linux-kernel - Re: [PATCH v4] uart: fix race between uart_put_char() and uart

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Message-ID: <20180713092828.GA6589@kroah.com>
Date:   Fri, 13 Jul 2018 11:28:28 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, "Serge E . Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH v4] uart: fix race between uart_put_char() and
 uart_shutdown()

On Thu, Jul 12, 2018 at 12:30:01PM -0600, Tycho Andersen wrote:
> On Thu, Jul 12, 2018 at 08:25:45PM +0200, Greg Kroah-Hartman wrote:
> > On Thu, Jul 12, 2018 at 12:18:46PM -0600, Tycho Andersen wrote:
> > > On Thu, Jul 12, 2018 at 05:40:15PM +0200, Greg Kroah-Hartman wrote:
> > > > On Thu, Jul 12, 2018 at 09:08:22AM -0600, Tycho Andersen wrote:
> > > > > On Thu, Jul 12, 2018 at 05:04:38PM +0200, Greg Kroah-Hartman wrote:
> > > > > > On Wed, Jul 11, 2018 at 10:07:44AM -0600, Tycho Andersen wrote:
> > > > > > > +	if (uport)
> > > > > > > +		spin_lock_irqsave(&uport->lock, flags);
> > > > > > 
> > > > > > That's the same thing as just calling uart_port_lock(), why aren't you
> > > > > > doing that?
> > > > > 
> > > > > Because the compiler can't seem to "see" through the macros/ref calls,
> > > > > and I get the warning I mentioned here if I use them:
> > > > > 
> > > > > https://lkml.org/lkml/2018/7/6/840
> > > > 
> > > > What horrible version of gcc are you using that give you that?  Don't
> > > > open-code things just because of a broken compiler.
> > > 
> > > I've tried with both 7.3.0 and 5.4.0. I think the reason we see this
> > > here but not elsewhere in the file is because there's an actual
> > > function call (free_page()) in the critical section.
> > > 
> > > If we move that out, something like the below patch, it all works for
> > > me.
> > 
> > Ick.  Which version of this series had the problem?  Let me test it out
> > here...
> 
> v3, if you remove the initialization of flags from both functions you
> should see it.

Ok, I tried v3 out and yes, you are right, removing the "= 0" causes gcc
to complain.  The compiler is being dumb here, so I'll just leave it
as-is.  I've queued up the v3 version now, thanks for sticking with
this.

greg k-h