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From: Naoya Horiguchi To: Tong Tiangen Cc: Andrew Morton , Naoya Horiguchi , Miaohe Lin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, wangkefeng.wang@huawei.com, Guohanjun Subject: Re: [RFC PATCH -next] mm: fix softlockup by replacing tasklist_lock with RCU in for_each_process() Message-ID: <20230821005610.GA2251898@ik1-406-35019.vs.sakura.ne.jp> References: <20230815130154.1100779-1-tongtiangen@huawei.com> <20230817053630.GA461822@ik1-406-35019.vs.sakura.ne.jp> <4b91e2c7-cd54-d5e9-472e-6ece369b0651@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4b91e2c7-cd54-d5e9-472e-6ece369b0651@huawei.com> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: F24C780014 X-Stat-Signature: irbi539kt3iofwu6kmnz4iuq4cndsopb X-HE-Tag: 1692579380-292182 X-HE-Meta: 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 pZAvhJHY WO0kg7B8BaNdvEfD0rAYNgKAX3b1KdW4lfXotRY9sEl4agmGTan3yXlv1nIQLEHbpxI34wTj8imF/aG3CdwZr+XdadaLU39CHVNQ8SnL8BcR1LfQNt9sddf3HOA== 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 Fri, Aug 18, 2023 at 05:26:34PM +0800, Tong Tiangen wrote: > > > 在 2023/8/17 13:36, Naoya Horiguchi 写道: > > On Tue, Aug 15, 2023 at 09:01:54PM +0800, Tong Tiangen wrote: > > > We found a softlock issue in our test, analyzed the logs, and found that > > > the relevant CPU call trace as follows: > > > > > > CPU0: > > > _do_fork > > > -> copy_process() > > > -> write_lock_irq(&tasklist_lock) //Disable irq,waiting for > > > //tasklist_lock > > > > > > CPU1: > > > wp_page_copy() > > > ->pte_offset_map_lock() > > > -> spin_lock(&page->ptl); //Hold page->ptl > > > -> ptep_clear_flush() > > > -> flush_tlb_others() ... > > > -> smp_call_function_many() > > > -> arch_send_call_function_ipi_mask() > > > -> csd_lock_wait() //Waiting for other CPUs respond > > > //IPI > > > > > > CPU2: > > > collect_procs_anon() > > > -> read_lock(&tasklist_lock) //Hold tasklist_lock > > > ->for_each_process(tsk) > > > -> page_mapped_in_vma() > > > -> page_vma_mapped_walk() > > > -> map_pte() > > > ->spin_lock(&page->ptl) //Waiting for page->ptl > > > > > > We can see that CPU1 waiting for CPU0 respond IPI，CPU0 waiting for CPU2 > > > unlock tasklist_lock, CPU2 waiting for CPU1 unlock page->ptl. As a result, > > > softlockup is triggered. > > > > > > For collect_procs_anon(), we will not modify the tasklist, but only perform > > > read traversal. Therefore, we can use rcu lock instead of spin lock > > > tasklist_lock, from this, we can break the softlock chain above. > > > > > > The same logic can also be applied to: > > > - collect_procs_file() > > > - collect_procs_fsdax() > > > - collect_procs_ksm() > > > - find_early_kill_thread() > > > > > > Signed-off-by: Tong Tiangen > > > > Hello Tiangen, thank you for finding the issue. > > mm/filemap.c mentions tasklist_lock in the comment about locking order, > > > > * ->i_mmap_rwsem > > * ->tasklist_lock (memory_failure, collect_procs_ao) > > > > so you can update this together? > > Otherwise looks good to me. > > > > Thanks, > > Naoya Horiguchi > > Thank you for your reply. Since tasklist_lock is no longer used in > collect_procs_xxx(), Should I delete these two comments in mm/filemap.c? Yes, I think you should. - Naoya Horiguchi