Re: [PATCH 1/3] make sparse keep its promise about context tracking

[Josh Triplett <josh@freedesktop.org>]

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Johannes Berg wrote:
>> And as for how to solve it: I think alias analysis might work.
> 
> How would I do that?

Real compilers have alias analysis code, which builds up information
on which variables and expressions may/must alias each other.  I think
that that alias information might prove sufficient to compare
contexts: if two contexts alias each other then they represent the
same context.

Basic type-based alias analysis should prove sufficient to handle
cases like:

spinlock_t s;
struct something_with_a_spinlock st;

spin_lock(&st.s);
spin_unlock(&st.s); /* contexts match */

spin_lock(&st.s);
spin_unlock(&s); /* warn */

Flow-based alias analysis may also handle cases like you mentioned,
where you assign a lock to a variable and then use it through that
variable.


Alternatively, some simple dataflow analysis and symbolic execution
might let you figure out the lock a variable points to in most cases.


As a simpler solution, you could just do what lockdep does: make
assumptions that handle the common cases, and allow explicit overrides
for the uncommon cases.  I suspect that most code does not assign a
lock to a variable and use the lock through that variable.

In particular, you could perhaps make the assumption that any
expression which resolves to the "foo" field of a "struct bar"
represents the same context, rather than attempting to distinguish
between them.

- Josh Triplett


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