linux-kernel - Re: [RFC PATCH v5 03/12] __wr_after

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Date:   Fri, 15 Feb 2019 09:57:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Igor Stoppa <igor.stoppa@...il.com>
Cc:     Igor Stoppa <igor.stoppa@...wei.com>,
        Andy Lutomirski <luto@...capital.net>,
        Nadav Amit <nadav.amit@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ahmed Soliman <ahmedsoliman@...a.vt.edu>,
        linux-integrity@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v5 03/12] __wr_after_init: Core and default arch

On Fri, Feb 15, 2019 at 01:10:33AM +0200, Igor Stoppa wrote:
> 
> 
> On 14/02/2019 13:28, Peter Zijlstra wrote:
> > On Thu, Feb 14, 2019 at 12:41:32AM +0200, Igor Stoppa wrote:
> 
> [...]
> 
> > > +#define wr_rcu_assign_pointer(p, v) ({	\
> > > +	smp_mb();			\
> > > +	wr_assign(p, v);		\
> > > +	p;				\
> > > +})
> > 
> > This requires that wr_memcpy() (through wr_assign) is single-copy-atomic
> > for native types. There is not a comment in sight that states this.
> 
> Right, I kinda expected native-aligned <-> atomic, but it's not necessarily
> true. It should be confirmed when enabling write rare on a new architecture.
> I'll add the comment.
> 
> > Also, is this true of x86/arm64 memcpy ?
> 
> 
> For x86_64:
> https://elixir.bootlin.com/linux/v5.0-rc6/source/arch/x86/include/asm/uaccess.h#L462
> the mov"itype"  part should deal with atomic copy of native, aligned types.
> 
> 
> For arm64:
> https://elixir.bootlin.com/linux/v5.0-rc6/source/arch/arm64/lib/copy_template.S#L110
> .Ltiny15 deals with copying less than 16 bytes, which includes pointers.
> When the data is aligned, the copy of a pointer should be atomic.
> 

Where are the comments and Changelog notes ? How is an arch maintainer
to be aware of this requirement when adding support for his/her arch?