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Message-ID: <20080408155259.GA8381@linux.vnet.ibm.com>
Date: Tue, 8 Apr 2008 08:52:59 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Johannes Berg <johannes@...solutions.net>
Cc: Linux Kernel list <linux-kernel@...r.kernel.org>,
linux-sparse <linux-sparse@...r.kernel.org>
Subject: Re: Using sparse to catch invalid RCU dereferences?
On Tue, Apr 08, 2008 at 12:04:16AM +0200, Johannes Berg wrote:
> Hi,
>
> Just a thought, I haven't tried this yet because I'm not entirely sure
> it's actually correct. I was just thinking it should be possible to
> introduce something like
>
> #define __rcu __attribute__((address_space(3)))
>
> (for sparse only, of course) and then be able to say
>
> struct myfoo *foo __rcu;
>
> and sparse would warn on
>
> struct myfoo *bar = foo;
>
> but not on
>
> struct myfoo *bar = rcu_dereference(foo);
Ah, "address_space" is a sparse-ism, no wonder I couldn't find it in
the gcc docs...
So the address_space attribute says what the pointer points to rather
than where the pointer resides, correct?
> by way of using __force inside rcu_dereference(), rcu_assign_pointer()
> etc.
>
> Would this be feasible? Or should one actually use __bitwise/__force to
> also get the warning when assigning between two variables both marked
> __rcu?
It might be. There are a number of places where it is legal to access
RCU-protected pointers directly, and all of these would need to be
changed. For example, in the example above, one could do:
foo = NULL;
I recently tried to modify rcu_assign_pointer() to issue the memory
memory barrier only when the pointer was non-NULL, but this ended badly.
Probably because I am not the greatest gcc expert around... We ended
up having to define an rcu_assign_index() to handle the possibility of
assigning a zero-value array index, but my attempts to do type-checking
backfired, and I eventually gave it up. Again, someone a bit more clued
in to gcc than I am could probably pull it off.
In addition, it is legal to omit rcu_dereference() and rcu_assign_pointer()
when holding the update-side lock.
So I very much like this approach in general, but it will require some
care to implement. I would be very happy to review and comment!!!
Thanx, Paul
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