linux-kernel - Re: [PATCH] ACPI/OSL: Remove RCU in the osl.c to avoid dead lock with cpu hot plug

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Date:	Thu, 07 Aug 2014 17:31:06 +0800
From:	Lan Tianyu <tianyu.lan@...el.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	lenb@...nel.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI/OSL: Remove RCU in the osl.c to avoid dead lock
 with cpu hot plug

On 2014年08月07日 03:09, Rafael J. Wysocki wrote:
> On Wednesday, August 06, 2014 10:22:45 AM Lan Tianyu wrote:
>> On 2014年08月06日 09:24, Rafael J. Wysocki wrote:
>>> On Monday, August 04, 2014 04:40:08 PM Lan Tianyu wrote:
> 
> [cut]
> 
>>>> @@ -298,29 +298,29 @@ void __iomem *acpi_os_get_iomem(acpi_physical_address phys, unsigned int size)
>>>>  {
>>>>  	struct acpi_ioremap *map;
>>>>  	void __iomem *virt = NULL;
>>>> +	unsigned long flags;
>>>>  
>>>> -	mutex_lock(&acpi_ioremap_lock);
>>>> +	spin_lock_irqsave(&acpi_ioremap_lock, flags);
>>>
>>> Why do you need to do _irqsave here?  It was a mutex before, after all,
>>> so it can't be called from interrupt context.
>>>
>>> In other places below too.
>>
>> Original code uses RCU lock to protect acpi_ioremaps list in the
>> acpi_os_read/write_memory() which will be called in apei_read/write().
>> apei_read/write() will be called in the interrupt from APEI comments.
> 
> But acpi_os_get_iomem() won't be called from interrupt context and should use
> spin_lock_irq() instead of _irqsave.  This also applies to the other places
> that use the mutex.

Yes, that's correct. Sorry. I misunderstood what you meant.

> 
>> Now replace RCU with acpi_ioremap_lock and the lock will be called in
>> the interrupt. So redefine it to spin lock. From history,
>> acpi_ioremap_lock was spin lock before adding RCU support.
> 
> And it had scalability problems IIRC.
> 
> Did you consider using SRCU instead of going back to the spinlock?

No, I will have a look at SRCU.

> 
> Rafael
> 


-- 
Best regards
Tianyu Lan
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