Re: How should we do a 64-bit jiffies?

[torvalds@transmeta.com (Linus Torvalds)]

In article <3BD52454.218387D9@mvista.com>,
george anzinger  <george@mvista.com> wrote:
>
>I am beginning to think that defining a u64 and casting, i.e.:
>
>#define jiffies (unsigned long volitial)jiffies_u64
>
>is the way to go.

..except for gcc being bad at even 64->32-bit casts like the above.  It
will usually still load the full 64-bit value, and then only use the low
bits. 

The efficient and sane way to do it is:

	/*
	 * The 64-bit value is not volatile - you MUST NOT read it
	 * without holding the spinlock
	 */
	u64 jiffies_64;

	/*
	 * Most people don't necessarily care about the full 64-bit
	 * value, so we can just get the "unstable" low bits without
	 * holding the lock. For historical reasons we also mark
	 * it volatile so that busy-waiting doesn't get optimized
	 * away in old drivers.
	 */
	#if defined(__LITTLE_ENDIAN) || (BITS_PER_LONG > 32)
	#define jiffies (((volatile unsigned long *)&jiffies_64)[0])
	#else
	#define jiffies (((volatile unsigned long *)&jiffies_64)[1])
	#endif
	
which looks ugly, but the ugliness is confined to that one place, and
none of the users will ever have to care..

		Linus