Messages in this thread |  | | | Date | Fri, 21 Jan 2011 11:07:16 -0600 (CST) | | From | Christoph Lameter <> | | Subject | Re: [cpuops cmpxchg double V2 1/4] Generic support for this_cpu_cmpxchg_double |
| |
On Fri, 21 Jan 2011, Mathieu Desnoyers wrote:
> I'm again probably missing something, but what is "clumsy" about defining a > structure like the following to ensure proper alignment of the target > pointer (instead of adding a runtime test) ? > > struct cmpxchg_double { > #if __BYTE_ORDER == __LITTLE_ENDIAN > unsigned long low, high; > #else > unsigned long high, low; > #endif > } __attribute__((packed, aligned(2 * sizeof(unsigned long)))); > > (note: packed here along with "aligned" does _not_ generate ugly bytewise > read/write memory ops like "packed" alone. The use of "packed" is to let the > compiler down-align the structure to the value requested, instead of uselessly > aligning it on 32-byte if it chooses to.) > > The prototype could then look like: > > bool __this_cpu_generic_cmpxchg_double(pcp, oval_low, oval_high, nval_low, nval_high); > > With: > struct cmpxchg_double *pcp
That does not conform to the parameter conventions in other this_cpu_ops. The first parameter is a variable because the notion of a pointer is problematic given that percpu operations use a segment prefix to relocate pointers. You would be implicitly passing a 128 bit argument although the compiler may not need to generate code for that.
> I think Christoph's point is that he wants to alias this with a pointer. Well, > this can be done cleanly with: > > union { > struct cmpxchg_double casdbl; > struct { > void *ptr; > unsigned long cpuid_tid; > } t; > }
There is no need for aliases with the existing implementation.
How will the macro check the parameters now?
> Thoughts ?
Could you actually try to write a patch instead running through points that we have discussed earlier?
|  |