linux-kernel - Re: [PATCH 2/2] ARM: domains: add memory dependencies to get_domain/set

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Date:	Fri, 11 Sep 2015 16:10:17 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:	Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] ARM: domains: add memory dependencies to
 get_domain/set_domain

On Fri, Sep 11, 2015 at 04:56:27PM +0200, Robert Jarzmik wrote:
> Russell King <rmk+kernel@....linux.org.uk> writes:
> > We need to have memory dependencies on get_domain/set_domain to avoid
> > the compiler over-optimising these inline assembly instructions.
> >
> > Loads/stores must not be reordered across a set_domain(), so introduce
> > a compiler barrier for that assembly.
> >
> > The value of get_domain() must not be cached across a set_domain(), but
> > we still want to allow the compiler to optimise it away.  Introduce a
> > dependency on current_thread_info()->cpu_domain to avoid this; the new
> > memory clobber in set_domain() should therefore cause the compiler to
> > re-load this.  The other advantage of using this is we should have its
> > address in the register set already, or very soon after at most call
> > sites.
> >
> > Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
> Tested-by: Robert Jarzmik <robert.jarzmik@...e.fr>
> 
> The test is failing without these 2 patches, while with them, an unaligned
> access is fixed, and the generated code looks good :
>      7ac:       ee13cf10        mrc     15, 0, ip, cr3, cr0, {0}
>      7b0:       e3cc300c        bic     r3, ip, #12
>      7b4:       e58dc014        str     ip, [sp, #20]
>      7b8:       e3833004        orr     r3, r3, #4
>      7bc:       ee033f10        mcr     15, 0, r3, cr3, cr0, {0}

Thanks Robert, I've queued these as fixes now.

If you want to put the bug alignment patch in the patch system, I'll get
that off to Linus this weekend too.

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