linux-kernel - Re: [PATCH v2 tip/core/rcu 01/13] rcu: Make rcu_assign_pointer's assignment volatile and type-safe

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Message-ID: <20131009222656.GC11709@jtriplet-mobl1>
Date:	Wed, 9 Oct 2013 15:26:56 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH v2 tip/core/rcu 01/13] rcu: Make rcu_assign_pointer's
 assignment volatile and type-safe

On Wed, Oct 09, 2013 at 02:29:34PM -0700, Paul E. McKenney wrote:
> From: Josh Triplett <josh@...htriplett.org>
> 
> rcu_assign_pointer needs to use ACCESS_ONCE to make the assignment to
> the destination pointer volatile, to protect against compilers too
> clever for their own good.
> 
> In addition, since rcu_assign_pointer force-casts the source pointer to
> add the __rcu address space (overriding any existing address space), add
> an explicit check that the source pointer has the __kernel address space
> to start with.
> 
> This new check produces warnings like this, when attempting to assign
> from a __user pointer:
> 
> test.c:25:9: warning: incorrect type in argument 2 (different address spaces)
> test.c:25:9:    expected struct foo *<noident>
> test.c:25:9:    got struct foo [noderef] <asn:1>*badsrc
> 
> Signed-off-by: Josh Triplett <josh@...htriplett.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

See my responses to the cover letter; I'd suggest making this the last
patch of the series to avoid introducing and subsequently fixing
warnings.

I'd suggest adding the comment I mentioned in my second response as the
new first patch of the series, since it applies to the old version as
well; thus, the series would first add the comment documenting the
evaluate-once constraint, then add all the calls to rcu_access_pointer,
and finally add this patch introducing the sparse checking.

- Josh Triplett
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