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(p200300cbc705fb00c6c01fe6bfa1e868.dip0.t-ipconnect.de. [2003:cb:c705:fb00:c6c0:1fe6:bfa1:e868]) by smtp.gmail.com with ESMTPSA id a27sm21089edj.17.2022.01.19.07.15.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 19 Jan 2022 07:15:17 -0800 (PST) Message-ID: Date: Wed, 19 Jan 2022 16:15:16 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH v1] proc/vmcore: fix false positive lockdep warning To: Boqun Feng Cc: linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Baoquan He , Andrew Morton , Vivek Goyal , Dave Young , "Paul E. McKenney" , Josh Triplett , Peter Zijlstra References: <20220119113702.102567-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 3E25F20018 X-Stat-Signature: nje84x43d89dh9manbb9ontxwygm8usb Authentication-Results: imf31.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="U/wPP1qd"; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf31.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com X-Rspamd-Server: rspam02 X-HE-Tag: 1642605321-484328 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 19.01.22 16:08, Boqun Feng wrote: > Hi, > > On Wed, Jan 19, 2022 at 12:37:02PM +0100, David Hildenbrand wrote: >> Lockdep complains that we do during mmap of the vmcore: >> down_write(mmap_lock); >> down_read(vmcore_cb_rwsem); >> And during read of the vmcore: >> down_read(vmcore_cb_rwsem); >> down_read(mmap_lock); >> >> We cannot possibly deadlock when only taking vmcore_cb_rwsem in read >> mode, however, it's hard to teach that to lockdep. >> > > Lockdep warned about the above sequences because rw_semaphore is a fair > read-write lock, and the following can cause a deadlock: > > TASK 1 TASK 2 TASK 3 > ====== ====== ====== > down_write(mmap_lock); > down_read(vmcore_cb_rwsem) > down_write(vmcore_cb_rwsem); // blocked > down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness > down_read(mmap_lock); // blocked > > IOW, a reader can block another read if there is a writer queued by the > second reader and the lock is fair. > > So there is a deadlock possiblity. Task 3 will never take the mmap_lock before doing a down_write(vmcore_cb_rwsem). How would this happen? -- Thanks, David / dhildenb