Re: [PATCH] "volatile considered harmful" document

[Robert Hancock <hancockr@shaw.ca>]

Bill Davidsen wrote:
> Jonathan Corbet wrote:
>
> > +There are still a few rare situations where volatile makes sense in the
> > +kernel:
> > +
> > +  - The above-mentioned accessor functions might use volatile on
> > +    architectures where direct I/O memory access does work.  
> > Essentially,
> > +    each accessor call becomes a little critical section on its own and
> > +    ensures that the access happens as expected by the programmer.
> > +
> > +  - Inline assembly code which changes memory, but which has no other
> > +    visible side effects, risks being deleted by GCC.  Adding the 
> > volatile
> > +    keyword to asm statements will prevent this removal.
> > +
> > +  - The jiffies variable is special in that it can have a different 
> > value
> > +    every time it is referenced, but it can be read without any special
> > +    locking.  So jiffies can be volatile, but the addition of other
> > +    variables of this type is frowned upon.  Jiffies is considered to 
> > be a
> > +    "stupid legacy" issue in this regard.
>
> It would seem that any variable which is (a) subject to change by other 
> threads or hardware, and (b) the value of which is going to be used 
> without writing the variable, would be a valid use for volatile.

You don't need volatile in that case, rmb() can be used.

--
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/