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Thu, 21 May 2026 09:58:51 -0700 (PDT) Message-ID: <469287b6-201a-497d-ac67-03e1336dd81a@kernel.dk> Date: Thu, 21 May 2026 10:58:51 -0600 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] io_uring/io-wq: avoid repeated task_work scans during teardown To: Fengnan Chang , io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, peterz@infradead.org, rostedt@goodmis.org References: <20260520031221.83210-1-changfengnan@bytedance.com> Content-Language: en-US From: Jens Axboe In-Reply-To: <20260520031221.83210-1-changfengnan@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/19/26 9:12 PM, Fengnan Chang wrote: > We hit hard-lockup reports from iou-wrk threads stuck in > task_work_cancel_match() during io-wq teardown in syzkaller test. > The root cause is that teardown repeatedly rescans the submitter task's > full task_work list under pi_lock, once per matched item. > > Two spots are problematic: > > 1) io_wq_cancel_tw_create() loops calling task_work_cancel_match() to > remove worker-creation callbacks one at a time. Each call re-walks > the entire list from scratch while holding pi_lock. > > 2) io_worker_exit() unconditionally scans the submitter task_work list > for its own create_work, even when it never queued one. With many > workers exiting simultaneously against a large unrelated task_work > list, this adds up fast. > > Fix (1) by adding task_work_cancel_match_all() that unlinks all matching > callbacks in a single traversal, then iterating the returned list locally. > Same try_cmpxchg() synchronisation as before, stops at the work_exited > sentinel. > > Fix (2) by skipping the cancel entirely unless create_state indicates a > pending create_work. Since create_state is exclusively owned via > test_and_set_bit_lock, at most one callback can be queued per worker, so > the cancel is also simplified from a loop to a single call. > > With this fix the reproducer (FIFO-open + MSG_RING SEND_FD stress) no > longer triggers hard-lockup reports, and task_work_cancel_match samples > drop to microseconds. Looks good to me, nicer way to do this too. -- Jens Axboe