Re: [PATCH] irqchip/gic-v3: Use wmb() instead of smb_wmb() in gic_raise_softirq()

[Adam Wallis]

On 2/1/2018 8:24 AM, Marc Zyngier wrote:
> On 01/02/18 12:55, Shanker Donthineni wrote:
>> Hi Will, Thanks for your quick reply.
>>
>> On 02/01/2018 04:33 AM, Will Deacon wrote:
>>> Hi Shanker,
>>>
>>> On Wed, Jan 31, 2018 at 06:03:42PM -0600, Shanker Donthineni wrote:
>>>> A DMB instruction can be used to ensure the relative order of only
>>>> memory accesses before and after the barrier. Since writes to system
>>>> registers are not memory operations, barrier DMB is not sufficient
>>>> for observability of memory accesses that occur before ICC_SGI1R_EL1
>>>> writes.
>>>>
>>>> A DSB instruction ensures that no instructions that appear in program
>>>> order after the DSB instruction, can execute until the DSB instruction
>>>> has completed.
>>>>
>>>> Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
>>>> ---
>>>>  drivers/irqchip/irq-gic-v3.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
>>>> index b56c3e2..980ae8e 100644
>>>> --- a/drivers/irqchip/irq-gic-v3.c
>>>> +++ b/drivers/irqchip/irq-gic-v3.c
>>>> @@ -688,7 +688,7 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>>>>  	 * Ensure that stores to Normal memory are visible to the
>>>>  	 * other CPUs before issuing the IPI.
>>>>  	 */
>>>> -	smp_wmb();
>>>> +	wmb();
>>>
>>> I think this is the right thing to do and the smp_wmb() was accidentally
>>> pulled in here as a copy-paste from the GICv2 driver where it is sufficient
>>> in practice.
>>>
>>> Did you spot this by code inspection, or did the DMB actually cause
>>> observable failures? (trying to figure out whether or not this need to go
>>> to -stable).
>>>
>>
>> We've inspected the code because kernel was causing failures in scheduler/IPI_RESCHDULE.
>> After some time of debugging, we landed in GIC driver and found that the issue was due
>> to the DMB barrier. 
> 
> OK. I've applied this with a cc: stable and Will's Ack.
> 
>> Side note, we're also missing synchronization barriers in GIC driver after writing some
>> of the ICC_XXX system registers. I'm planning to post those changes for comments.
>>
>> e.g: gic_write_sgi1r(val) and gic_write_eoir(irqnr);
> 
> Thanks,
> 
> 	M.
> 

Tested-by: Adam Wallis <awallis@xxxxxxxxxxxxxx>

-- 
Adam Wallis
Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum,
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