Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

Ralf Baechle
2006-12-07T16:51:58Z

On Thu, Dec 07, 2006 at 08:31:08PM +1100, Nick Piggin wrote:

> Wrong. Your ll/sc implementation with cmpxchg is buggy. The cmpxchg
> load_locked is not locked at all, and there can be interleaving writes
> between the load and cmpxchg which do not cause the store_conditional
> to fail.
>
> It might be reasonable to implement this watered down version, but:
> don't some architectures have restrictions on what instructions can
> be issued between the ll and the sc?

On MIPS the restriction is no loads or stores or sync instructions between
ll/sc. Also there may be at most 2048 bytes between the address of the
ll and sc instructions. Which means ll/sc sequences should better be
written in assembler or gcc might do a bit too creative things ...

Ralf