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

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Sat, Apr 20, 2019 at 11:50:14PM +0900, Akira Yokosawa wrote:
> On Fri, 19 Apr 2019 11:06:41 -0700, Paul E. McKenney wrote:
> > On Sat, Apr 20, 2019 at 12:06:58AM +0900, Akira Yokosawa wrote:
> >> Hi Paul,
> >>
> [...]
> > 
> >>> +     (1) The compiler can reorder the load from a to precede the
> >>> +     atomic_dec(), (2) Because x86 smp_mb__before_atomic() is only a
> >>> +     compiler barrier, the CPU can reorder the preceding store to
> >>> +     obj->dead with the later load from a.
> >>> +
> >>> +     This could be avoided by using READ_ONCE(), which would prevent the
> >>> +     compiler from reordering due to both atomic_dec() and READ_ONCE()
> >>> +     being volatile accesses, and is usually preferable for loads from
> >>> +     shared variables.  However, weakly ordered CPUs would still be
> >>> +     free to reorder the atomic_dec() with the load from a, so a more
> >>> +     readable option is to also use smp_mb__after_atomic() as follows:
> >>
> >> The point here is not just "readability", but also the portability of the
> >> code, isn't it?
> > 
> > As Andrea noted, in this particular case, the guarantee that the
> > store to obj->dead precedes the load from x is portable.  Either the
> > smp_mb__before_atomic() or the atomic_dec() must provide the ordering.
> 
> I think I understood this. What I wanted to say was the code for x86 implied
> in the subjunctive sentence:
> 
> 	obj->dead = 1;
> 	smp_mb__before_atomic();
> 	atomic_dec(&obj->ref_count);
> 	r1 = READ_ONCE(x);
> 
> , which was not spelled out, is not portable if we expect the ordering of
> atomic_dec() with READ_ONCE().

I now understand that you understood.  ;-)

> > However, you are right that there is some non-portability.  But this
> > non-portability involves the order of the atomic_dec() and the store to x.
> 
> Yes, you've guessed it right.

Don't worry, it won't happen again!

> > So what I did was ...
> > 
> >>         Thanks, Akira
> >>
> >>> +
> >>> +	WRITE_ONCE(obj->dead, 1);
> >>> +	smp_mb__before_atomic();
> >>> +	atomic_dec(&obj->ref_count);
> >>> +	smp_mb__after_atomic();
> >>> +	r1 = READ_ONCE(a);
> >>> +
> >>> +     This orders all three accesses against each other, and also makes
> >>> +     the intent quite clear.
> > 
> > ... change the above paragraph to read as follows:
> > 
> >      In addition, the example without the smp_mb__after_atomic() does
> >      not necessarily order the atomic_dec() with the load from x.
> >      In contrast, the example with both smp_mb__before_atomic() and
> >      smp_mb__after_atomic() orders all three accesses against each other,
> >      and also makes the intent quite clear.
> > 
> > Does that help?
> 
> This looks a little bit redundant to me. The original one is clear
> enough.
> 
> How about editing the leading sentence above:
> 
> >>> +     shared variables.  However, weakly ordered CPUs would still be
> >>> +     free to reorder the atomic_dec() with the load from a, so a more
> >>> +     readable option is to also use smp_mb__after_atomic() as follows:
> 
> to read as follows?
> 
>      shared variables.  However, weakly ordered CPUs would still be
>      free to reorder the atomic_dec() with the load from x, so a
>      portable and more readable option is to also use
>      smp_mb__after_atomic() as follows:

Adding "portable and", correct?  Makes sense, so I applied this change.

> Obviously, the interesting discussion going on in another thread will
> surely affect this patch.

Quite possibly!  ;-)


							Thanx, Paul

> >>>       See Documentation/atomic_{t,bitops}.txt for more information.
> >>>  
> >>> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> >>> index 8dcb37835b61..b6866f93abb8 100644
> >>> --- a/tools/memory-model/linux-kernel.cat
> >>> +++ b/tools/memory-model/linux-kernel.cat
> >>> @@ -28,8 +28,8 @@ include "lock.cat"
> >>>  let rmb = [R \ Noreturn] ; fencerel(Rmb) ; [R \ Noreturn]
> >>>  let wmb = [W] ; fencerel(Wmb) ; [W]
> >>>  let mb = ([M] ; fencerel(Mb) ; [M]) |
> >>> -	([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
> >>> -	([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
> >>> +	([M] ; fencerel(Before-atomic) ; [RMW]) |
> >>> +	([RMW] ; fencerel(After-atomic) ; [M]) |
> >>>  	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
> >>>  	([M] ; po ; [UL] ; (co | po) ; [LKW] ;
> >>>  		fencerel(After-unlock-lock) ; [M])
> >>>
> >>
> > 
>