linux-kernel - Re: [Patch v7 14/18] x86, irq, ACPI: Introduce a rwsem to protect IOAPIC operations from hotplug

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Date:	Sun, 02 Nov 2014 13:24:04 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	x86@...nel.org, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tony Luck <tony.luck@...el.com>,
	Joerg Roedel <joro@...tes.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [Patch v7 14/18] x86, irq, ACPI: Introduce a rwsem to protect
 IOAPIC operations from hotplug

On 2014/11/2 2:59, Thomas Gleixner wrote:
> On Mon, 27 Oct 2014, Jiang Liu wrote:
>> We are going to support ACPI based IOAPIC hotplug, so introduce a rwsem
>> to protect IOAPIC data structures from IOAPIC hotplug. We choose to
>> serialize in ACPI instead of in the IOAPIC core because:
>> 1) currently we are only plan to support ACPI based IOAPIC hotplug
>> 2) it's much more cleaner and easier
>> 3) It does't affect IOAPIC discovered by devicetree, SFI and mpparse.
> 
> I had a last intensive look at this series as I was about to merge
> it. So I looked at the locking rules here again
>  
>> +/*
>> + * Locks related to IOAPIC hotplug
>> + * Hotplug side:
>> + * 	->lock_device_hotplug()	//device_hotplug_lock
>> + *		->acpi_ioapic_rwsem
>> + *			->ioapic_lock
>> + * Interrupt mapping side:
>> + *	->acpi_ioapic_rwsem
>> + *		->ioapic_mutex
>> + *			->ioapic_lock
>> + */
> 
> This looks sane, but I cannot figure out at all why this needs to be a
> rwsem.
> 
>> +static DECLARE_RWSEM(acpi_ioapic_rwsem);
> 
> I think it should be a simple mutex because the rwsem does not protect
> against concurrent execution what taken for read.
> 
> And the site which takes it for write is in the early boot process
> where nothing runs in parallel AFAICT.
Hi Thomas,
	You are right. It's not on hot path, so a mutex is better than
a rwsem here. I will send out an updated version soon.
Regards!
Gerry
> 
> Thanks,
> 
> 	tglx
> 
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