7H93IpIOkw21RvXYvnOd/ZKTSCO7qYv4GJMMA7jhwsV8pzejG8jYiv2alKejhaBayzNrK33xtctaciiogbQ6rrwGM4CwEWiD6j407pFd52ym+U0vMr+j3dCkY= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=sGai/H0a; 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="sGai/H0a" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5631BC4AF08; Tue, 4 Jun 2024 22:26:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1717540014; bh=TioFm82Snd8j9pH8jLZE3PHQ5qk2yV0osr+xyGd7g+c=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=sGai/H0ahb0euEOElAvwYeBDGe9Jz07a85uy4SPB+l6ilTdewzdtN5Ae4hLz4OI+j I2GqRslKomd29+Tmkzt6rrykXqrevKlx5MJnvHL5bOROOoBT8k4kBeGGv8FCJgF3OG YBFxN5on/nWq/stc5hGRnSuOhvyl1YQDKE/VcgYOIbxfqbWQIN+kYk51IYMcCYvkg3 Y8k4H6jedHeqI3RYTeXCltZM6pwzXrla3xfHIo8jHnCjgCrAsnHc+Kg5A1i6tNJ5jw zNoGl1Gk27gYNYooerRsbrmx909YHi1YeiXdB994wWE8uY3VqchImLi18fEtViVyUG TodZ+5mr+0Ixg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id F0EB8CE3F26; Tue, 4 Jun 2024 15:26:53 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, Frederic Weisbecker , "Paul E . McKenney" Subject: [PATCH rcu 3/6] rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot Date: Tue, 4 Jun 2024 15:26:49 -0700 Message-Id: <20240604222652.2370998-3-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <81f4e616-8125-4934-a8e2-fd5beae90995@paulmck-laptop> References: <81f4e616-8125-4934-a8e2-fd5beae90995@paulmck-laptop> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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(). + */ + snap = ct_dynticks_cpu_acquire(cpu); if (rcu_dynticks_in_eqs(snap)) mask_ofl_test |= mask; else