From patchwork Tue Dec 18 22:10:42 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sebastian Andrzej Siewior X-Patchwork-Id: 10736389 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id AEB056C2 for ; Tue, 18 Dec 2018 22:12:44 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 9BDB528537 for ; Tue, 18 Dec 2018 22:12:44 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 8F5AD2AFE8; Tue, 18 Dec 2018 22:12:44 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED autolearn=ham version=3.3.1 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id EDC8628537 for ; Tue, 18 Dec 2018 22:12:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Message-Id:Date:Subject:To:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=FrgL1HH3RtSrPLsg9PfWTEBAtu6KgKlJH/PSzGfKp9U=; b=nrw2NGkG84mMlp yQqdvuH7KsMz4nW0GHtQBmJ0MbLFDgC6JtsIs+spqHRqPXjZsCRy/wixqrexo5kUzNEBXG7f8AX5J 08ng8BxQtMliSANQNsOvcqezPlWH7Rw/ujjtegRUN+x+tcipe/838S6/aLlu59Rmkex/oXwiFnmaF hZKQUBidsp1Lmy/E9hT3gxu1RsUQOZJpfiBX4OqGsqPzMAKfQzGEkiW9V6e6kKFHro7+sIOe0C6wo riO9cKzNDBHPu4Hk1GLpK3fw/iZquxYF8FA2YogKADsIih4VICh4gbyJUhENj8mLRdA6acgKxO1nA rMLXMKq3ygX1PVBID3YA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gZNbV-0005Pw-Vi; Tue, 18 Dec 2018 22:12:41 +0000 Received: from galois.linutronix.de ([2a01:7a0:2:106d:700::1]) by bombadil.infradead.org with esmtps (Exim 4.90_1 #2 (Red Hat Linux)) id 1gZNac-0004WK-IC for linux-arm-kernel@lists.infradead.org; Tue, 18 Dec 2018 22:11:57 +0000 Received: from localhost ([127.0.0.1] helo=bazinga.breakpoint.cc) by Galois.linutronix.de with esmtp (Exim 4.80) (envelope-from ) id 1gZNaB-000889-Lg; Tue, 18 Dec 2018 23:11:20 +0100 From: Sebastian Andrzej Siewior To: stable@vger.kernel.org Subject: [PATCH STABLE v4.9 03/10] locking/qspinlock: Bound spinning on pending->locked transition in slowpath Date: Tue, 18 Dec 2018 23:10:42 +0100 Message-Id: <20181218221049.6816-4-bigeasy@linutronix.de> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20181218221049.6816-1-bigeasy@linutronix.de> References: <20181218221049.6816-1-bigeasy@linutronix.de> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20181218_141147_193575_DADDBC03 X-CRM114-Status: GOOD ( 14.76 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Zijlstra , bigeasy@linutronix.de, Will Deacon , Ingo Molnar , Daniel Wagner , Waiman Long , Thomas Gleixner , paulmck@linux.vnet.ibm.com, Linus Torvalds , boqun.feng@gmail.com, linux-arm-kernel@lists.infradead.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org X-Virus-Scanned: ClamAV using ClamSMTP From: Will Deacon commit 6512276d97b160d90b53285bd06f7f201459a7e3 upstream. If a locker taking the qspinlock slowpath reads a lock value indicating that only the pending bit is set, then it will spin whilst the concurrent pending->locked transition takes effect. Unfortunately, there is no guarantee that such a transition will ever be observed since concurrent lockers could continuously set pending and hand over the lock amongst themselves, leading to starvation. Whilst this would probably resolve in practice, it means that it is not possible to prove liveness properties about the lock and means that lock acquisition time is unbounded. Rather than removing the pending->locked spinning from the slowpath altogether (which has been shown to heavily penalise a 2-threaded locking stress test on x86), this patch replaces the explicit spinning with a call to atomic_cond_read_relaxed and allows the architecture to provide a bound on the number of spins. For architectures that can respond to changes in cacheline state in their smp_cond_load implementation, it should be sufficient to use the default bound of 1. Suggested-by: Waiman Long Signed-off-by: Will Deacon Acked-by: Peter Zijlstra (Intel) Acked-by: Waiman Long Cc: Linus Torvalds Cc: Thomas Gleixner Cc: boqun.feng@gmail.com Cc: linux-arm-kernel@lists.infradead.org Cc: paulmck@linux.vnet.ibm.com Link: http://lkml.kernel.org/r/1524738868-31318-4-git-send-email-will.deacon@arm.com Signed-off-by: Ingo Molnar Signed-off-by: Sebastian Andrzej Siewior --- kernel/locking/qspinlock.c | 20 +++++++++++++++++--- 1 file changed, 17 insertions(+), 3 deletions(-) diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 6fce84401dba1..a8da1fc5222eb 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -75,6 +75,18 @@ #define MAX_NODES 4 #endif +/* + * The pending bit spinning loop count. + * This heuristic is used to limit the number of lockword accesses + * made by atomic_cond_read_relaxed when waiting for the lock to + * transition out of the "== _Q_PENDING_VAL" state. We don't spin + * indefinitely because there's no guarantee that we'll make forward + * progress. + */ +#ifndef _Q_PENDING_LOOPS +#define _Q_PENDING_LOOPS 1 +#endif + /* * Per-CPU queue node structures; we can never have more than 4 nested * contexts: task, softirq, hardirq, nmi. @@ -422,13 +434,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) return; /* - * wait for in-progress pending->locked hand-overs + * Wait for in-progress pending->locked hand-overs with a bounded + * number of spins so that we guarantee forward progress. * * 0,1,0 -> 0,0,1 */ if (val == _Q_PENDING_VAL) { - while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL) - cpu_relax(); + int cnt = _Q_PENDING_LOOPS; + val = smp_cond_load_acquire(&lock->val.counter, + (VAL != _Q_PENDING_VAL) || !cnt--); } /*