[PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier

[Boqun Feng]

On Wed, Oct 21, 2015 at 09:48:25PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 21, 2015 at 12:35:23PM -0700, Paul E. McKenney wrote:
> > > > > > I ask this because I recall Peter once bought up a discussion:
> > > > > > 
> > > > > > https://lkml.org/lkml/2015/8/26/596
> 
> > > So a full barrier on one side of these operations is enough, I think.
> > > IOW, there is no need to strengthen these operations.
> > 
> > Do we need to also worry about other futex use cases?
> 
> Worry, always!
> 
> But yes, there is one more specific usecase, which is that of a
> condition variable.
> 
> When we go sleep on a futex, we might want to assume visibility of the
> stores done by the thread that woke us by the time we wake up.
> 

But the thing is futex atomics in PPC are already RELEASE(pc)+ACQUIRE
and imply a full barrier, is an RELEASE(sc) semantics really needed
here?

Further more, is this condition variable visibility guaranteed by other
part of futex? Because in futex_wake_op:

	futex_wake_op()
	  ...
	  double_unlock_hb(hb1, hb2);  <- RELEASE(pc) barrier here.
	  wake_up_q(&wake_q);

and in futex_wait():

	futex_wait()
	  ...
	  futex_wait_queue_me(hb, &q, to); <- schedule() here
	  ...
	  unqueue_me(&q)
	    drop_futex_key_refs(&q->key);
	    	iput()/mmdrop(); <- a full barrier
	  

The RELEASE(pc) barrier pairs with the full barrier, therefore the
userspace wakee can observe the condition variable modification.

> 
> 
> And.. aside from the thoughts I outlined in the email referenced above,
> there is always the chance people accidentally rely on the strong
> ordering on their x86 CPU and find things come apart when ran on their
> ARM/MIPS/etc..
> 
> There are a fair number of people who use the raw futex call and we have
> 0 visibility into many of them. The assumed and accidental ordering
> guarantees will forever remain a mystery.
> 

Understood. That's truely a potential problem. Considering not all the
architectures imply a full barrier at user<->kernel boundries, maybe we
can use one bit in the opcode of the futex system call to indicate
whether userspace treats futex as fully ordered. Like:

#define FUTEX_ORDER_SEQ_CST  0
#define FUTEX_ORDER_RELAXED  64 (bit 7 and bit 8 are already used)

Therefore all existing code will run with a strong ordering version of
futex(of course, we need to check and modify kernel code first to
guarantee that), and if userspace code uses FUTEX_ORDER_RELAXED, it must
not rely on futex() for the strong ordering, and should add memory
barriers itself if necessary.

Regards,
Boqun
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