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SubjectRe: [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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