linux-kernel - Re: [patch 1/4] hotplug: Prevent alloc/free of irq descriptors during cpu up/down

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Message-ID: <55A532C2.4080306@oracle.com>
Date:	Tue, 14 Jul 2015 12:03:14 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Peter Anvin <hpa@...or.com>, xiao jin <jin.xiao@...el.com>,
	Joerg Roedel <jroedel@...e.de>, Borislav Petkov <bp@...e.de>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: [patch 1/4] hotplug: Prevent alloc/free of irq descriptors during
 cpu up/down

On 07/14/2015 11:44 AM, Thomas Gleixner wrote:
> On Tue, 14 Jul 2015, Boris Ostrovsky wrote:
>>> Prevent allocation and freeing of interrupt descriptors accross cpu
>>> hotplug.
>>
>> This breaks Xen guests that allocate interrupt descriptors in .cpu_up().
> And where exactly does XEN allocate those descriptors?

xen_cpu_up()
     xen_setup_timer()
         bind_virq_to_irqhandler()
             bind_virq_to_irq()
                 xen_allocate_irq_dynamic()
                     xen_allocate_irqs_dynamic()
                         irq_alloc_descs()


There is also a similar pass via xen_cpu_up() -> xen_smp_intr_init()


>   
>> Any chance this locking can be moved into arch code?
> No.
>
>> (The patch doesn't appear to have any side effects for the down path since Xen
>> guests deallocate descriptors in __cpu_die()).
>   
> Exact place please.

Whose place? Where descriptors are deallocated?

__cpu_die()
     xen_cpu_die()
         xen_teardown_timer()
             unbind_from_irqhandler()
                 unbind_from_irq()
                     __unbind_from_irq()
                         xen_free_irq()
                             irq_free_descs()
                                 free_desc()

-boris

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