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McKenney" , Andrew Morton , Naoya Horiguchi , Miaohe Lin , wangkefeng.wang@huawei.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm: memory-failure: use rcu lock instead of tasklist_lock when collect_procs() Message-ID: References: <20230821091312.2034844-1-tongtiangen@huawei.com> <0bbbb7d8-699b-30ac-9657-840112c41a78@huawei.com> <20230825060221.GA3948311@ik1-406-35019.vs.sakura.ne.jp> <9e205429-1f27-4d18-faca-8a4fe9d429e3@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <9e205429-1f27-4d18-faca-8a4fe9d429e3@huawei.com> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 76D2E14000D X-Stat-Signature: j6u6wjg4ezmu9rfeujf85qnsrehwakmx X-Rspam-User: X-HE-Tag: 1693081726-826139 X-HE-Meta: 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 uM4zwEwl zhz5VBYo1HJKW42ArLax2s/4HU+eaYyHF+XQhdTv+iJaVFTqxSYKYhaZg2tFJnQaIec/pyF7F7T9W+4WCxDf/uLqCMB7S7rwSgLKrTULih9VTS03WnI34VMIGrvyxJc34kqGSuCULc4VTkrSVdRoS0D6/DqvLXWHX85+kvsoJdQ8J3bPAdARtCRxoBg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000004, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, Aug 26, 2023 at 09:46:53AM +0800, Tong Tiangen wrote: > " the ``task_struct`` object is freed only after one or more > grace periods elapse, with the help of call_rcu(), which is invoked via > put_task_struct_rcu_user(). " > > Combined with the code,when the task exits： > > release_task() > __exit_signal() > __unhash_process() > list_del_rcu(&p->tasks) > > put_task_struct_rcu_user() > call_rcu(&task->rcu, delayed_put_task_struct); > > delayed_put_task_struct() > put_task_struct() > if (refcount_sub_and_test(nr, &t->usage)) > __put_task_struct() > free_task() > > The code is consistent with the description in the document. > > According to this understanding, i think for_each_process() under the > protection of rcu locl is safe, that is, task_struct in the list will not be > destroyed, and get_task_struct() is also safe. Aha! This is different from the usual pattern. What I'm used to seeing is: if (refcount_sub_and_test()) { list_del_rcu(); rcu_free(); } and then on the read side you need a refcount_inc_not_zero(), which we didn't have here. Given this new information you've found, I withdraw my objection. It'd be nice to include some of this analysis in an updated changelog (and maybe improved documentation for tasklist?).