From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS, USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D0AF6C282CF for ; Mon, 28 Jan 2019 18:04:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 944362148E for ; Mon, 28 Jan 2019 18:04:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548698649; bh=7qV054TafpO/A6Qe11Oo4BSboYCEjbTaPer/LE6GUIE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=nUEidz+6byCAAqWneCK4eLGW8ebhvP+EgZoy6RDjCHyR3yLiE5X2nYmxSggv2tpqz huQDKpIV0ztfAIkmm/VZ396GN/yGqruoyvb8moC1MKkPY60nXETmw66RXml19gfevu oWP8TFGHot1B8Q4ZCg7GOhMEmSCcRU2fRy1mXwfs= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726973AbfA1SEI (ORCPT ); Mon, 28 Jan 2019 13:04:08 -0500 Received: from mail.kernel.org ([198.145.29.99]:46530 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726266AbfA1SEH (ORCPT ); Mon, 28 Jan 2019 13:04:07 -0500 Received: from localhost (c-73-47-72-35.hsd1.nh.comcast.net [73.47.72.35]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D74C82148E; Mon, 28 Jan 2019 18:04:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548698646; bh=7qV054TafpO/A6Qe11Oo4BSboYCEjbTaPer/LE6GUIE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=X4lV/k+P0GunH1mhv4tY2qnjVdMN9EhSFtwVj4ZWINZF4blcOCXrcyCn1/L9HA/xU fh2PTi5ASDEV6C7sScF7Iu0qn31nBrsF8i4KcETMyjRkv/zDddFd9f3fudULEO+6S9 tYWzZlvQYl8QyVB9vMdQW0aQl4js9MXjI9BFA3+k= Date: Mon, 28 Jan 2019 13:04:04 -0500 From: Sasha Levin To: Doug Anderson Cc: LKML , stable@vger.kernel.org, Greg Kroah-Hartman Subject: Re: [PATCH AUTOSEL 4.20 035/304] serial: core: Allow processing sysrq at port unlock time Message-ID: <20190128180404.GF3973@sasha-vm> References: <20190128154341.47195-1-sashal@kernel.org> <20190128154341.47195-35-sashal@kernel.org> <20190128171614.GB3973@sasha-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 28, 2019 at 09:21:31AM -0800, Doug Anderson wrote: >Hi, > >On Mon, Jan 28, 2019 at 9:16 AM Sasha Levin wrote: >> >> On Mon, Jan 28, 2019 at 08:05:13AM -0800, Doug Anderson wrote: >> >Hi, >> > >> >On Mon, Jan 28, 2019 at 7:44 AM Sasha Levin wrote: >> >> >> >> From: Douglas Anderson >> >> >> >> [ Upstream commit d6e1935819db0c91ce4a5af82466f3ab50d17346 ] >> >> >> >> Right now serial drivers process sysrq keys deep in their character >> >> receiving code. This means that they've already grabbed their >> >> port->lock spinlock. This can end up getting in the way if we've go >> >> to do serial stuff (especially kgdb) in response to the sysrq. >> >> >> >> Serial drivers have various hacks in them to handle this. Looking at >> >> '8250_port.c' you can see that the console_write() skips locking if >> >> we're in the sysrq handler. Looking at 'msm_serial.c' you can see >> >> that the port lock is dropped around uart_handle_sysrq_char(). >> >> >> >> It turns out that these hacks aren't exactly perfect. If you have >> >> lockdep turned on and use something like the 8250_port hack you'll get >> >> a splat that looks like: >> >> >> >> WARNING: possible circular locking dependency detected >> >> [...] is trying to acquire lock: >> >> ... (console_owner){-.-.}, at: console_unlock+0x2e0/0x5e4 >> >> >> >> but task is already holding lock: >> >> ... (&port_lock_key){-.-.}, at: serial8250_handle_irq+0x30/0xe4 >> >> >> >> which lock already depends on the new lock. >> >> >> >> the existing dependency chain (in reverse order) is: >> >> >> >> -> #1 (&port_lock_key){-.-.}: >> >> _raw_spin_lock_irqsave+0x58/0x70 >> >> serial8250_console_write+0xa8/0x250 >> >> univ8250_console_write+0x40/0x4c >> >> console_unlock+0x528/0x5e4 >> >> register_console+0x2c4/0x3b0 >> >> uart_add_one_port+0x350/0x478 >> >> serial8250_register_8250_port+0x350/0x3a8 >> >> dw8250_probe+0x67c/0x754 >> >> platform_drv_probe+0x58/0xa4 >> >> really_probe+0x150/0x294 >> >> driver_probe_device+0xac/0xe8 >> >> __driver_attach+0x98/0xd0 >> >> bus_for_each_dev+0x84/0xc8 >> >> driver_attach+0x2c/0x34 >> >> bus_add_driver+0xf0/0x1ec >> >> driver_register+0xb4/0x100 >> >> __platform_driver_register+0x60/0x6c >> >> dw8250_platform_driver_init+0x20/0x28 >> >> ... >> >> >> >> -> #0 (console_owner){-.-.}: >> >> lock_acquire+0x1e8/0x214 >> >> console_unlock+0x35c/0x5e4 >> >> vprintk_emit+0x230/0x274 >> >> vprintk_default+0x7c/0x84 >> >> vprintk_func+0x190/0x1bc >> >> printk+0x80/0xa0 >> >> __handle_sysrq+0x104/0x21c >> >> handle_sysrq+0x30/0x3c >> >> serial8250_read_char+0x15c/0x18c >> >> serial8250_rx_chars+0x34/0x74 >> >> serial8250_handle_irq+0x9c/0xe4 >> >> dw8250_handle_irq+0x98/0xcc >> >> serial8250_interrupt+0x50/0xe8 >> >> ... >> >> >> >> other info that might help us debug this: >> >> >> >> Possible unsafe locking scenario: >> >> >> >> CPU0 CPU1 >> >> ---- ---- >> >> lock(&port_lock_key); >> >> lock(console_owner); >> >> lock(&port_lock_key); >> >> lock(console_owner); >> >> >> >> *** DEADLOCK *** >> >> >> >> The hack used in 'msm_serial.c' doesn't cause the above splats but it >> >> seems a bit ugly to unlock / lock our spinlock deep in our irq >> >> handler. >> >> >> >> It seems like we could defer processing the sysrq until the end of the >> >> interrupt handler right after we've unlocked the port. With this >> >> scheme if a whole batch of sysrq characters comes in one irq then we >> >> won't handle them all, but that seems like it should be a fine >> >> compromise. >> >> >> >> Signed-off-by: Douglas Anderson >> >> Signed-off-by: Greg Kroah-Hartman >> >> Signed-off-by: Sasha Levin >> >> --- >> >> include/linux/serial_core.h | 37 ++++++++++++++++++++++++++++++++++++- >> >> 1 file changed, 36 insertions(+), 1 deletion(-) >> > >> >FWIW this patch shouldn't hurt to be backported (I haven't heard any >> >problems report with it), but it is effectively a no-op unless you >> >also pick a patch that uses the new API. For instance commit >> >596f63da42b9 ("serial: 8250: Process sysrq at port unlock time"). >> >...and if you want that patch I think you also need commit >> >3e6f88068314 ("serial: core: Include console.h from serial_core.h"). >> > >> >In theory you could think about adding the "qcom_geni_serial" patches >> >related to sysrq processing too--dunno if anyone really cares about >> >those on 4.20 stable... >> >> Since no one actually tagged it for stable, probably not... I'll drop >> it, thanks! > >OK. Whatever behavior you decide on, please apply it across the >board. I got pings that this same patch was being picked to lots and >lots of different stable kernels and it is equally a no-op (without >the followup patches) everywhere. Yup, I send a mail for each branch it was added on. It was now dropped from all of them. -- Thanks, Sasha