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

On Wed, Dec 06, 2006 at 11:29:42AM -0800, Christoph Lameter wrote:
> On Wed, 6 Dec 2006, Matthew Wilcox wrote:
> 
> > It's just been pointed out to me that the parisc one isn't safe.
> > 
> > <dhowells> imagine variable X is set to 3
> > <dhowells> CPU A issues cmpxchg(&X, 3, 5)
> > <dhowells> you'd expect that to change X to 5
> > <dhowells> but what if CPU B assigns 6 to X between cmpxchg reading X
> > and it setting X?
> 
> The same could happen with a regular cmpxchg. Cmpxchg changes it to 5 and 
> then other cpu performs a store before the next instruction.

For someone who's advocating use of cmpxchg, it seems you don't
understand its semantics!  In the scenario dhowells pointed out, X would
be left set to 5.  X should have the value 6 under any legitimate
implementation:

CPU A		CPU B
cmpxchg(3,5)
		X = 6


CPU A		CPU B
		X = 6
cmpxhcg(3,5)


CPU A
cmpxchg(3,
		X = 6
5)


Given that even yourself got confused about how to use it, perhaps it's
not a good idea to expose this primitive to most programmers anyway?
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

---

• Follow-Ups:
  • Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
    • From: Christoph Lameter <[email protected]>

• References:
  • [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
    • From: David Howells <[email protected]>
  • Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
    • From: Christoph Lameter <[email protected]>
  • Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
    • From: Linus Torvalds <[email protected]>
  • Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
    • From: Matthew Wilcox <[email protected]>
  • Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
    • From: Christoph Lameter <[email protected]>

• Prev by Date: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
• Next by Date: Re: -mm merge plans for 2.6.20
• Previous by thread: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
• Next by thread: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
• Index(es):
  • Date
  • Thread

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]