linux-kernel - Re: bit fields && data tearing

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Date:	Fri, 05 Sep 2014 14:50:31 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	paulmck@...ux.vnet.ibm.com, Michael Cree <mcree@...on.net.nz>,
	"H. Peter Anvin" <hpa@...or.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Laight <David.Laight@...LAB.COM>,
	Jakub Jelinek <jakub@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Miroslav Franc <mfranc@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	linux-alpha@...r.kernel.org
Subject: Re: bit fields && data tearing

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@...ux.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?
> 

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