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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Russell King wrote:
> On Wed, Dec 06, 2006 at 11:16:55AM -0800, Christoph Lameter wrote:

> No.  If you read what I said, you'll see that you can _cheaply_ use
> cmpxchg in a ll/sc based implementation.  Take an atomic increment
> operation.
> 
> 	do {
> 		old = load_locked(addr);
> 	} while (store_exclusive(old, old + 1, addr);
> 
> On a cmpxchg, that "store_exclusive" (loosely) becomes your cmpxchg
> instruction, comparing the first arg, and if equal storing the second.
> The "load_locked" macro becomes a standard pointer deref.  Ergo, x86
> becomes:
> 
> 	do {
> 		load value
> 		manipulate it
> 		conditional store
> 	} while not stored
> 
> On ll/sc, the load_locked() macro is the load locked instruction.  The
> store_exclusive() macro is the exclusive store and it doesn't need to
> use the first parameter at all.  Ergo, ARM becomes:
> 
> 	do {
> 		ldrex r1, [r2]
> 		manipulate r1
> 		strex r0, r1, [r2]
> 	} while failed
> 
> Notice that both are optimal.
> 
> Now let's consider the cmpxchg case.
> 
> 	do {
> 		val = *addr;
> 	} while (cmpxchg(val, val + 1, addr);
> 
> The x86 case is _identical_ to the ll/sc based implementation.  Absolutely
> entirely.  No impact what so ever.
> 
> Let's look at the ll/sc case.  The cmpxchg code implemented on this has
> to reload the original value, compare it, if equal store the new value.
> So:
> 
> 	do {
> 		val = *addr;
> 		(r2 = addr, 
> 		ldrex r1, [r2]
> 		compare r1, r0
> 		strexeq r4, r3, [r2] (store exclusive if equal)
> 	} while store failed or comparecondition failed
> 
> Note how the cmpxchg has _forced_ the ll/sc implementation to become
> more complex.
> 
> So, let's recap.
> 
> Implementing ll/sc based accessor macros allows both ll/sc _and_ cmpxchg
> architectures to produce optimal code.
> 
> Implementing an cmpxchg based accessor macro allows cmpxchg architectures
> to produce optimal code and ll/sc non-optimal code.
> 
> See my point?

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?

But in general I agree with you, in that a higher level primitive is
preferable (eg. atomic_add_unless).

-- 
SUSE Labs, Novell Inc.
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