Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

From: Leonid Yegoshin <[email-masked]>
Date: Thu, 14 Jan 2016 11:28:18 -0800

On 01/14/2016 04:14 AM, Will Deacon wrote:
> On Wed, Jan 13, 2016 at 02:26:16PM -0800, Leonid Yegoshin wrote:
>
>>      Moreover, there are voices against guarantee that it will be in future
>> and that voices point me to Documentation/memory-barriers.txt section "DATA
>> DEPENDENCY BARRIERS" examples which require SYNC_RMB between loading
>> address/index and using that for loading data based on that address or index
>> for shared data (look on CPU2 pseudo-code):
>>> To deal with this, a data dependency barrier or better must be inserted
>>> between the address load and the data load:
>>>
>>>         CPU 1                 CPU 2
>>>         ===============       ===============
>>>         { A == 1, B == 2, C = 3, P == &A, Q == &C }
>>>         B = 4;
>>>         <write barrier>
>>>         WRITE_ONCE(P, &B);
>>>                               Q = READ_ONCE(P);
>>>                               <data dependency barrier> <-----------
>>> SYNC_RMB is here
>>>                               D = *Q;
>> ...
>>> Another example of where data dependency barriers might be required is
>>> where a
>>> number is read from memory and then used to calculate the index for an
>>> array
>>> access:
>>>
>>>         CPU 1                 CPU 2
>>>         ===============       ===============
>>>         { M[0] == 1, M[1] == 2, M[3] = 3, P == 0, Q == 3 }
>>>         M[1] = 4;
>>>         <write barrier>
>>>         WRITE_ONCE(P, 1);
>>>                               Q = READ_ONCE(P);
>>>                               <data dependency barrier> <------------
>>> SYNC_RMB is here
>>>                               D = M[Q];
>> That voices say that there is a legitimate reason to relax HW here for
>> performance if SYNC_RMB is needed anyway to work with this sequence of
>> shared data.
> Are you saying that MIPS needs to implement [smp_]read_barrier_depends?

It is not me, it is Documentation/memory-barriers.txt from kernel sources.

HW team can't work on voice statements, it should do a work on written 
documents. If that is written (see above the lines which I marked by 
"SYNC_RMB") then anybody should use it and never mind how many 
CPUs/Threads are in play. This examples explicitly requires to insert 
"data dependency barrier" between reading a shared pointer/index and 
using it to fetch a shared data. So, your WRC+addr+addr test is a 
violation of that recommendation.

- Leonid.