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

Akira Yokosawa
2019-04-20T14:50:27Z

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().

> 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.

>
> 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:

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

Thanks, Akira

>
> 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])
>>>
>>
>