Re: [RFC][PATCH 0/5] arch: atomic rework

[Paul E. McKenney]

On Thu, Feb 06, 2014 at 10:17:03PM +0100, Torvald Riegel wrote:
> On Thu, 2014-02-06 at 11:27 -0800, Paul E. McKenney wrote:
> > On Thu, Feb 06, 2014 at 06:59:10PM +0000, Will Deacon wrote:
> > > On Thu, Feb 06, 2014 at 06:55:01PM +0000, Ramana Radhakrishnan wrote:
> > > > On 02/06/14 18:25, David Howells wrote:
> > > > >
> > > > > Is it worth considering a move towards using C11 atomics and barriers and
> > > > > compiler intrinsics inside the kernel?  The compiler _ought_ to be able to do
> > > > > these.
> > > > 
> > > > 
> > > > It sounds interesting to me, if we can make it work properly and 
> > > > reliably. + gcc@xxxxxxxxxxx for others in the GCC community to chip in.
> > > 
> > > Given my (albeit limited) experience playing with the C11 spec and GCC, I
> > > really think this is a bad idea for the kernel. It seems that nobody really
> > > agrees on exactly how the C11 atomics map to real architectural
> > > instructions on anything but the trivial architectures. For example, should
> > > the following code fire the assert?
> > > 
> > > 
> > > extern atomic<int> foo, bar, baz;
> > > 
> > > void thread1(void)
> > > {
> > > 	foo.store(42, memory_order_relaxed);
> > > 	bar.fetch_add(1, memory_order_seq_cst);
> > > 	baz.store(42, memory_order_relaxed);
> > > }
> > > 
> > > void thread2(void)
> > > {
> > > 	while (baz.load(memory_order_seq_cst) != 42) {
> > > 		/* do nothing */
> > > 	}
> > > 
> > > 	assert(foo.load(memory_order_seq_cst) == 42);
> > > }
> > > 
> > > 
> > > To answer that question, you need to go and look at the definitions of
> > > synchronises-with, happens-before, dependency_ordered_before and a whole
> > > pile of vaguely written waffle to realise that you don't know. Certainly,
> > > the code that arm64 GCC currently spits out would allow the assertion to fire
> > > on some microarchitectures.
> > 
> > Yep!  I believe that a memory_order_seq_cst fence in combination with the
> > fetch_add() would do the trick on many architectures, however.  All of
> > this is one reason that any C11 definitions need to be individually
> > overridable by individual architectures.
> 
> "Overridable" in which sense?  Do you want to change the semantics on
> the language level in the sense of altering the memory model, or rather
> use a different implementation under the hood to, for example, fix
> deficiencies in the compilers?

We need the architecture maintainer to be able to select either an
assembly-language implementation or a C11-atomics implementation for any
given Linux-kernel operation.  For example, a given architecture might
be able to use fetch_add(1, memory_order_relaxed) for atomic_inc() but
assembly for atomic_add_return().  This is because atomic_inc() is not
required to have any particular ordering properties, while as discussed
previously, atomic_add_return() requires tighter ordering than the C11
standard provides.

> > > There are also so many ways to blow your head off it's untrue. For example,
> > > cmpxchg takes a separate memory model parameter for failure and success, but
> > > then there are restrictions on the sets you can use for each. It's not hard
> > > to find well-known memory-ordering experts shouting "Just use
> > > memory_model_seq_cst for everything, it's too hard otherwise". Then there's
> > > the fun of load-consume vs load-acquire (arm64 GCC completely ignores consume
> > > atm and optimises all of the data dependencies away) as well as the definition
> > > of "data races", which seem to be used as an excuse to miscompile a program
> > > at the earliest opportunity.
> > 
> > Trust me, rcu_dereference() is not going to be defined in terms of
> > memory_order_consume until the compilers implement it both correctly and
> > efficiently.  They are not there yet, and there is currently no shortage
> > of compiler writers who would prefer to ignore memory_order_consume.
> 
> Do you have any input on
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448?  In particular, the
> language standard's definition of dependencies?

Let's see...  1.10p9 says that a dependency must be carried unless:

â B is an invocation of any specialization of std::kill_dependency (29.3), or
â A is the left operand of a built-in logical AND (&&, see 5.14) or logical OR (||, see 5.15) operator,
or
â A is the left operand of a conditional (?:, see 5.16) operator, or
â A is the left operand of the built-in comma (,) operator (5.18);

So the use of "flag" before the "?" is ignored.  But the "flag - flag"
after the "?" will carry a dependency, so the code fragment in 59448
needs to do the ordering rather than just optimizing "flag - flag" out
of existence.  One way to do that on both ARM and Power is to actually
emit code for "flag - flag", but there are a number of other ways to
make that work.

BTW, there is some discussion on 1.10p9's handling of && and ||, and
that clause is likely to change.  And yes, I am behind on analyzing
usage in the Linux kernel to find out if Linux cares...

> > And rcu_dereference() will need per-arch overrides for some time during
> > any transition to memory_order_consume.
> > 
> > > Trying to introduce system concepts (writes to devices, interrupts,
> > > non-coherent agents) into this mess is going to be an uphill battle IMHO. I'd
> > > just rather stick to the semantics we have and the asm volatile barriers.
> > 
> > And barrier() isn't going to go away any time soon, either.  And
> > ACCESS_ONCE() needs to keep volatile semantics until there is some
> > memory_order_whatever that prevents loads and stores from being coalesced.
> 
> I'd be happy to discuss something like this in ISO C++ SG1 (or has this
> been discussed in the past already?).  But it needs to have a paper I
> suppose.

The current position of the usual suspects other than me is that this
falls into the category of forward-progress guarantees, which are
considers (again, by the usual suspects other than me) to be out
of scope.

> Will you be in Issaquah for the C++ meeting next week?

Weather permitting, I will be there!

							Thanx, Paul

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