Re: Kernel Concurrency Sanitizer (KCSAN)

[Boqun Feng]

On Fri, Sep 20, 2019 at 04:54:21PM +0100, Will Deacon wrote:
> Hi Marco,
> 
> On Fri, Sep 20, 2019 at 04:18:57PM +0200, Marco Elver wrote:
> > We would like to share a new data-race detector for the Linux kernel:
> > Kernel Concurrency Sanitizer (KCSAN) --
> > https://github.com/google/ktsan/wiki/KCSAN  (Details:
> > https://github.com/google/ktsan/blob/kcsan/Documentation/dev-tools/kcsan.rst)
> > 
> > To those of you who we mentioned at LPC that we're working on a
> > watchpoint-based KTSAN inspired by DataCollider [1], this is it (we
> > renamed it to KCSAN to avoid confusion with KTSAN).
> > [1] http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf
> 
> Oh, spiffy!
> 
> > In the coming weeks we're planning to:
> > * Set up a syzkaller instance.
> > * Share the dashboard so that you can see the races that are found.
> > * Attempt to send fixes for some races upstream (if you find that the
> > kcsan-with-fixes branch contains an important fix, please feel free to
> > point it out and we'll prioritize that).
> 
> Curious: do you take into account things like alignment and/or access size
> when looking at READ_ONCE/WRITE_ONCE? Perhaps you could initially prune
> naturally aligned accesses for which __native_word() is true?
> 
> > There are a few open questions:
> > * The big one: most of the reported races are due to unmarked
> > accesses; prioritization or pruning of races to focus initial efforts
> > to fix races might be required. Comments on how best to proceed are
> > welcome. We're aware that these are issues that have recently received
> > attention in the context of the LKMM
> > (https://lwn.net/Articles/793253/).
> 
> This one is tricky. What I think we need to avoid is an onslaught of
> patches adding READ_ONCE/WRITE_ONCE without a concrete analysis of the
> code being modified. My worry is that Joe Developer is eager to get their
> first patch into the kernel, so runs this tool and starts spamming
> maintainers with these things to the point that they start ignoring KCSAN
> reports altogether because of the time they take up.
> 
> I suppose one thing we could do is to require each new READ_ONCE/WRITE_ONCE
> to have a comment describing the racy access, a bit like we do for memory
> barriers. Another possibility would be to use atomic_t more widely if
> there is genuine concurrency involved.
> 

Instead of commenting READ_ONCE/WRITE_ONCE()s, how about adding
anotations for data fields/variables that might be accessed without
holding a lock? Because if all accesses to a variable are protected by
proper locks, we mostly don't need to worry about data races caused by
not using READ_ONCE/WRITE_ONCE(). Bad things happen when we write to a
variable using locks but read it outside a lock critical section for
better performance, for example, rcu_node::qsmask. I'm thinking so maybe
we can introduce a new annotation similar to __rcu, maybe call it
__lockfree ;-) as follow:

	struct rcu_node {
		...
		unsigned long __lockfree qsmask;
		...
	}

, and __lockfree indicates that by design the maintainer of this data
structure or variable believe there will be accesses outside lock
critical sections. Note that not all accesses to __lockfree field, need
to be READ_ONCE/WRITE_ONCE(), if the developer manages to build a
complex but working wake/wait state machine so that it could not be
accessed in the same time, READ_ONCE()/WRITE_ONCE() is not needed.

If we have such an annotation, I think it won't be hard for configuring
KCSAN to only examine accesses to variables with this annotation. Also 
this annotation could help other checkers in the future.

If KCSAN (at the least the upstream version) only check accesses with
such an anotation, "spamming with KCSAN warnings/fixes" will be the
choice of each maintainer ;-) 

Thoughts?

Regards,
Boqun

> > * How/when to upstream KCSAN?
> 
> Start by posting the patches :)
> 
> Will

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