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

[Akira Yokosawa]

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/

>     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//

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

> +     (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?

        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.
>  
>       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])
>