From mboxrd@z Thu Jan  1 00:00:00 1970
From: gdavis@mvista.com (George G. Davis)
Date: Thu, 6 Oct 2011 08:35:08 -0400
Subject: [RFC/PATCH 7/7] ARM: ARM11 MPCore: cpu_v6_set_pte_ext is not
	preempt safe
In-Reply-To: <20111006074647.GA26349@n2100.arm.linux.org.uk>
References: <05942148-7AFB-4755-A22F-355E0360B098@mvista.com>
	<1317877714-11355-1-git-send-email-gdavis@mvista.com>
	<1317877714-11355-8-git-send-email-gdavis@mvista.com>
	<20111006074647.GA26349@n2100.arm.linux.org.uk>
Message-ID: <232DB229-FDB4-4E59-AA06-D10DFF02604C@mvista.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org


On Oct 6, 2011, at 3:46 AM, Russell King - ARM Linux wrote:

> On Thu, Oct 06, 2011 at 01:08:34AM -0400, gdavis at mvista.com wrote:
>> diff --git a/arch/arm/mm/proc-v6.S b/arch/arm/mm/proc-v6.S
>> index a923aa0..dd2e5e5 100644
>> --- a/arch/arm/mm/proc-v6.S
>> +++ b/arch/arm/mm/proc-v6.S
>> @@ -122,7 +122,18 @@ ENTRY(cpu_v6_switch_mm)
>> 
>> ENTRY(cpu_v6_set_pte_ext)
>> #ifdef CONFIG_MMU
>> +#ifdef CONFIG_PREEMPT
>> +	ALT_SMP(stmdb	sp!, {r4, r10, r11})
>> +	ALT_SMP(get_thread_info r10)
>> +	ALT_SMP(ldr	r4, [r10, #TI_PREEMPT])	@ get preempt count
>> +	ALT_SMP(add	r11, r4, #1)		@ increment it
>> +	ALT_SMP(str	r11, [r10, #TI_PREEMPT])	@ disable preempt
>> +#endif
>> 	armv6_set_pte_ext cpu_v6
>> +#ifdef CONFIG_PREEMPT
>> +	ALT_SMP(str	r4, [r10, #TI_PREEMPT])	@ restore preempt count
>> +	ALT_SMP(ldmia	sp!, {r4, r10, r11})
>> +#endif
>> #endif
> 
> And what effect do you expect the above to have?

My goal was to be proactive, heading off a request to use this macro for
the SMP case since preempt disable is only required on ARM11 MPCore.
I thought this would achieve that goal, only using those instructions on SMP,
not UP machines.

> Do you understand the purpose of the ALT_SMP macro?

Clearly I don't understand the use case.

> Do you understand that it should be paired with ALT_UP ?

No, I did not know that.  I'll remove use of ALT_SMP and add CONFIG_SMP
as a conditional compile dependency then.  Is that OK?

Thanks!

--
Regards,
George