Messages in this thread | | | Date | Fri, 17 Aug 2007 12:08:31 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures |
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On Fri, 17 Aug 2007, Chris Friesen wrote: > > I assume you mean "except for IO-related code and 'random' values like > jiffies" as you mention later on?
Yes. There *are* valid uses for "volatile", but they have remained the same for the last few years: - "jiffies" - internal per-architecture IO implementations that can do them as normal stores.
> I assume other values set in interrupt handlers would count as "random" > from a volatility perspective?
I don't really see any valid case. I can imagine that you have your own "jiffy" counter in a driver, but what's the point, really? I'd suggest not using volatile, and using barriers instead.
> > > So anybody who argues for "volatile" fixing bugs is fundamentally > > incorrect. It does NO SUCH THING. By arguing that, such people only > > show that you have no idea what they are talking about.
> What about reading values modified in interrupt handlers, as in your > "random" case? Or is this a bug where the user of atomic_read() is > invalidly expecting a read each time it is called?
Quite frankly, the biggest reason for using "volatile" on jiffies was really historic. So even the "random" case is not really a very strong one. You'll notice that anybody who is actually careful will be using sequence locks for the jiffy accesses, if only because the *full* jiffy count is actually a 64-bit value, and so you cannot get it atomically on a 32-bit architecture even on a single CPU (ie a timer interrupt might happen in between reading the low and the high word, so "volatile" is only used for the low 32 bits).
So even for jiffies, we actually have:
extern u64 __jiffy_data jiffies_64; extern unsigned long volatile __jiffy_data jiffies;
where the *real* jiffies is not volatile: the volatile one is using linker tricks to alias the low 32 bits:
- arch/i386/kernel/vmlinux.lds.S:
... jiffies = jiffies_64; ...
and the only reason we do all these games is (a) it works and (b) it's legacy.
Note how I do *not* say "(c) it's a good idea".
Linus
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