Re: Adding plain accesses and detecting data races in the LKMM

[Andrea Parri <andrea.parri@amarulasolutions.com>]

> Another question is "should the kernel permit smp_mb__{before,after}*()
> anywhere other than immediately before or after the primitive being
> strengthened?"

Mmh, I do think that keeping these barriers "immediately before or after
the primitive being strengthened" is a good practice (readability, and
all that), if this is what you're suggesting.

However, a first auditing of the callsites showed that this practice is
in fact not always applied, notably... ;-)

	kernel/rcu/tree_exp.h:sync_exp_work_done
	kernel/sched/cpupri.c:cpupri_set

So there appear, at least, to be some exceptions/reasons for not always
following it?  Thoughts?

BTW, while auditing these callsites, I've stumbled across the following
snippet (from kernel/futex.c):

	*futex = newval;
	sys_futex(WAKE, futex);
          futex_wake(futex);
          smp_mb(); (B)
	  if (waiters)
	    ...

where B is actually (c.f., futex_get_mm()):

	atomic_inc(...->mm_count);
	smp_mb__after_atomic();

It seems worth mentioning the fact that, AFAICT, this sequence does not
necessarily provide ordering when plain accesses are involved: consider,
e.g., the following variant of the snippet:

	A:*x = 1;
	/*
	 * I've "ignored" the syscall, which should provide
	 * (at least) a compiler barrier...
	 */
	atomic_inc(u);
	smp_mb__after_atomic();
	B:r0 = *y;

On x86, AFAICT, the compiler can do this:

	atomic_inc(u);
	A:*x = 1;
	smp_mb__after_atomic();
	B:r0 = *y;

(the implementation of atomic_inc() contains no compiler barrier), then
the CPU can "reorder" A and B (smp_mb__after_atomic() being #defined to
a compiler barrier).

The mips implementation seems also affected by such "reorderings": I am
not familiar with this implementation but, AFAICT, it does not enforce
ordering from A to B in the following snippet:

	A:*x = 1;
	atomic_inc(u);
	smp_mb__after_atomic();
	B:WRITE_ONCE(*y, 1);

when CONFIG_WEAK_ORDERING=y, CONFIG_WEAK_REORDERING_BEYOND_LLSC=n.

Do these observations make sense to you?  Thoughts?

  Andrea