Re: [patch] spinlocks: remove 'volatile'

linux-os (Dick Johnson)
2006-07-06T12:29:57Z

On Thu, 6 Jul 2006, Arjan van de Ven wrote:

> On Thu, 2006-07-06 at 07:59 -0400, linux-os (Dick Johnson) wrote:
>> On Thu, 6 Jul 2006, Ingo Molnar wrote:
>>
>>>
>>> * Linus Torvalds <torvalds@osdl.org> wrote:
>>>
>>>> I wonder if we should remove the "volatile". There really isn't
>>>> anything _good_ that gcc can do with it, but we've seen gcc code
>>>> generation do stupid things before just because "volatile" seems to
>>>> just disable even proper normal working.
>>
>> Then GCC must be fixed. The keyword volatile is correct. It should
>> force the compiler to read the variable every time it's used.
>
> this is not really what the C standard says.
>
>
>
>> This is not pointless. If GCC generates bad code, tell the
>> GCC people. The volatile keyword is essential.
>
> no the "volatile" semantics are vague, trecherous and evil. It's a LOT
> better to insert the well defined "barrier()" in the right places.

Look at:

http://en.wikipedia.org/wiki/Volatile_variable

This is just what is needed to prevent the compiler from making non working
code during optimization.

Also look at:

http://en.wikipedia.org/wiki/Memory_barrier

This is used to prevent out-of-order execution, not at all what is
necessary.

Also, just because it 'works' means nothing. When SMP processors
came about, we had many drivers with no spin-locks at all. Sometimes
some files had a byte or two changed. That was the only obvious
problem. If this had continued, the entire file system would be
trashed, but fortunately we learned about spin-locks and the
volatile keyword. You must not destroy the spin locks by removing
the volatile keyword.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.88 BogoMips).
New book: http://www.AbominableFirebug.com/
_
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