linux-kernel - Re: [PATCH v2] Documentation: rewrite confusing statement about memory barriers

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Message-Id: <24b095bd-39f2-4a9b-2064-9ca38fa77380@linux.vnet.ibm.com>
Date:   Thu, 21 Sep 2017 16:53:45 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...ux.vnet.ibm.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        corbet@....net
Subject: Re: [PATCH v2] Documentation: rewrite confusing statement about
 memory barriers

On 09/21/2017 04:50 PM, Paul E. McKenney wrote:
> On Thu, Sep 21, 2017 at 04:29:01PM -0300, Guilherme G. Piccoli wrote:
>> In this specific portion of the write memory barriers description,
>> the documentation mentions sequential order of stores, which is
>> confusing since sequential ordering is not guaranteed.
>>
>> This patch tries to improve the doc in order to avoid any
>> mis-understanding.
>>
>> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...ux.vnet.ibm.com>
> 
> Good catch, and you are quite correct, a write barrier orders only
> before and after itself, doing nothing to impose order on preceding
> writes among themselves.

That's nice, thanks a lot Paul! :)

> 
> Applied, thank you!
> 
> 							Thanx, Paul
> 
>> ---
>>
>> v2: added Paul in CC.
>>
>>  Documentation/memory-barriers.txt | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
>> index b759a60624fd..a4bbbd1b63a0 100644
>> --- a/Documentation/memory-barriers.txt
>> +++ b/Documentation/memory-barriers.txt
>> @@ -383,8 +383,8 @@ Memory barriers come in four basic varieties:
>>       to have any effect on loads.
>>
>>       A CPU can be viewed as committing a sequence of store operations to the
>> -     memory system as time progresses.  All stores before a write barrier will
>> -     occur in the sequence _before_ all the stores after the write barrier.
>> +     memory system as time progresses.  All stores _before_ a write barrier
>> +     will occur _before_ all the stores after the write barrier.
>>
>>       [!] Note that write barriers should normally be paired with read or data
>>       dependency barriers; see the "SMP barrier pairing" subsection.
>> -- 
>> 2.14.1
>>