From patchwork Mon Aug 2 13:46:19 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Zhen Lei X-Patchwork-Id: 490340 Delivered-To: patch@linaro.org Received: by 2002:a05:6638:1185:0:0:0:0 with SMTP id f5csp1940463jas; Mon, 2 Aug 2021 06:48:02 -0700 (PDT) X-Google-Smtp-Source: ABdhPJwL97GrecQfZLcMKJFtsXWqNZQzhoIr/kZLtNVVgmtWHH4/O9XYhgrNpEo8OVFKbUnoLNuj X-Received: by 2002:a92:d083:: with SMTP id h3mr607217ilh.157.1627912082298; Mon, 02 Aug 2021 06:48:02 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1627912082; cv=none; d=google.com; s=arc-20160816; b=UhI770vWDCYgaaHxMCIyCdsX/A/2eM2MF8LyM2S481dTr/5hFimnkVqlCea2izmj41 oRXvjVjkEqzLKdu92/GPjxBZnOQFWsVNg+jt0CMgaybnFPn4D7liaNeRkermkiqPdCTG SL/jx5kuzb1lj8A3ZogA36IyI8Ma5jwYdjvwlWQXdEjcxb8Lab+DXf0VzKcCMHG8OTxF HeVSFIAqDKysqcm3+k6CtwuLxFz7arXpF4ARg17HfyVoU8ofvuBujpbu756M9HnuxFMd sSqbXNeoy+oLpQehNqgkiCvZ2LOCS1yXZoKyiCJLKQTCZC4XuGCamB5aOJUuxzigHTSy CihA== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:content-transfer-encoding:mime-version :references:in-reply-to:message-id:date:subject:cc:to:from; bh=LV75ZxDJp5PItjneTAOAp8r2rnrtZWx9tCc3eIJRIPI=; b=LTlawlsG/jK+22QTRQgcgAF6P/ey4hXyhOeAbynOh/RthAEMUqYtRY1ivoftbZiCyg v9yJkLra530OaPtjbYIUPpyf0SVVCjey80+XiK6ofbTknrxrk+R1j+z6rQa7hKuouPgY 72Oy8jZql0VNQ32mlKseqVU2e7esrh0YBLXaK9IOsEttB1fF4v+/EpFAArLMGpFbDC55 /bdft7nT3/GhG/w7h/AEuWENhD4O9LsjkErdbxmg0FyShDrOoSFyWfrOnibHiaxuMQNj cTjw9wcnWeHPp+V3B82R7UQivojsYdsPrdoEe2tHEuYRvPYVHbpaL7zsYvgjIlT2ineH lfSg== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of stable-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=stable-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=huawei.com Return-Path: Received: from vger.kernel.org (vger.kernel.org. [23.128.96.18]) by mx.google.com with ESMTP id m9si12547472jaj.89.2021.08.02.06.48.02; Mon, 02 Aug 2021 06:48:02 -0700 (PDT) Received-SPF: pass (google.com: domain of stable-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of stable-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=stable-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=huawei.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234109AbhHBNsI (ORCPT + 12 others); Mon, 2 Aug 2021 09:48:08 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:16033 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234358AbhHBNrg (ORCPT ); Mon, 2 Aug 2021 09:47:36 -0400 Received: from dggemv711-chm.china.huawei.com (unknown [172.30.72.56]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4GdfNb1kVWzZwdR; Mon, 2 Aug 2021 21:43:43 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggemv711-chm.china.huawei.com (10.1.198.66) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Mon, 2 Aug 2021 21:47:15 +0800 Received: from thunder-town.china.huawei.com (10.174.179.0) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Mon, 2 Aug 2021 21:47:14 +0800 From: Zhen Lei To: Greg Kroah-Hartman , stable CC: Zhen Lei , Anna-Maria Gleixner , Mike Galbraith , Sasha Levin , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , linux-kernel Subject: [PATCH 4.4 06/11] futex: Futex_unlock_pi() determinism Date: Mon, 2 Aug 2021 21:46:19 +0800 Message-ID: <20210802134624.1934-7-thunder.leizhen@huawei.com> X-Mailer: git-send-email 2.26.0.windows.1 In-Reply-To: <20210802134624.1934-1-thunder.leizhen@huawei.com> References: <20210802134624.1934-1-thunder.leizhen@huawei.com> MIME-Version: 1.0 X-Originating-IP: [10.174.179.0] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Peter Zijlstra [ Upstream commit bebe5b514345f09be2c15e414d076b02ecb9cce8 ] The problem with returning -EAGAIN when the waiter state mismatches is that it becomes very hard to proof a bounded execution time on the operation. And seeing that this is a RT operation, this is somewhat important. While in practise; given the previous patch; it will be very unlikely to ever really take more than one or two rounds, proving so becomes rather hard. However, now that modifying wait_list is done while holding both hb->lock and wait_lock, the scenario can be avoided entirely by acquiring wait_lock while still holding hb-lock. Doing a hand-over, without leaving a hole. Signed-off-by: Peter Zijlstra (Intel) Cc: juri.lelli@arm.com Cc: bigeasy@linutronix.de Cc: xlpang@redhat.com Cc: rostedt@goodmis.org Cc: mathieu.desnoyers@efficios.com Cc: jdesfossez@efficios.com Cc: dvhart@infradead.org Cc: bristot@redhat.com Link: http://lkml.kernel.org/r/20170322104152.112378812@infradead.org Signed-off-by: Thomas Gleixner Signed-off-by: Zhen Lei --- kernel/futex.c | 24 +++++++++++------------- 1 file changed, 11 insertions(+), 13 deletions(-) -- 2.26.0.106.g9fadedd diff --git a/kernel/futex.c b/kernel/futex.c index 45f00a2fb59c554..8f6372d3a1feea0 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1555,15 +1555,10 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_ WAKE_Q(wake_q); int ret = 0; - raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); - if (!new_owner) { + if (WARN_ON_ONCE(!new_owner)) { /* - * Since we held neither hb->lock nor wait_lock when coming - * into this function, we could have raced with futex_lock_pi() - * such that we might observe @this futex_q waiter, but the - * rt_mutex's wait_list can be empty (either still, or again, - * depending on which side we land). + * As per the comment in futex_unlock_pi() this should not happen. * * When this happens, give up our locks and try again, giving * the futex_lock_pi() instance time to complete, either by @@ -3020,15 +3015,18 @@ retry: if (pi_state->owner != current) goto out_unlock; + get_pi_state(pi_state); /* - * Grab a reference on the pi_state and drop hb->lock. + * Since modifying the wait_list is done while holding both + * hb->lock and wait_lock, holding either is sufficient to + * observe it. * - * The reference ensures pi_state lives, dropping the hb->lock - * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to - * close the races against futex_lock_pi(), but in case of - * _any_ fail we'll abort and retry the whole deal. + * By taking wait_lock while still holding hb->lock, we ensure + * there is no point where we hold neither; and therefore + * wake_futex_pi() must observe a state consistent with what we + * observed. */ - get_pi_state(pi_state); + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); spin_unlock(&hb->lock); ret = wake_futex_pi(uaddr, uval, pi_state);