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

[Paul E. McKenney]

On Sat, Apr 20, 2019 at 12:06:58AM +0900, Akira Yokosawa wrote:
> Hi Paul,
> 
> Please find inline comments below.
> 
> On Fri, 19 Apr 2019 05:47:20 -0700, Paul E. McKenney wrote:
> > On Fri, Apr 19, 2019 at 02:53:02AM +0200, Andrea Parri wrote:
> >>> Are you saying that on x86, atomic_inc() acts as a full memory barrier 
> >>> but not as a compiler barrier, and vice versa for 
> >>> smp_mb__after_atomic()?  Or that neither atomic_inc() nor 
> >>> smp_mb__after_atomic() implements a full memory barrier?
> >>
> >> I'd say the former; AFAICT, these boil down to:
> >>
> >>   https://elixir.bootlin.com/linux/v5.1-rc5/source/arch/x86/include/asm/atomic.h#L95
> >>   https://elixir.bootlin.com/linux/v5.1-rc5/source/arch/x86/include/asm/barrier.h#L84
> > 
> > OK, how about the following?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 19d166dadc4e1bba4b248fb46d32ca4f2d10896b
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> > Date:   Fri Apr 19 05:20:30 2019 -0700
> > 
> >     tools/memory-model: Make smp_mb__{before,after}_atomic() match x86
> >     
> >     Read-modify-write atomic operations that do not return values need not
> >     provide any ordering guarantees, and this means that both the compiler
> >     and the CPU are free to reorder accesses across things like atomic_inc()
> >     and atomic_dec().  The stronger systems such as x86 allow the compiler
> >     to do the reordering, but prevent the CPU from so doing, and these
> >     systems implement smp_mb__{before,after}_atomic() as compiler barriers.
> >     The weaker systems such as Power allow both the compiler and the CPU
> >     to reorder accesses across things like atomic_inc() and atomic_dec(),
> >     and implement smp_mb__{before,after}_atomic() as full memory barriers.
> >     
> >     This means that smp_mb__before_atomic() only orders the atomic operation
> >     itself with accesses preceding the smp_mb__before_atomic(), and does
> >     not necessarily provide any ordering whatsoever against accesses
> >     folowing the atomic operation.  Similarly, smp_mb__after_atomic()
> 
> s/folowing/following/

Good eyes, fixed!

> >     only orders the atomic operation itself with accesses following the
> >     smp_mb__after_atomic(), and does not necessarily provide any ordering
> >     whatsoever against accesses preceding the atomic operation.  Full ordering
> >     therefore requires both an smp_mb__before_atomic() before the atomic
> >     operation and an smp_mb__after_atomic() after the atomic operation.
> >     
> >     Therefore, linux-kernel.cat's current model of Before-atomic
> >     and After-atomic is too strong, as it guarantees ordering of
> >     accesses on the other side of the atomic operation from the
> >     smp_mb__{before,after}_atomic().  This commit therefore weakens
> >     the guarantee to match the semantics called out above.
> >     
> >     Reported-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
> >     Suggested-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> > 
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index 169d938c0b53..e5b97c3e8e39 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -1888,7 +1888,37 @@ There are some more advanced barrier functions:
> >  	atomic_dec(&obj->ref_count);
> >  
> >       This makes sure that the death mark on the object is perceived to be set
> > -     *before* the reference counter is decremented.
> > +     *before* the reference counter is decremented.  However, please note
> > +     that smp_mb__before_atomic()'s ordering guarantee does not necessarily
> > +     extend beyond the atomic operation.  For example:
> > +
> > +	obj->dead = 1;
> > +	smp_mb__before_atomic();
> > +	atomic_dec(&obj->ref_count);
> > +	r1 = a;
> > +
> > +     Here the store to obj->dead is not guaranteed to be ordered with
> > +     with the load from a.  This reordering can happen on x86 as follows:
> 
> s/with//

Fixed fixed.  ;-)

> And I beg you to avoid using the single letter variable "a".
> It's confusing.

Good point!  I changed it to "x".

> > +     (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.
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.

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?

							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])
> > 
>