good explanation of __read_mostly, __init, __exit macros,

[Gaurav Jain]

Forgot to mention that this 'overlaying' is where 'linker magic' comes into
picture. I myself don't know the details though.

On Mon, Jul 16, 2012 at 7:56 PM, Gaurav Jain <gjainroorkee at gmail.com> wrote:

> One instance where this 'linker magic' is employed is in the treatment of
> the variable jiffies. jiffies has been traditionally an 'unsigned long'.
> Therefore, it occupies 32 bits on 32 bit arch. and 64 bits on 64-bit arch.
> However, having jiffies just 32-bit long can result in overflows pretty
> quickly (few days) if the HZ value is large. So, nowadays (2.6 kernel), we
> have one more variable - jiffies_64, which is 64 bit long. To maintain code
> compatibility, this 'jiffies_64' is overlayed on the original 'jiffies'
> variable, so that both variables begin at the same address. Older code can
> continue to operate as is, while new code can make use of the 'jiffies_64'
> variable, as and when required.
>
> Please refer Chapter:11 (Timers and Time Management), Topic: Internal
> Representation of Jiffies
> Linux Kernel Development, 3rd Edition - Robert Love to read about this.
>
> Best Regards
> Gaurav
>
>
> On Mon, Jul 16, 2012 at 6:10 PM, Robert P. J. Day <rpjday at crashcourse.ca>wrote:
>
>> On Mon, 16 Jul 2012, Filipe Rinaldi wrote:
>>
>> > On 16 July 2012 12:22, Aft nix <aftnix at gmail.com> wrote:
>> > > Hi,
>> > >
>> > > The macro expansion of __read_mostly :
>> > >
>> > > #define __read_mostly
>> __attribute__((__section__(".data..read_mostly"))
>> > >
>> > > This one is from cache.h
>> > >
>> > > __init:
>> > > #define __init          __section(.init.text) __cold notrace
>> > >
>> > > from init.h
>> > >
>> > > __exit:
>> > >
>> > > #define __exit          __section(.exit.text) __exitused __cold
>> notrace
>> > >
>> > > After searching through net i have not found any good explanation of
>> > > what is happening there.
>> > >
>> > > Additonal question : I have heard about various "linker magic"
>> > > employed in kernel development. Any information
>> > > regarding this will be wonderful.
>> > >
>> > > cheers
>> > >
>> > > --
>> > > -aft
>> > >
>> > > _______________________________________________
>> > > Kernelnewbies mailing list
>> > > Kernelnewbies at kernelnewbies.org
>> > > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>> >
>> >
>> > Hi Aft,
>> >
>> > These macros are used to assign a function or initialised variable
>> > content to a specific section in memory. If you search for the linker
>> > script of any architecture (*.lds.S files), you will see that these
>> > variables and functions are placed in specific sections with names
>> > like "__init_begin" and "__init_end". After the initialisation, Linux
>> > can re-use for example the "init" memory.
>>
>>   that's what's happening when you see that boot-time message:
>>
>> "Freeing unused memory ..."
>>
>> or whatever it is, something to that effect.
>>
>> rday
>>
>> --
>>
>> ========================================================================
>> Robert P. J. Day                                 Ottawa, Ontario, CANADA
>>                         http://crashcourse.ca
>>
>> Twitter:                                       http://twitter.com/rpjday
>> LinkedIn:                               http://ca.linkedin.com/in/rpjday
>> ========================================================================
>>
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>>
>
>
>
> --
> Gaurav Jain
> Associate Software Engineer
> VxVM Escalations Team, SAMG
> Symantec Software India Pvt. Ltd.
>
>
>
>


-- 
Gaurav Jain
Associate Software Engineer
VxVM Escalations Team, SAMG
Symantec Software India Pvt. Ltd.
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