Re: [patch] spinlocks: remove 'volatile'

[Avi Kivity <avi@argo.co.il>]

linux-os (Dick Johnson) wrote:
> >> The 'C' compiler has no choice but to actually make that memory access
> >> and read the variable because the variable is in another module 
> (a.k.a.
> >> file).

[code showing the compiler may cache the access]

> So read the code; you have "10:   jne 10", jumping to itself
> forever, without even doing anything else to set the flags, much
> less reading a variable.

Short attention span, eh? He's proven you wrong and you go on and talk 
about something else.

> >
> >> However, if I have the same code, but the variable is visible during
> >> compile time, i.e.,
> >>
> >> int spinner=0;
> >>
> >> funct(){
> >>      while(spinner)
> >>          ;
> >>
> >> ... the compiler may eliminate that code altogether because it
> >> 'knows' that spinner is FALSE, having initialized it to zero
> >> itself.
> >

[code showing that defining the variable in the same translation unit 
makes no difference]

> Then, you have exactly the same thing here:
>    10:   75 fe                   jne    10 <funct+0x10>
>
> Same bad code.

You seem to have forgotten that you claimed different code would be 
generated.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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