From patchwork Thu Apr 15 17:27:11 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Ali Saidi X-Patchwork-Id: 423285 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=-18.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER, INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham 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 CF3EBC433B4 for ; Thu, 15 Apr 2021 17:27:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A141D610F7 for ; Thu, 15 Apr 2021 17:27:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233520AbhDOR2V (ORCPT ); Thu, 15 Apr 2021 13:28:21 -0400 Received: from smtp-fw-33001.amazon.com ([207.171.190.10]:32430 "EHLO smtp-fw-33001.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233343AbhDOR2U (ORCPT ); Thu, 15 Apr 2021 13:28:20 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1618507678; x=1650043678; h=from:to:cc:subject:date:message-id:mime-version: content-transfer-encoding; bh=h6nyQCQiH/sJBYBieZaGVxNQkuHLEIB52xWGhkpFXi8=; b=mBPlGx6MKbRBes0ySiQOEZ6vtQHNx/hmb2l0u+3uBaDmn0T1tMvIR0k/ d9+4VC7CkqXSZHBg63pJqbMRTvjrOH+eBw6c8OxAYfYeuZ5U6c4ACknOd T7xQ11+ViuEhKryLpNTXp4zaVzMO4lK2aApQIquB+vmX9F/KXG/xaCPII A=; X-IronPort-AV: E=Sophos;i="5.82,225,1613433600"; d="scan'208";a="118783787" Received: from pdx4-co-svc-p1-lb2-vlan2.amazon.com (HELO email-inbound-relay-2c-397e131e.us-west-2.amazon.com) ([10.25.36.210]) by smtp-border-fw-out-33001.sea14.amazon.com with ESMTP; 15 Apr 2021 17:27:57 +0000 Received: from EX13MTAUWC002.ant.amazon.com (pdx1-ws-svc-p6-lb9-vlan3.pdx.amazon.com [10.236.137.198]) by email-inbound-relay-2c-397e131e.us-west-2.amazon.com (Postfix) with ESMTPS id 98A15A258B; Thu, 15 Apr 2021 17:27:54 +0000 (UTC) Received: from EX13D02UWC003.ant.amazon.com (10.43.162.199) by EX13MTAUWC002.ant.amazon.com (10.43.162.240) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Thu, 15 Apr 2021 17:27:52 +0000 Received: from EX13MTAUWC001.ant.amazon.com (10.43.162.135) by EX13D02UWC003.ant.amazon.com (10.43.162.199) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Thu, 15 Apr 2021 17:27:52 +0000 Received: from dev-dsk-alisaidi-i31e-9f3421fe.us-east-1.amazon.com (10.200.138.153) by mail-relay.amazon.com (10.43.162.232) with Microsoft SMTP Server id 15.0.1497.2 via Frontend Transport; Thu, 15 Apr 2021 17:27:52 +0000 Received: by dev-dsk-alisaidi-i31e-9f3421fe.us-east-1.amazon.com (Postfix, from userid 5131138) id 27122228E4; Thu, 15 Apr 2021 17:27:51 +0000 (UTC) From: Ali Saidi To: CC: , , , , , Will Deacon , Peter Zijlstra , Ingo Molnar , Waiman Long , Boqun Feng Subject: [PATCH v2] locking/qrwlock: Fix ordering in queued_write_lock_slowpath Date: Thu, 15 Apr 2021 17:27:11 +0000 Message-ID: <20210415172711.15480-1-alisaidi@amazon.com> X-Mailer: git-send-email 2.24.4.AMZN MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org While this code is executed with the wait_lock held, a reader can acquire the lock without holding wait_lock. The writer side loops checking the value with the atomic_cond_read_acquire(), but only truly acquires the lock when the compare-and-exchange is completed successfully which isn’t ordered. This exposes the window between the acquire and the cmpxchg to an A-B-A problem which allows reads following the lock acquisition to observe values speculatively before the write lock is truly acquired. We've seen a problem in epoll where the reader does a xchg while holding the read lock, but the writer can see a value change out from under it. Writer | Reader 2 -------------------------------------------------------------------------------- ep_scan_ready_list() | |- write_lock_irq() | |- queued_write_lock_slowpath() | |- atomic_cond_read_acquire() | | read_lock_irqsave(&ep->lock, flags); --> (observes value before unlock)| chain_epi_lockless() | | epi->next = xchg(&ep->ovflist, epi); | | read_unlock_irqrestore(&ep->lock, flags); | | | atomic_cmpxchg_relaxed() | |-- READ_ONCE(ep->ovflist); | A core can order the read of the ovflist ahead of the atomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire semantics addresses this issue at which point the atomic_cond_read can be switched to use relaxed semantics. Fixes: b519b56e378ee ("locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwlock") Signed-off-by: Ali Saidi Cc: stable@vger.kernel.org Acked-by: Will Deacon Tested-by: Steve Capper Reviewed-by: Steve Capper --- kernel/locking/qrwlock.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c index 4786dd271b45..10770f6ac4d9 100644 --- a/kernel/locking/qrwlock.c +++ b/kernel/locking/qrwlock.c @@ -73,8 +73,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock) /* When no more readers or writers, set the locked flag */ do { - atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING); - } while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING, + atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING); + } while (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING, _QW_LOCKED) != _QW_WAITING); unlock: arch_spin_unlock(&lock->wait_lock);