RE: [PATCH] refcount_t: documentation for memory ordering differences

[Reshetova, Elena]

 On Wed, Nov 29, 2017 at 4:36 AM, Elena Reshetova
> <elena.reshetova@xxxxxxxxx> wrote:
> > Some functions from refcount_t API provide different
> > memory ordering guarantees that their atomic counterparts.
> > This adds a document outlining these differences.
> >
> > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> 
> Thanks for the improvements!
> 
> I have some markup changes to add, but I'll send that as a separate patch.

Thank you Kees! I guess I was too minimal on my markup use, so doc was pretty plain 
before. I have just joined your changes with mine and put both of our sign-off
to the resulting patch. I think this way it is easier for reviewers since ultimately
content is the same. 
I will now fix one more thing Randy noticed and then send it to linux-doc and 
Jon Corbet. 

Best Regards,
Elena.
> 
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> -Kees
> 
> > ---
> >  Documentation/core-api/index.rst              |   1 +
> >  Documentation/core-api/refcount-vs-atomic.rst | 129
> ++++++++++++++++++++++++++
> >  2 files changed, 130 insertions(+)
> >  create mode 100644 Documentation/core-api/refcount-vs-atomic.rst
> >
> > diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
> > index d5bbe03..d4d54b0 100644
> > --- a/Documentation/core-api/index.rst
> > +++ b/Documentation/core-api/index.rst
> > @@ -14,6 +14,7 @@ Core utilities
> >     kernel-api
> >     assoc_array
> >     atomic_ops
> > +   refcount-vs-atomic
> >     cpu_hotplug
> >     local_ops
> >     workqueue
> > diff --git a/Documentation/core-api/refcount-vs-atomic.rst
> b/Documentation/core-api/refcount-vs-atomic.rst
> > new file mode 100644
> > index 0000000..5619d48
> > --- /dev/null
> > +++ b/Documentation/core-api/refcount-vs-atomic.rst
> > @@ -0,0 +1,129 @@
> > +===================================
> > +refcount_t API compared to atomic_t
> > +===================================
> > +
> > +The goal of refcount_t API is to provide a minimal API for implementing
> > +an object's reference counters. While a generic architecture-independent
> > +implementation from lib/refcount.c uses atomic operations underneath,
> > +there are a number of differences between some of the refcount_*() and
> > +atomic_*() functions with regards to the memory ordering guarantees.
> > +This document outlines the differences and provides respective examples
> > +in order to help maintainers validate their code against the change in
> > +these memory ordering guarantees.
> > +
> > +memory-barriers.txt and atomic_t.txt provide more background to the
> > +memory ordering in general and for atomic operations specifically.
> > +
> > +Relevant types of memory ordering
> > +=================================
> > +
> > +**Note**: the following section only covers some of the memory
> > +ordering types that are relevant for the atomics and reference
> > +counters and used through this document. For a much broader picture
> > +please consult memory-barriers.txt document.
> > +
> > +In the absence of any memory ordering guarantees (i.e. fully unordered)
> > +atomics & refcounters only provide atomicity and
> > +program order (po) relation (on the same CPU). It guarantees that
> > +each atomic_*() and refcount_*() operation is atomic and instructions
> > +are executed in program order on a single CPU.
> > +This is implemented using READ_ONCE()/WRITE_ONCE() and
> > +compare-and-swap primitives.
> > +
> > +A strong (full) memory ordering guarantees that all prior loads and
> > +stores (all po-earlier instructions) on the same CPU are completed
> > +before any po-later instruction is executed on the same CPU.
> > +It also guarantees that all po-earlier stores on the same CPU
> > +and all propagated stores from other CPUs must propagate to all
> > +other CPUs before any po-later instruction is executed on the original
> > +CPU (A-cumulative property). This is implemented using smp_mb().
> > +
> > +A RELEASE memory ordering guarantees that all prior loads and
> > +stores (all po-earlier instructions) on the same CPU are completed
> > +before the operation. It also guarantees that all po-earlier
> > +stores on the same CPU and all propagated stores from other CPUs
> > +must propagate to all other CPUs before the release operation
> > +(A-cumulative property). This is implemented using smp_store_release().
> > +
> > +A control dependency (on success) for refcounters guarantees that
> > +if a reference for an object was successfully obtained (reference
> > +counter increment or addition happened, function returned true),
> > +then further stores are ordered against this operation.
> > +Control dependency on stores are not implemented using any explicit
> > +barriers, but rely on CPU not to speculate on stores. This is only
> > +a single CPU relation and provides no guarantees for other CPUs.
> > +
> > +
> > +Comparison of functions
> > +=======================
> > +
> > +case 1) - non-"Read/Modify/Write" (RMW) ops
> > +-------------------------------------------
> > +
> > +Function changes:
> > +                atomic_set() --> refcount_set()
> > +                atomic_read() --> refcount_read()
> > +
> > +Memory ordering guarantee changes:
> > +                none (both fully unordered)
> > +
> > +case 2) - increment-based ops that return no value
> > +--------------------------------------------------
> > +
> > +Function changes:
> > +                atomic_inc() --> refcount_inc()
> > +                atomic_add() --> refcount_add()
> > +
> > +Memory ordering guarantee changes:
> > +                none (both fully unordered)
> > +
> > +
> > +case 3) - decrement-based RMW ops that return no value
> > +------------------------------------------------------
> > +Function changes:
> > +                atomic_dec() --> refcount_dec()
> > +
> > +Memory ordering guarantee changes:
> > +                fully unordered --> RELEASE ordering
> > +
> > +
> > +case 4) - increment-based RMW ops that return a value
> > +-----------------------------------------------------
> > +
> > +Function changes:
> > +                atomic_inc_not_zero() --> refcount_inc_not_zero()
> > +                no atomic counterpart --> refcount_add_not_zero()
> > +
> > +Memory ordering guarantees changes:
> > +                fully ordered --> control dependency on success for stores
> > +
> > +*Note*: we really assume here that necessary ordering is provided as a result
> > +of obtaining pointer to the object!
> > +
> > +
> > +case 5) - decrement-based RMW ops that return a value
> > +-----------------------------------------------------
> > +
> > +Function changes:
> > +                atomic_dec_and_test() --> refcount_dec_and_test()
> > +                atomic_sub_and_test() --> refcount_sub_and_test()
> > +                no atomic counterpart --> refcount_dec_if_one()
> > +                atomic_add_unless(&var, -1, 1) --> refcount_dec_not_one(&var)
> > +
> > +Memory ordering guarantees changes:
> > +                fully ordered --> RELEASE ordering + control dependency
> > +
> > +Note: atomic_add_unless() only provides full order on success.
> > +
> > +
> > +case 6) - lock-based RMW
> > +------------------------
> > +
> > +Function changes:
> > +
> > +                atomic_dec_and_lock() --> refcount_dec_and_lock()
> > +                atomic_dec_and_mutex_lock() --> refcount_dec_and_mutex_lock()
> > +
> > +Memory ordering guarantees changes:
> > +                fully ordered --> RELEASE ordering + control dependency +
> > +                                  hold spin_lock() on success
> > --
> > 2.7.4
> >
> 
> 
> 
> --
> Kees Cook
> Pixel Security