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=-10.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=unavailable 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 BD27CC2D0B1 for ; Tue, 10 Dec 2019 04:02:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 93B97207FF for ; Tue, 10 Dec 2019 04:02:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575950544; bh=cHLTLZeys6gPXi7kdLlViUglgETxAyHwd8fFYGAZ/Iw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=Ea6hlVB5VyDR5eeJsUx51Oz5x4jRU7ynKVR5w7Z/NZzGKYme6SmDOYrMOM5zuq4Tl XGDFZRmm5XK12fm/8WuDR0DLyo0kUICHYukuaAZJus7AK5krYWzw01Mp4UWbXvIC1l /+uoWbewNNqgUitSxNiWuVEERas2I/+DC8NwVM+Y= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727143AbfLJECR (ORCPT ); Mon, 9 Dec 2019 23:02:17 -0500 Received: from mail.kernel.org ([198.145.29.99]:41606 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726959AbfLJECB (ORCPT ); Mon, 9 Dec 2019 23:02:01 -0500 Received: from paulmck-ThinkPad-P72.home (199-192-87-166.static.wiline.com [199.192.87.166]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 6B42C2465B; Tue, 10 Dec 2019 04:01:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575950519; bh=cHLTLZeys6gPXi7kdLlViUglgETxAyHwd8fFYGAZ/Iw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=IzJeNo/9m0pOYg1n5q24g4Hor/GFr9eZ+ERGWoDN5419b7lolK8RxTmyACAB3N4zu z893QFTjqDxDTn3A4idz3eT5wESuoT/0jiB8uhMrEFg1Cu/NCeqBZnbRO26mnGZFqC tu4JxAIhlXkjaNczaqHPuikDwzcxvfEmvk2vHCs4= From: paulmck@kernel.org To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.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, Neeraj Upadhyay , "Paul E . McKenney" Subject: [PATCH tip/core/rcu 06/10] rcu: Allow only one expedited GP to run concurrently with wakeups Date: Mon, 9 Dec 2019 20:01:50 -0800 Message-Id: <20191210040154.2498-6-paulmck@kernel.org> X-Mailer: git-send-email 2.9.5 In-Reply-To: <20191210040122.GA2419@paulmck-ThinkPad-P72> References: <20191210040122.GA2419@paulmck-ThinkPad-P72> Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org From: Neeraj Upadhyay The current expedited RCU grace-period code expects that a task requesting an expedited grace period cannot awaken until that grace period has reached the wakeup phase. However, it is possible for a long preemption to result in the waiting task never sleeping. For example, consider the following sequence of events: 1. Task A starts an expedited grace period by invoking synchronize_rcu_expedited(). It proceeds normally up to the wait_event() near the end of that function, and is then preempted (or interrupted or whatever). 2. The expedited grace period completes, and a kworker task starts the awaken phase, having incremented the counter and acquired the rcu_state structure's .exp_wake_mutex. This kworker task is then preempted or interrupted or whatever. 3. Task A resumes and enters wait_event(), which notes that the expedited grace period has completed, and thus doesn't sleep. 4. Task B starts an expedited grace period exactly as did Task A, complete with the preemption (or whatever delay) just before the call to wait_event(). 5. The expedited grace period completes, and another kworker task starts the awaken phase, having incremented the counter. However, it blocks when attempting to acquire the rcu_state structure's .exp_wake_mutex because step 2's kworker task has not yet released it. 6. Steps 4 and 5 repeat, resulting in overflow of the rcu_node structure's ->exp_wq[] array. In theory, this is harmless. Tasks waiting on the various ->exp_wq[] array will just be spuriously awakened, but they will just sleep again on noting that the rcu_state structure's ->expedited_sequence value has not advanced far enough. In practice, this wastes CPU time and is an accident waiting to happen. This commit therefore moves the rcu_exp_gp_seq_end() call that officially ends the expedited grace period (along with associate tracing) until after the ->exp_wake_mutex has been acquired. This prevents Task A from awakening prematurely, thus preventing more than one expedited grace period from being in flight during a previous expedited grace period's wakeup phase. Fixes: 3b5f668e715b ("rcu: Overlap wakeups with next expedited grace period") Signed-off-by: Neeraj Upadhyay [ paulmck: Added updated comment. ] Signed-off-by: Paul E. McKenney --- kernel/rcu/tree_exp.h | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index fa143e4..7a1f093 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -539,14 +539,13 @@ static void rcu_exp_wait_wake(unsigned long s) struct rcu_node *rnp; synchronize_sched_expedited_wait(); - rcu_exp_gp_seq_end(); - trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end")); - /* - * Switch over to wakeup mode, allowing the next GP, but -only- the - * next GP, to proceed. - */ + // Switch over to wakeup mode, allowing the next GP to proceed. + // End the previous grace period only after acquiring the mutex + // to ensure that only one GP runs concurrently with wakeups. mutex_lock(&rcu_state.exp_wake_mutex); + rcu_exp_gp_seq_end(); + trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end")); rcu_for_each_node_breadth_first(rnp) { if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) { -- 2.9.5