From nobody Mon Feb 9 01:51:26 2026 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 E66E3235346; Thu, 8 May 2025 23:49:50 +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=1746748191; cv=none; b=t0vNlicwTHs04eyMMjOdfJqCXwigVT9gfdP8L2lKD8v5npb0y/dlWLDHVPjEZIhWBhheQLkVgKfeOPB2cvR43+QsGy8indo7y6AXJrDWLgf/4QkINLquoROZRX3UgF9gzEzreXwgHzoVEs6+UyASCgLCsSNqexxYwhFQ779mFDg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746748191; c=relaxed/simple; bh=lL0EFNIiIcqpQh3uUwyWJQ72+MbHAoPc+7fJeD9cv2c=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition; b=Jy/7gHtU8kAwn+5VORaJ0nLoLSRFRrIpJ3EpHoegss1QSUDWPwlhGp0NoHAQqi/OFmHHP0ab8fKXCStdaDjNiHx7StB2X3745l3B+YJBfxUpOMoA4GP6KrxODioS/cnui7PQPpaYYVyMVVGM//i6Jy2dC8MMBcAhDXdRcuHQzJ4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ki+sy1Uu; 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="ki+sy1Uu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BAF69C4CEE7; Thu, 8 May 2025 23:49:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1746748190; bh=lL0EFNIiIcqpQh3uUwyWJQ72+MbHAoPc+7fJeD9cv2c=; h=Date:From:To:Cc:Subject:Reply-To:From; b=ki+sy1Uu2mXoOb83Es1O9jlyKZ8DOL5O0FDVjqRzJmVmYO7bClLEoK6+rf9tFDJHX 989tcSDKrbrHzZzAo5OWaQ6GJvf03NNSAuLrVc7MQvE58SmV0BDuK40VyiX8d5MJRT 1ZpTD71OCSKNh4YvbtcnnZfoC8MGtzB11rOARYhwUOm9ZTZypsrhssky7XfS0KjdtY 6RNPBpk/wX31icPWOmX+K7Gc6UP8cMe+JdICUVlskLdPgzh6FYmp89nRiANVywY1uQ aXctuEnz28hfbZspPODTfbmUDeTsIY7JM6K2S2qrWsiO98iQ3WUH8jWOn0MMCoOXR0 oGh9PDqIQcdWQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 53BB8CE11A4; Thu, 8 May 2025 16:49:50 -0700 (PDT) Date: Thu, 8 May 2025 16:49:50 -0700 From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org Subject: [PATCH] rcu: Protect ->defer_qs_iw_pending from data race Message-ID: Reply-To: paulmck@kernel.org Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" On kernels built with CONFIG_IRQ_WORK=3Dy, 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=3Dy, 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+0x129/0= xd70 softirqs last enabled at (0): [] copy_process+0x4df/0x1c= c0 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 diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 3c0bbbbb686fe..003e549f65141 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -624,10 +624,13 @@ notrace void rcu_preempt_deferred_qs(struct task_stru= ct *t) */ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp) { + unsigned long flags; struct rcu_data *rdp; =20 rdp =3D container_of(iwp, struct rcu_data, defer_qs_iw); + local_irq_save(flags); rdp->defer_qs_iw_pending =3D false; + local_irq_restore(flags); } =20 /*