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 0EDEC29E0F9; Wed, 9 Jul 2025 11:27:51 +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=1752060472; cv=none; b=FT6P+W/y7lP/+kZshVzFk7jrZPuEnBvo1qcVFxkkiW4z20pECwP2jxKFhhZHETz3ZQyod60XIP4esxNQIUI4zxrKdQFPqpkZ4Wv5YZ5b8/vm+IsieJIIBI/MIpblWqjRx6+PeW3UwAkVNZ9sJPtPC6vXXrosagxbi7y4Mh8uqxQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752060472; c=relaxed/simple; bh=HhFX0RfL+sP2Um7Qtfv4Vig0hKSodNaNvx4L1cYbfvI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=audKyN+4hmVZhj6v7FmFg6DB9tBw5/PB8Nx0mlK043kV0/2E5JY92BNBoP7ys6wFawQN6iwwUouh6SzYkb5CP3YijmPYWOX3WM1vbX/6ZDG4co91LuNoHkmpp9mqTPSF2+NEeBFbvUp1lFTh/jWFlXy17by5RDbztAKIGnaBi6s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Et+FLqWX; 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="Et+FLqWX" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 108BAC4CEEF; Wed, 9 Jul 2025 11:27:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1752060471; bh=HhFX0RfL+sP2Um7Qtfv4Vig0hKSodNaNvx4L1cYbfvI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Et+FLqWXFP09TYCJoCHwJOIOkHlm6h8Fy5CtahYOJ7SxLK0uorMPQP4D/ryjED6lY lan7U/lkvkM8aE6vR0jCKhx47JIMwc6wvRfKompMdHwzDmXdR0aCEJnCDw2+q/wzs/ QeUuqGjvWgLVSV4pai/dwofK3gjgYdxiDuH9mhfo98ocFBoSwdHjoQZ6qMeQpySsou 8Olt4UowWOh0Z+LCcL7fmM8CVkrJLFYuAhPajpTcUnaYxvKyEzx4Q/Ki8bZQTdyk6M krU9MQU2mbSicrIj7Aw8OjmNMNvpW6TbvyfTU498deuhG1sSwLWkavitcoFyief60t Cj4Cg12mo0SqA== Date: Wed, 9 Jul 2025 13:27:48 +0200 From: Frederic Weisbecker To: neeraj.upadhyay@kernel.org Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@kernel.org, joelagnelf@nvidia.com, boqun.feng@gmail.com, urezki@gmail.com, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, qiang.zhang1211@gmail.com, neeraj.iitr10@gmail.com, neeraj.upadhyay@amd.com Subject: Re: [PATCH rcu 2/5] rcu: Protect ->defer_qs_iw_pending from data race Message-ID: References: <20250709104118.15532-1-neeraj.upadhyay@kernel.org> <20250709104118.15532-3-neeraj.upadhyay@kernel.org> 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=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20250709104118.15532-3-neeraj.upadhyay@kernel.org> Le Wed, Jul 09, 2025 at 04:11:15PM +0530, neeraj.upadhyay@kernel.org a écrit : > From: "Paul E. McKenney" > > On kernels built with CONFIG_IRQ_WORK=y, when rcu_read_unlock() is > invoked within an interrupts-disabled region of code [1], it will invoke > rcu_read_unlock_special(), which uses an irq-work handler to force the > system to notice when the RCU read-side critical section actually ends. > That end won't happen until interrupts are enabled at the soonest. > > In some kernels, such as those booted with rcutree.use_softirq=y, the > irq-work handler is used unconditionally. > > The per-CPU rcu_data structure's ->defer_qs_iw_pending field is > updated by the irq-work handler and is both read and updated by > rcu_read_unlock_special(). This resulted in the following KCSAN splat: > > ------------------------------------------------------------------------ > > BUG: KCSAN: data-race in rcu_preempt_deferred_qs_handler / rcu_read_unlock_special > > read to 0xffff96b95f42d8d8 of 1 bytes by task 90 on cpu 8: > rcu_read_unlock_special+0x175/0x260 > __rcu_read_unlock+0x92/0xa0 > rt_spin_unlock+0x9b/0xc0 > __local_bh_enable+0x10d/0x170 > __local_bh_enable_ip+0xfb/0x150 > rcu_do_batch+0x595/0xc40 > rcu_cpu_kthread+0x4e9/0x830 > smpboot_thread_fn+0x24d/0x3b0 > kthread+0x3bd/0x410 > ret_from_fork+0x35/0x40 > ret_from_fork_asm+0x1a/0x30 > > write to 0xffff96b95f42d8d8 of 1 bytes by task 88 on cpu 8: > rcu_preempt_deferred_qs_handler+0x1e/0x30 > irq_work_single+0xaf/0x160 > run_irq_workd+0x91/0xc0 > smpboot_thread_fn+0x24d/0x3b0 > kthread+0x3bd/0x410 > ret_from_fork+0x35/0x40 > ret_from_fork_asm+0x1a/0x30 > > no locks held by irq_work/8/88. > irq event stamp: 200272 > hardirqs last enabled at (200272): [] finish_task_switch+0x131/0x320 > hardirqs last disabled at (200271): [] __schedule+0x129/0xd70 > softirqs last enabled at (0): [] copy_process+0x4df/0x1cc0 > softirqs last disabled at (0): [<0000000000000000>] 0x0 > > ------------------------------------------------------------------------ > > The problem is that irq-work handlers run with interrupts enabled, which > means that rcu_preempt_deferred_qs_handler() could be interrupted, > and that interrupt handler might contain an RCU read-side critical > section, which might invoke rcu_read_unlock_special(). In the strict > KCSAN mode of operation used by RCU, this constitutes a data race on > the ->defer_qs_iw_pending field. > > This commit therefore disables interrupts across the portion of the > rcu_preempt_deferred_qs_handler() that updates the ->defer_qs_iw_pending > field. This suffices because this handler is not a fast path. > > Signed-off-by: Paul E. McKenney > Signed-off-by: Neeraj Upadhyay (AMD) Reviewed-by: Frederic Weisbecker -- Frederic Weisbecker SUSE Labs