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From: Muchun Song In-Reply-To: <20220820135955.1520aa480fe04ab31d4fce1f@linux-foundation.org> Date: Wed, 24 Aug 2022 14:59:45 +0800 Cc: Kefeng Wang , Linux MM , Qian Cai , linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: References: <20220820032506.126860-1-wangkefeng.wang@huawei.com> <1E87F09C-4904-49E2-B45C-C408DD5F6F62@linux.dev> <20220820135955.1520aa480fe04ab31d4fce1f@linux-foundation.org> To: Andrew Morton X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661324424; a=rsa-sha256; cv=none; b=0iSHtjduoGHsJRWADksbeJdMHHOOdAhCin8fhmoZ9ipKRTTziLzep1o62yp2nUCK5waNWR 5GKv/7j2C8ujP8/T4xWInPww1MeFUbbeICVxsUvmAzauilaJBTh8kljPzR3S+xX68Wpo8v UOL+PEu8oaSdE00MoTxQbrQybeQKu3E= ARC-Authentication-Results: i=1; imf09.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=NrZDTTvp; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf09.hostedemail.com: domain of muchun.song@linux.dev designates 94.23.1.103 as permitted sender) smtp.mailfrom=muchun.song@linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661324424; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=8wTEkRmEhfEAMOHgr9KTE38V3eHehsSikHxHeXkBHng=; b=irEtNgXWF3H+Qv24l6Nh1MMW08sgYFISE4lGRxUTqavWpMYf/Hf1FyGn0jMKP1JPDLBR8H LylGuZpQWXdS1IPg8Oi2EiW2V9S1MqZ8YeqvupiSFYpRjI9vMsZ6U2cp7mWBlAQ0eBRvkS NLdNk7mGgSR1IxoXFB0NhT6X8LehTts= X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 716A6140043 X-Rspam-User: Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=NrZDTTvp; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf09.hostedemail.com: domain of muchun.song@linux.dev designates 94.23.1.103 as permitted sender) smtp.mailfrom=muchun.song@linux.dev X-Stat-Signature: 944cdrbrpwwipbg7hjuwww57mbioabdz X-HE-Tag: 1661324424-671782 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > On Aug 21, 2022, at 04:59, Andrew Morton = wrote: >=20 > On Sat, 20 Aug 2022 15:33:04 +0800 Muchun Song = wrote: >=20 >>=20 >>=20 >>> + if (IS_ERR(t)) { >>> /* failure at boot is fatal */ >>> BUG_ON(system_state < SYSTEM_RUNNING); >>> pr_err("Failed to start kswapd on node %d\n", nid); >>> - pgdat->kswapd =3D NULL; >>> + WRITE_ONCE(pgdat->kswapd, NULL); >>> + } else { >>> + WRITE_ONCE(pgdat->kswapd, t); >>> } >>> } >>=20 >> IIUC, the race is like the followings: >>=20 >> CPU 0: CPU 1: >>=20 >> kswapd_run() >> pgdat->kswapd =3D kthread_run() >> if (IS_ERR(pgdat->kswapd)) >> kswapd_is_running >> // load pgdat->kswapd = and it is NOT NULL. >> pgdat->kswapd =3D NULL >> = task_is_running(pgdat->kswapd); // NULL pointer dereference >>=20 >=20 > But don't we still have a bug? Sure, kswapd_is_running() will no > longer deref a null pointer. But it now runs kswapd_is_running() > against a task which has exited - a use-after-free? >=20 Agree. I missed that. Thanks, Muchun