From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 08EC42472A6; Thu, 9 Apr 2026 18:27:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775759244; cv=none; b=H73Axv9NeSiuI+MhoTH1ppvWG1Jr0kgHjiqpfP+EeINvPDhpRnaoXTIPUD3H4Khcb7YbIb7XgL8E1jx2SRjYAwuM9aiVgaAcjf0CfL76y58TbOMIYcWjidCMbRAWRLIz79gsbE4jLrPotKuLs8SxzSxXiDowxNIpThL1TEl1UfQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775759244; c=relaxed/simple; bh=q1dsgCRT/qzI4qDyl9z2gXVFEjDbUd69CCsY7/0NAGM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=QLfyxL/dveGdPSKZgHrxlWjIgIyxlzvLR24g9ReS5J8+IBgpWVxgdsnZp+KOBwjvajpKUfWzCqjcwSwlIZ9rNKmsssRzdeRTjQ6+PtGk5Cs4qqfAboe9N7B4J6JicWVTB+wdlwVtAyEIv1y9wfgIMrCoHzKFz/bVIjaMulyX0nU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=u4D8AzZV; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="u4D8AzZV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A2ACCC4CEF7; Thu, 9 Apr 2026 18:27:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775759243; bh=q1dsgCRT/qzI4qDyl9z2gXVFEjDbUd69CCsY7/0NAGM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=u4D8AzZVtdMSJSY3DHM0UHEopkz3umIE8I3rYWonrUyalMVs4trNeoWrpMJhfyz3f 6tb+f5eMwYrTZeyYmA9pKdUT7UmxOEJzSqajYNE/g/8XH6vmFrA+giwYkH5ZWltDQn ruZ9AQeqUMENrJ/KDBkjeZJnlxz8AdP8MBmUBQDZVm9Ay+2vl5QdyFUKzQjYDngul4 jGa4l1AKMGcpOyFuJ9C4dJrQsIV4NNVfC6cHye4C9VzESI46DRAkXFAFsX1ZoqEpih yQfjp7K1ht2vXTzZcoAJpEB2tGYs1ubC9dWrXE4rNY3VgQ0l8yBNOzvXA/CRmdIAjx UO4aL6Q+MoShg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 375FFCE0975; Thu, 9 Apr 2026 11:27:23 -0700 (PDT) Date: Thu, 9 Apr 2026 11:27:23 -0700 From: "Paul E. McKenney" To: Boqun Feng Cc: Tejun Heo , Vasily Gorbik , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Uladzislau Rezki , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, Lai Jiangshan Subject: Re: BUG: workqueue lockup - SRCU schedules work on not-online CPUs during size transition Message-ID: Reply-To: paulmck@kernel.org References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Apr 09, 2026 at 11:10:04AM -0700, Boqun Feng wrote: > On Thu, Apr 09, 2026 at 07:47:09AM -1000, Tejun Heo wrote: > > On Thu, Apr 09, 2026 at 10:40:05AM -0700, Boqun Feng wrote: > > > On Thu, Apr 09, 2026 at 10:26:49AM -0700, Boqun Feng wrote: > > > > On Thu, Apr 09, 2026 at 03:08:45PM +0200, Vasily Gorbik wrote: > > > > > Commit 61bbcfb50514 ("srcu: Push srcu_node allocation to GP when > > > > > non-preemptible") defers srcu_node tree allocation when called under > > > > > raw spinlock, putting SRCU through ~6 transitional grace periods > > > > > (SRCU_SIZE_ALLOC to SRCU_SIZE_BIG). During this transition srcu_gp_end() > > > > > uses mask = ~0, which makes srcu_schedule_cbs_snp() call queue_work_on() > > > > > for every possible CPU. Since rcu_gp_wq is WQ_PERCPU, work targets > > > > > per-CPU pools directly - pools for not-online CPUs have no workers, > > > > > > > > [Cc workqueue] > > > > > > > > Hmm.. I thought for offline CPUs the corresponding worker pools become a > > > > unbound one hence there are still workers? > > > > > > > > > > Ah, as Paul replied in another email, the problem was because these CPUs > > > had never been onlined, so they don't even have unbound workers? > > > > Hahaha, we do initialize worker pool for every possible CPU but the > > transition to unbound operation happens in the hot unplug callback. We > > ;-) ;-) ;-) > > > probably need to do some of the hot unplug operation during init if the CPU > > Seems that we (mostly Paul) have our own trick to track whether a CPU > has ever been onlined in RCU, see rcu_cpu_beenfullyonline(). Paul also > used it in his fix [1]. And I think it won't be that hard to copy it > into workqueue and let queue_work_on() use it so that if the user queues > a work on a never-onlined CPU, it can detect it (with a warning?) and do > something? > > [1]: https://lore.kernel.org/rcu/073abb55-197a-4519-b177-f9f776624fed@paulmck-laptop/ It might be that my patch (or something like it) will be required in addition to Tejun's fix because the current Tree SRCU code is happy to schedule a workqueue handler on a CPU that does not even have a bit set in the cpu_possible_mask. This could happen on a system with the first 50 CPUs, as in 0-49, in cpu_possible_mask. Tree SRCU would then be quite happy to schedule workqueue handlers on the mythical CPUs 50-63. Which, now that I think on it, does seem a bit more brave than absolutely warranted. ;-) Thanx, Paul > Regards, > Boqun > > > is possible but not online. That said, what kind of machine is it? Is the > > firmware just reporting bogus possible mask? How come the CPUs weren't > > online during boot? > > > > Thanks. > > > > -- > > tejun