linux-kernel - Re: [PATCH v3] uart: fix race between uart_put_char() and uart

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Date:   Fri, 6 Jul 2018 15:22:20 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Serge E . Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH v3] uart: fix race between uart_put_char() and
 uart_shutdown()

On Fri, Jul 06, 2018 at 11:48:58PM +0300, Andy Shevchenko wrote:
> On Fri, Jul 6, 2018 at 9:39 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> > On Fri, Jul 06, 2018 at 07:49:09PM +0300, Andy Shevchenko wrote:
> >> On Fri, Jul 6, 2018 at 7:24 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> 
> >  but without the initialization I get,
> >
> >   CC      drivers/tty/serial/serial_core.o
> > In file included from ./include/linux/seqlock.h:36:0,
> >                  from ./include/linux/time.h:6,
> >                  from ./include/linux/stat.h:19,
> >                  from ./include/linux/module.h:10,
> >                  from drivers/tty/serial/serial_core.c:10:
> > drivers/tty/serial/serial_core.c: In function ‘uart_startup.part.20’:
> > ./include/linux/spinlock.h:260:3: warning: ‘flags’ may be used uninitialized in this function  -Wmaybe-uninitialized]
> >    _raw_spin_unlock_irqrestore(lock, flags); \
> >    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> > drivers/tty/serial/serial_core.c:184:22: note: ‘flags’ was declared here
> >   unsigned long page, flags;
> >                       ^~~~~
> 
> Hmm... I didn't see such warning. How you run make?

Just with `make`, although using the specific object file works too.
Perhaps it's gcc versions?

~/packages/linux uart-fix-v4 make drivers/tty/serial/serial_core.o
  CALL    scripts/checksyscalls.sh
  DESCEND  objtool
  CC      drivers/tty/serial/serial_core.o
In file included from ./include/linux/seqlock.h:36:0,
                 from ./include/linux/time.h:6,
                 from ./include/linux/stat.h:19,
                 from ./include/linux/module.h:10,
                 from drivers/tty/serial/serial_core.c:10:
drivers/tty/serial/serial_core.c: In function ‘uart_startup.part.20’:
./include/linux/spinlock.h:260:3: warning: ‘flags’ may be used uninitialized in this function  -Wmaybe-uninitialized]
   _raw_spin_unlock_irqrestore(lock, flags); \
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/tty/serial/serial_core.c:184:22: note: ‘flags’ was declared here
  unsigned long page, flags;
                      ^~~~~
In file included from ./include/linux/seqlock.h:36:0,
                 from ./include/linux/time.h:6,
                 from ./include/linux/stat.h:19,
                 from ./include/linux/module.h:10,
                 from drivers/tty/serial/serial_core.c:10:
drivers/tty/serial/serial_core.c: In function ‘uart_shutdown’:
./include/linux/spinlock.h:260:3: warning: ‘flags’ may be used uninitialized in this function  -Wmaybe-uninitialized]
   _raw_spin_unlock_irqrestore(lock, flags); \
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/tty/serial/serial_core.c:269:16: note: ‘flags’ was declared here
  unsigned long flags;
                ^~~~~
~/packages/linux uart-fix-v4 gcc --version
gcc (Ubuntu 7.3.0-16ubuntu3) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

> Btw, you adding the only places with such assignments in this file.
> So, I would not do in your case, until entire file would be fixed.
> 
> (But warning looks bogus, or you have some patches on top of current
> vanilla / next)

I don't have any patches, but I do admit to not thinking about it very hard and
adding initializations. I'll see if I can figure out what's going on.

Thanks,

Tycho