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[209.85.221.173]) by smtp.gmail.com with ESMTPSA id y20sm7586vsq.16.2019.01.28.08.05.25 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Jan 2019 08:05:26 -0800 (PST) Received: by mail-vk1-f173.google.com with SMTP id 197so3782917vkf.4 for ; Mon, 28 Jan 2019 08:05:25 -0800 (PST) X-Received: by 2002:a1f:bfd6:: with SMTP id p205mr9160551vkf.70.1548691525478; Mon, 28 Jan 2019 08:05:25 -0800 (PST) MIME-Version: 1.0 References: <20190128154341.47195-1-sashal@kernel.org> <20190128154341.47195-35-sashal@kernel.org> In-Reply-To: <20190128154341.47195-35-sashal@kernel.org> From: Doug Anderson Date: Mon, 28 Jan 2019 08:05:13 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH AUTOSEL 4.20 035/304] serial: core: Allow processing sysrq at port unlock time To: Sasha Levin Cc: LKML , stable@vger.kernel.org, Greg Kroah-Hartman Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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... -Doug