Re: [PATCH] mm: prevent mmap_cache race in find_vma()

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Ian Lance Taylor <iant@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	David Rientjes <rientjes@google.com>,
	Hugh Dickins <hughd@google.com>,
	Jan Stancek <jstancek@redhat.com>,
	linux-mm@kvack.org
Subject: Re: [PATCH] mm: prevent mmap_cache race in find_vma()
Date: Wed, 3 Apr 2013 15:11:29 -0700	[thread overview]
Message-ID: <20130403221129.GL28522@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAKOQZ8wd24AUCN2c6p9iLFeHMpJy=jRO2xoiKkH93k=+iYQpEA@mail.gmail.com>

On Wed, Apr 03, 2013 at 10:47:28AM -0700, Ian Lance Taylor wrote:
> On Wed, Apr 3, 2013 at 9:33 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Wed, Apr 03, 2013 at 06:45:51AM -0700, Ian Lance Taylor wrote:
> >
> >> The C language standard only describes how access to
> >> volatile-qualified objects behave.  In this case x is (presumably) not
> >> a volatile-qualifed object.  The standard never defines the behaviour
> >> of volatile-qualified pointers.  That might seem like an oversight,
> >> but it is not: using a non-volatile-qualified pointer to access a
> >> volatile-qualified object is undefined behaviour.
> >>
> >> In short, casting a pointer to a non-volatile-qualified object to a
> >> volatile-qualified pointer has no specific meaning in C.  It's true
> >> that most compilers will behave as you wish, but there is no
> >> guarantee.
> >
> > But we are not using a non-volatile-qualified pointer to access a
> > volatile-qualified object.  We are doing the opposite.  I therefore
> > don't understand the relevance of your comment about undefined behavior.
> 
> That was just a digression to explain why the standard does not need
> to define the behaviour of volatile-qualified pointers.
> 
> 
> >> If using a sufficiently recent version of GCC, you can get the
> >> behaviour that I think you want by using
> >>     __atomic_load(&x, __ATOMIC_RELAXED)
> >
> > If this maps to the memory_order_relaxed token defined in earlier versions
> > of the C11 standard, then this absolutely does -not-, repeat -not-, work
> > for ACCESS_ONCE().
> 
> Yes, I'm sorry, you are right.  It will work in practice today but
> you're quite right that there is no reason to think that it will work
> in principle.
> 
> This need suggests that GCC needs a new builtin function to implement
> the functionality that you want.  Would you consider opening a request
> for that at http://gcc.gnu.org/bugzilla/ ?

How about a request for gcc to formally honor the current uses of volatile?

							Thanx, Paul

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