Re: bit fields && data tearing

Peter Hurley
2014-09-05T18:50:31Z

On 09/05/2014 02:09 PM, Paul E. McKenney wrote:
> On Fri, Sep 05, 2014 at 08:16:48PM +1200, Michael Cree wrote:
>> On Thu, Sep 04, 2014 at 07:08:48PM -0700, H. Peter Anvin wrote:
>>> On 09/04/2014 05:59 PM, Peter Hurley wrote:
>>>> I have no idea how prevalent the ev56 is compared to the ev5.
>>>> Still we're talking about a chip that came out in 1996.
>>>
>>> Ah yes, I stand corrected. According to Wikipedia, the affected CPUs
>>> were all the 2106x CPUs (EV4, EV45, LCA4, LCA45) plus the 21164 with no
>>> suffix (EV5). However, we're still talking about museum pieces here.
>>
>> Yes, that is correct, EV56 is the first Alpha CPU to have the byte-word
>> extension (BWX) CPU instructions.
>>
>> It would not worry me if the kernel decided to assume atomic aligned
>> scalar accesses for all arches, thus terminating support for Alphas
>> without BWX.
>>
>> The X server, ever since the libpciaccess change, does not work on
>> Alphas without BWX.
>>
>> Debian Alpha (pretty much up to date at Debian-Ports) is still compiled
>> for all Alphas, i.e., without BWX. The last attempt to start compiling
>> Debian Alpha with BWX, about three years ago when Alpha was kicked out
>> to Debian-Ports resulted in a couple or so complaints so got nowhere.
>> It's frustrating supporting the lowest common demoninator as many of
>> the bugs specific to Alpha can be resolved by recompiling with the BWX.
>> The kernel no longer supporting Alphas without BWX might just be the
>> incentive we need to switch Debian Alpha to compiling with BWX.
>
> Very good, then I update my patch as follows. Thoughts?
>
> Thanx, Paul

Minor [optional] edits.

Thanks,
Peter Hurley

> ------------------------------------------------------------------------
>
> documentation: Record limitations of bitfields and small variables
>
> This commit documents the fact that it is not safe to use bitfields as
> shared variables in synchronization algorithms. It also documents that
> CPUs must provide one-byte and two-byte load and store instructions
^
atomic
> in order to be supported by the Linux kernel. (Michael Cree
> has agreed to the resulting non-support of pre-EV56 Alpha CPUs:
> https://lkml.org/lkml/2014/9/5/143.
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 87be0a8a78de..455df6b298f7 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -269,6 +269,30 @@ And there are a number of things that _must_ or _must_not_ be assumed:
> STORE *(A + 4) = Y; STORE *A = X;
> STORE {*A, *(A + 4) } = {X, Y};
>
> +And there are anti-guarantees:
> +
> + (*) These guarantees do not apply to bitfields, because compilers often
> + generate code to modify these using non-atomic read-modify-write
> + sequences. Do not attempt to use bitfields to synchronize parallel
> + algorithms.
> +
> + (*) Even in cases where bitfields are protected by locks, all fields
> + in a given bitfield must be protected by one lock. If two fields
> + in a given bitfield are protected by different locks, the compiler's
> + non-atomic read-modify-write sequences can cause an update to one
> + field to corrupt the value of an adjacent field.
> +
> + (*) These guarantees apply only to properly aligned and sized scalar
> + variables. "Properly sized" currently means variables that are the
> + same size as "char", "short", "int" and "long". "Properly aligned"
> + means the natural alignment, thus no constraints for "char",
> + two-byte alignment for "short", four-byte alignment for "int",
> + and either four-byte or eight-byte alignment for "long", on 32-bit
> + and 64-bit systems, respectively. Note that this means that the
> + Linux kernel does not support pre-EV56 Alpha CPUs, because these
> + older CPUs do not provide one-byte and two-byte loads and stores.
^
non-atomic
> + Alpha EV56 and later Alpha CPUs are still supported.
> +
>
> ============> WHAT ARE MEMORY BARRIERS?
>