Re: [PATCH 19/19] lockd: clean up 64-bit alignment fix in nsm_init_private()

["Måns Rullgård" <mans-2StjZFpD7GcAvxtiuMwx3w@public.gmane.org>]

Chuck Lever <chuck.lever@oracle.com> writes:

> On Apr 28, 2009, at 6:31 PM, M=E5ns Rullg=E5rd wrote:
>> Chuck Lever <chuck.lever@oracle.com> writes:
>>> On Apr 28, 2009, at 5:36 PM, J. Bruce Fields wrote:
>>>
>>>> On Tue, Apr 28, 2009 at 01:24:24PM -0400, Chuck Lever wrote:
>>>>> On Apr 28, 2009, at 12:40 PM, J. Bruce Fields wrote:
>>>>>> On Tue, Apr 28, 2009 at 12:35:50PM -0400, Chuck Lever wrote:
>>>>>>> On Apr 28, 2009, at 12:31 PM, J. Bruce Fields wrote:
>>>>>>>> On Thu, Apr 23, 2009 at 07:33:40PM -0400, Chuck Lever wrote:
>>>>>>>>> Recently, commit ad5b365c fixed a data type alignment issue i=
n
>>>>>>>>> nsm_init_private() by introducing put_unaligned().  We don't
>>>>>>>>> actually _require_ an unaligned access to nsm_private here.
>>>>>>>>>
>>>>>>>>> Instead, we should always use memcpy/memcmp to access the
>>>>>>>>> contents of RPC opaque data types.  This permits us to
>>>>>>>>> continue to define these as simple character arrays, as most
>>>>>>>>> legacy RPC code does, and to rely on gcc to pack the fields i=
n
>>>>>>>>> in-core structures optimally without breaking on platforms
>>>>>>>>> that require strict alignment.
>>>>>>>>
>>>>>>>> OK, I'm confused.  What structures will get packed differently=
?
>>>>>>>
>>>>>>> Any struct that has an nsm_private embedded in it, such as stru=
ct
>>>>>>> nlm_reboot.
>>>>>>
>>>>>> I don't see how that or any structure is changed by this patch.
>>>>>
>>>>> It's not.  Note the phrase above in the description: "permits us =
to
>>>>> _continue_ to define these" -- meaning, I'm not changing the
>>>>> structures.
>>>>
>>>> Err, but that's not right either, is it?:  We don't need to apply
>>>> this
>>>> patch in order to continue to define the structures as they're
>>>> currently
>>>> defined.
>>>>
>>>> Help! I'm confused!
>>>
>>> This patch is simply a clean up.  We don't need to use put_unaligne=
d
>>> in nsm_init_private.  There is absolutely nothing special about the
>>> nsm_private data type that would require this.  It should be access=
ed
>>
>> The "special" thing is has not guaranteed alignment.  Hence, any
>> access to it must use unaligned-safe methods.
>>
>>> and modified the way all other RPC opaques are, via memset/memcpy.
>>
>> What is so special about put_unaligned() that you insist on
>> replacing it?
>>
>>> Take a look at http://thread.gmane.org/gmane.linux.nfs/25607 and
>>> commit ad5b365c.
>>>
>>> The controversy is over how to define opaques so they are accessibl=
e
>>> on both 32- and 64-bit hardware platforms.  My first pass at
>>> nsm_init_private worked on 32-bit systems, but broke on 64-bit
>>> systems.  An expedient fix for this was to add the put_unaligned in
>>> there so 64-bit systems could access the field no matter how it was
>>> aligned.  I argue this is unneeded complexity, and inconsistent wit=
h
>>> the way most other RPC opaques are treated in the kernel.
>>>
>>> Andrew Morton proposed making RPC opaques a union of u8, u32 (or
>>> __be32), and u64 -- the u8 would allow us to treat an opaque as a
>>> byte
>>> array when needed, the u32 would allow access via XDR quads, and th=
e
>>> u64 would force 64-bit alignment.  The issues with this are:
>>>
>>> 1.  Defined this way, opaque fields in data structures will force t=
he
>>> encompassing structures to be large enough to honor the alignment
>>> requirements of the fields, and
>>>
>>> 2.  Most other RPC opaques are already defined as character
>>> arrays, so we would have to visit all of them to see if there were
>>> issues.
>>>
>>> If we insist on accessing opaques only via memset() and memcpy()
>>> problem 1 goes away and we remain compatible with the traditional
>>> definition of an RPC opaque as an array of bytes, on both 64- and 3=
2-
>>> bit systems.
>>
>> I still don't see what problem put_unaligned() poses.  Think of it a=
s
>> a more efficient memcpy().  We don't want the code to be larger and
>> slower than necessary, do we?
>
> The problem put_unaligned() poses is that it's harder to read and
> comprehend this code.  Why call put_unaligned() here, but not for
> other RPC opaques, such as nfs4_verifier?  With put_unaligned()
> readers have to chase down asm/unaligned.h.  With memcpy() it is
> precisely clear what is going on.

I thought put_unaligned() was precisely clear when I wrote that code.
I would have thought anyone working on the kernel would be familiar
with it, or at the very least be able to guess what it did.

Your reasoning above is the kind of thing I'd expect in some silly
corporate environment, not from Linux kernel developers.

> Again, this is just a clean up.  The current code works.  But I think
> it's inconsistent with other areas of the code, and harder to
> understand.

I'll leave the decision to the maintainers of the code.

--=20
M=E5ns Rullg=E5rd
mans-2StjZFpD7GcAvxtiuMwx3w@public.gmane.org