Re: BUG: iomap_dio_rw() accesses freed memory

[Christoph Hellwig <hch@lst.de>]

On Wed, Jan 16, 2019 at 07:51:41AM +1100, Dave Chinner wrote:
> Atomic operations don't imply a memory barrier for dependent data,
> right?

Documentation/atomic_t.txt says:

-------------------------- snip --------------------------
The rule of thumb:

 - non-RMW operations are unordered;

 - RMW operations that have no return value are unordered;

 - RMW operations that have a return value are fully ordered;

[...]

Fully ordered primitives are ordered against everything prior and everything
subsequent. Therefore a fully ordered primitive is like having an smp_mb()
before and an smp_mb() after the primitive.


-------------------------- snip --------------------------

I think atomic_dec_and_test clearly falls into the third category,
and I can't see how much of the kernel could work if that wasn't the
case.