Re: [PATCH] Document Linux's memory barriers

[Sergei Organov]

"linux-os \(Dick Johnson\)" <linux-os@xxxxxxxxxxxx> writes:

> On Tue, 7 Mar 2006, Matthew Wilcox wrote:
>
>> On Tue, Mar 07, 2006 at 01:54:33PM -0500, linux-os (Dick Johnson) wrote:
>>> This might be a good place to document:
>>>     dummy = readl(&foodev->ctrl);
>>>
>>> Will flush all pending writes to the PCI bus and that:
>>>     (void) readl(&foodev->ctrl);
>>> ... won't because `gcc` may optimize it away. In fact, variable
>>> "dummy" should be global or `gcc` may make it go away as well.
>>
>> static inline unsigned int readl(const volatile void __iomem *addr)
>> {
>> 	return *(volatile unsigned int __force *) addr;
>> }
>>
>> The cast is volatile, so gcc knows not to optimise it away.
>>
>
> When the assignment is not made a.k.a., cast to void, or when the
> assignment is made to an otherwise unused variable, `gcc` does,
> indeed make it go away.

Wrong. From the GCC texinfo documentation:

" Less obvious expressions are where something which looks like an access
is used in a void context.  An example would be,

     volatile int *src = SOMEVALUE;
     *src;

 With C, such expressions are rvalues, and as rvalues cause a read of
the object, GCC interprets this as a read of the volatile being pointed
to. "

So, did you report the bug to the GCC maintainers?

-- Sergei.

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