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bh=uBK5OITzTpB4IVgjdOW97uEOhFLeY9qY+4HyFM8sGYY=; b=2cbUnOyFpszm38F6MhRwDqEDHNnJfTEajLb66hytSoSY9O7HLdqWZJPtzhYcUotsUTvk5p 3Vuc305xnGhomCCQ== Received: from quack3.suse.cz (unknown [10.163.28.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id F0D8FA3B85; Thu, 24 Feb 2022 10:22:39 +0000 (UTC) Received: by quack3.suse.cz (Postfix, from userid 1000) id 4D488A0605; Thu, 24 Feb 2022 11:22:39 +0100 (CET) Date: Thu, 24 Feb 2022 11:22:39 +0100 From: Jan Kara To: Byungchul Park Cc: Jan Kara , torvalds@linux-foundation.org, damien.lemoal@opensource.wdc.com, linux-ide@vger.kernel.org, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, mingo@redhat.com, linux-kernel@vger.kernel.org, peterz@infradead.org, will@kernel.org, tglx@linutronix.de, rostedt@goodmis.org, joel@joelfernandes.org, sashal@kernel.org, daniel.vetter@ffwll.ch, chris@chris-wilson.co.uk, duyuyang@gmail.com, johannes.berg@intel.com, tj@kernel.org, tytso@mit.edu, willy@infradead.org, david@fromorbit.com, amir73il@gmail.com, bfields@fieldses.org, gregkh@linuxfoundation.org, kernel-team@lge.com, linux-mm@kvack.org, akpm@linux-foundation.org, mhocko@kernel.org, minchan@kernel.org, hannes@cmpxchg.org, vdavydov.dev@gmail.com, sj@kernel.org, jglisse@redhat.com, dennis@kernel.org, cl@linux.com, penberg@kernel.org, rientjes@google.com, vbabka@suse.cz, ngupta@vflare.org, linux-block@vger.kernel.org, axboe@kernel.dk, paolo.valente@linaro.org, josef@toxicpanda.com, linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk, jack@suse.com, jlayton@kernel.org, dan.j.williams@intel.com, hch@infradead.org, djwong@kernel.org, dri-devel@lists.freedesktop.org, airlied@linux.ie, rodrigosiqueiramelo@gmail.com, melissa.srw@gmail.com, hamohammed.sa@gmail.com Subject: Re: Report 2 in ext4 and journal based on v5.17-rc1 Message-ID: <20220224102239.n7nzyyekuacgpnzg@quack3.lan> References: <1645095472-26530-1-git-send-email-byungchul.park@lge.com> <1645096204-31670-1-git-send-email-byungchul.park@lge.com> <1645096204-31670-2-git-send-email-byungchul.park@lge.com> <20220221190204.q675gtsb6qhylywa@quack3.lan> <20220223003534.GA26277@X58A-UD3R> <20220223144859.na2gjgl5efgw5zhn@quack3.lan> <20220224011102.GA29726@X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220224011102.GA29726@X58A-UD3R> X-Rspamd-Queue-Id: F211E100008 X-Stat-Signature: nwgmhc8f4n8pt7qms5zezmbun9mj9ebz Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=OVrcPBP1; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=2cbUnOyF; dmarc=none; spf=pass (imf05.hostedemail.com: domain of jack@suse.cz designates 195.135.220.29 as permitted sender) smtp.mailfrom=jack@suse.cz X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1645698163-698499 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 Thu 24-02-22 10:11:02, Byungchul Park wrote: > On Wed, Feb 23, 2022 at 03:48:59PM +0100, Jan Kara wrote: > > > KJOURNALD2(kthread) TASK1(ksys_write) TASK2(ksys_write) > > > > > > wait A > > > --- stuck > > > wait B > > > --- stuck > > > wait C > > > --- stuck > > > > > > wake up B wake up C wake up A > > > > > > where: > > > A is a wait_queue, j_wait_commit > > > B is a wait_queue, j_wait_transaction_locked > > > C is a rwsem, mapping.invalidate_lock > > > > I see. But a situation like this is not necessarily a guarantee of a > > deadlock, is it? I mean there can be task D that will eventually call say > > 'wake up B' and unblock everything and this is how things were designed to > > work? Multiple sources of wakeups are quite common I'd say... What does > > Yes. At the very beginning when I desgined Dept, I was thinking whether > to support multiple wakeup sources or not for a quite long time. > Supporting it would be a better option to aovid non-critical reports. > However, I thought anyway we'd better fix it - not urgent tho - if > there's any single circle dependency. That's why I decided not to > support it for now and wanted to gather the kernel guys' opinions. Thing > is which policy we should go with. I see. So supporting only a single wakeup source is fine for locks I guess. But for general wait queues or other synchronization mechanisms, I'm afraid it will lead to quite some false positive reports. Just my 2c. > > Dept do to prevent false reports in cases like this? > > > > > The above is the simplest form. And it's worth noting that Dept focuses > > > on wait and event itself rather than grabing and releasing things like > > > lock. The following is the more descriptive form of it. > > > > > > KJOURNALD2(kthread) TASK1(ksys_write) TASK2(ksys_write) > > > > > > wait @j_wait_commit > > > ext4_truncate_failed_write() > > > down_write(mapping.invalidate_lock) > > > > > > ext4_truncate() > > > ... > > > wait @j_wait_transaction_locked > > > > > > ext_truncate_failed_write() > > > down_write(mapping.invalidate_lock) > > > > > > ext4_should_retry_alloc() > > > ... > > > __jbd2_log_start_commit() > > > wake_up(j_wait_commit) > > > jbd2_journal_commit_transaction() > > > wake_up(j_wait_transaction_locked) > > > up_write(mapping.invalidate_lock) > > > > > > I hope this would help you understand the report. > > > > I see, thanks for explanation! So the above scenario is impossible because > > My pleasure. > > > for anyone to block on @j_wait_transaction_locked the transaction must be > > committing, which is done only by kjournald2 kthread and so that thread > > cannot be waiting at @j_wait_commit. Essentially blocking on > > @j_wait_transaction_locked means @j_wait_commit wakeup was already done. > > kjournal2 repeatedly does the wait and the wake_up so the above scenario > looks possible to me even based on what you explained. Maybe I should > understand how the journal things work more for furhter discussion. Your > explanation is so helpful. Thank you really. OK, let me provide you with more details for better understanding :) In jbd2 we have an object called 'transaction'. This object can go through many states but for our case is important that transaction is moved to T_LOCKED state and out of it only while jbd2_journal_commit_transaction() function is executing and waiting on j_wait_transaction_locked waitqueue is exactly waiting for a transaction to get out of T_LOCKED state. Function jbd2_journal_commit_transaction() is executed only by kjournald. Hence anyone can see transaction in T_LOCKED state only if kjournald is running inside jbd2_journal_commit_transaction() and thus kjournald cannot be sleeping on j_wait_commit at the same time. Does this explain things? Honza -- Jan Kara SUSE Labs, CR