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:	Wed, 06 Aug 2014 21:09:25 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Lan Tianyu <tianyu.lan@...el.com>
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 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.

> 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?

Rafael

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