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([2620:10d:c092:600::2:f766]) by smtp.gmail.com with ESMTPSA id y10sm1730308edc.66.2021.06.07.10.38.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 07 Jun 2021 10:38:44 -0700 (PDT) To: "Zhang, Qiang" , Hillf Danton , "axboe@kernel.dk" Cc: "syzbot+6cb11ade52aa17095297@syzkaller.appspotmail.com" , "io-uring@vger.kernel.org" , "linux-kernel@vger.kernel.org" References: <20210524071844.24085-1-qiang.zhang@windriver.com> <20210524082536.2032-1-hdanton@sina.com> <916ad789-c996-258f-d3b7-b41d749618d8@gmail.com> From: Pavel Begunkov Subject: =?UTF-8?B?UmU6IOWbnuWkjTog5Zue5aSNOiBbUEFUQ0hdIGlvLXdxOiBGaXggVUFG?= =?UTF-8?Q?_when_wakeup_wqe_in_hash_waitqueue?= Message-ID: <9af68623-57e4-cef0-bb61-347207fb0c45@gmail.com> Date: Mon, 7 Jun 2021 18:38:34 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 5/25/21 3:01 AM, Zhang, Qiang wrote: [...] >> Haven't looked at the trace and description, but I do think >> there is a problem it solves. >> >> 1) io_wait_on_hash() -> __add_wait_queue(&hash->wait, &wqe->wait); >> 2) (note: wqe is a worker) wqe's workers exit dropping refs >> 3) refs are zero, free io-wq >> 4) @hash is shared, so other task/wq does wake_up(&wq->hash->wait); >> 5) it wakes freed wqe >> >> step 4) is a bit more trickier than that, tl;dr; >> wq3:worker1 | locks bit1 >> wq1:worker2 | waits bit1 >> wq2:worker1 | waits bit1 >> wq1:worker3 | waits bit1 >> >> wq3:worker1 | drop bit1 >> wq1:worker2 | locks bit1 >> wq1:worker2 | completes all wq1 bit1 work items >> wq1:worker2 | drop bit1, exit and free io-wq >> >> wq2:worker1 | locks bit1 >> wq1 | free complete >> wq2:worker1 | drops bit1 >> wq1:worker3 | waked up, even though freed >> >> Can be simplified, don't want to waste time on that > > Thanks Pavel > > Your description is better. I have another question: under what circumstances will three io-wq(wq1, wq2, wq3) be created to share this @hash? Oops, missed the email. It's created by io_uring, and passed to io-wq, which is per-task and created on demand by io_uring. Can be achieved by a snippet just below, where threads haven't had io_uring instances before. thread1: ring = create_io_uring(); thread2: submit_sqes(ring); thread3: submit_sqes(ring); > > This kind of problem also occurs between two io-wq(wq1, wq2). Is the following description OK？ Yep, and I feel like there are cases simpler (and more likely) than the one I described. > > wq1:worker2 | locks bit1 > wq2:worker1 | waits bit1 > wq1:worker3 | waits bit1 > > wq1:worker2 | completes all wq1 bit1 work items > wq1:worker2 | drop bit1, exit and free io-wq > > wq2:worker1 | locks bit1 > wq1 | free complete > wq2:worker1 | drops bit1 > wq1:worker3 | waked up, even though freed -- Pavel Begunkov