linux-kernel - Re: [PATCH v2 tip/core/rcu 0/13] Sparse-related updates for 3.13

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Message-ID: <20131009222357.GB11709@jtriplet-mobl1>
Date:	Wed, 9 Oct 2013 15:23:58 -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,
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Subject: Re: [PATCH v2 tip/core/rcu 0/13] Sparse-related updates for 3.13

On Wed, Oct 09, 2013 at 02:29:20PM -0700, Paul E. McKenney wrote:
> Hello!
> 
> This series features updates to allow sparse to do a better job of
> statically analyzing RCU usage:
> 
> 1.	Apply ACCESS_ONCE() to rcu_assign_pointer()'s target to prevent
> 	comiler mischief.  Also require that the source pointer be from
> 	the kernel address space.  Sometimes it can be from the RCU address
> 	space, which necessitates the remaining patches in this series.
> 	Which, it must be admitted, apply to a very small fraction of
> 	the rcu_assign_pointer() invocations in the kernel.  This commit
> 	courtesy of Josh Triplett.
> 
> 2-13.	Apply rcu_access_pointer() to avoid a number of false positives.

The use of rcu_access_pointer directly in the argument of
rcu_assign_pointer does add a new constraint to rcu_assign_pointer,
namely that it *must not* evaluate its argument more than once.
Currently, it expands its argument three times, but one expansion is
only visible to sparse (__CHECKER__), and one lives inside a typeof
(where it'll never be evaluated), so this is safe.  However, I'd add a
comment to that effect above rcu_assign_pointer, explicitly saying that
it must evaluate its argument exactly once; that way, if anyone ever
changes it, they'll know they have to introduce an appropriate local
temporary variable inside the macro.

- Josh Triplett
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