linux-kernel - Re: [PATCH v2 2/2] io: prevent compiler reordering on the default readX() implementation

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Message-ID: <587b59bb-2794-ffc2-3cd3-b77de85d3e7d@codeaurora.org>
Date:   Tue, 3 Apr 2018 09:06:23 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Mark Rutland <mark.rutland@....com>,
        Timur Tabi <timur@...eaurora.org>, sulrich@...eaurora.org,
        linux-arm-msm@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] io: prevent compiler reordering on the default
 readX() implementation

On 4/3/2018 8:56 AM, Arnd Bergmann wrote:
> On Tue, Apr 3, 2018 at 2:44 PM, Sinan Kaya <okaya@...eaurora.org> wrote:
>> On 4/3/2018 7:13 AM, Arnd Bergmann wrote:
>>> On Tue, Apr 3, 2018 at 12:49 PM, Mark Rutland <mark.rutland@....com> wrote:
>>>> Hi,
>>>>
>>>> On Fri, Mar 30, 2018 at 11:58:13AM -0400, Sinan Kaya wrote:
>>>>> The default implementation of mapping readX() to __raw_readX() is wrong.
>>>>> readX() has stronger ordering semantics. Compiler is allowed to reorder
>>>>> __raw_readX().
>>>>
>>>> Could you please specify what the compiler is potentially reordering
>>>> __raw_readX() against, and why this would be wrong?
>>>>
>>>> e.g. do we care about prior normal memory accesses, subsequent normal
>>>> memory accesses, and/or other IO accesses?
>>>>
>>>> I assume that the asm-generic __raw_{read,write}X() implementations are
>>>> all ordered w.r.t. each other (at least for a specific device).
>>>
>>> I think that is correct: the compiler won't reorder those because of the
>>> 'volatile' pointer dereference, but it can reorder access to a normal
>>> pointer against a __raw_readl()/__raw_writel(), which breaks the scenario
>>> of using writel to trigger a DMA, or using a readl to see if a DMA has
>>> completed.
>>
>> Yes, we are worried about memory update vs. IO update ordering here.
>> That was the reason why barrier() was introduced in this patch. I'll try to
>> clarify that better in the commit text.
>>
>>>
>>> The question is whether we should use a stronger barrier such
>>> as rmb() amd wmb() here rather than a simple compiler barrier.
>>>
>>> I would assume that on complex architectures with write buffers and
>>> out-of-order prefetching, those are required, while on architectures
>>> without those features, the barriers are cheap.
>>
>> That's my reasoning too. I'm trying to follow the x86 example here where there
>> is a compiler barrier in writeX() and readX() family of functions.
> 
> I think x86 is the special case here because it implicitly guarantees
> the strict ordering in the hardware, as long as the compiler gets it
> right. For the asm-generic version, it may be better to play safe and
> do the safest version, requiring architectures to override that barrier
> if they want to be faster.
> 
> We could use the same macros that riscv has, using __io_br(),
> __io_ar(), __io_bw() and __io_aw() for before/after read/write.

Sure, let me take a stab at it. 

> 
>       Arnd
> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.