lkml.org 
[lkml]   [2019]   [Jan]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [PATCH AUTOSEL 4.20 035/304] serial: core: Allow processing sysrq at port unlock time
    Hi,

    On Mon, Jan 28, 2019 at 9:16 AM Sasha Levin <sashal@kernel.org> 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 <sashal@kernel.org> wrote:
    > >>
    > >> From: Douglas Anderson <dianders@chromium.org>
    > >>
    > >> [ 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 <dianders@chromium.org>
    > >> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    > >> Signed-off-by: Sasha Levin <sashal@kernel.org>
    > >> ---
    > >> 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.

    -Doug

    \
     
     \ /
      Last update: 2019-01-28 18:23    [W:4.063 / U:0.036 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site