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[47.88.44.36]) by mx.google.com with ESMTPS id v35si27162266plg.187.2019.04.16.18.05.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Apr 2019 18:05:04 -0700 (PDT) Received-SPF: pass (google.com: domain of jiufei.xue@linux.alibaba.com designates 47.88.44.36 as permitted sender) client-ip=47.88.44.36; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jiufei.xue@linux.alibaba.com designates 47.88.44.36 as permitted sender) smtp.mailfrom=jiufei.xue@linux.alibaba.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=alibaba.com X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R201e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04423;MF=jiufei.xue@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0TPVWJz1_1555463088; Received: from ali-186590e05fa3.local(mailfrom:jiufei.xue@linux.alibaba.com fp:SMTPD_---0TPVWJz1_1555463088) by smtp.aliyun-inc.com(127.0.0.1); Wed, 17 Apr 2019 09:04:49 +0800 Subject: Re: [PATCH v2] fs/fs-writeback: wait isw_nr_in_flight to be zero when umount To: Tejun Heo Cc: cgroups@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, joseph.qi@linux.alibaba.com References: <20190416120902.18616-1-jiufei.xue@linux.alibaba.com> <20190416150415.GB374014@devbig004.ftw2.facebook.com> From: Jiufei Xue Message-ID: Date: Wed, 17 Apr 2019 09:04:48 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190416150415.GB374014@devbig004.ftw2.facebook.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit 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: Hi Tejun, On 2019/4/16 下午11:04, Tejun Heo wrote: > Hello, Jiufei. > > On Tue, Apr 16, 2019 at 08:09:02PM +0800, Jiufei Xue wrote: >> synchronize_rcu() didn't wait for call_rcu() callbacks, so inode wb >> switch may not go to the workqueue after synchronize_rcu(). Thus >> previous scheduled switches was not finished even flushing the >> workqueue, which will cause a NULL pointer dereferenced followed below. > > Isn't all that's needed replacing the synchronize_rcu() call with a > rcu_barrier() call? > Yes, it can be fixed if we replace synchronize_rcu() with rcu_barrier(). However, I'm worried that rcu_barrier() is too heavyweight and we have encountered some hung tasks that rcu_barrier() waiting for callbacks that other drivers queued but not handled correctly. Thanks, Jiufei > Thanks. >