Re: [Xen-devel] Question about the usage of spinlock

[Julien Grall <julien.grall@xxxxxxx>]

Hello,

On 26/05/2016 12:07, Xu, Quan wrote:
> On May 26, 2016 9:31 AM, 조현권 <chkwon91@xxxxxxxxx> wrote:
> > I am studying xen 4.6.0 version now and wonder the usage of spinlock which is 
> > located in free_heap_pages(xen/common/page_alloc. c)
> > As its memory setup is ahead of multicore initialization, spinlock seems to be 
> > unnecessary during booting because it uses only single core
> > spinlock looks required after multicore initialization finished and when xen 
> > needs to free memory space during work
> > However i experimented not to use spin lock in free_heap_pages and 
> > dom0(linux-kernel) was booted well. It does not cause any error using internet 
> > or application program even reboot
> > Are therr any special reason to use spinlock?
> > My experiment environment was ODROID-XU4 which uses arm architecture
> Ah, you can try to test more for DomU..
Please try to be helpful when answering to a question. I.e provide 
comment which will help to understand the need (or not) of spinlock.
In this case, booting a single domU will unlikely exacerbate the need of 
spinlock because you need concurrent call to {alloc,free}_heap_pages. 
The easiest way would be creating/destroying multiple guests at the same 
time.
Anyway, Xen will need to manage memory (with {alloc,free}_heap_pages) 
after boot, such as allocating/freeing internal structure associated to 
a domain/vCPU. Search for {alloc,free}_xenheap_pages, which are helpers 
for *_heap_pages.
After Xen has finished to boot, the memory can be managed from any CPUs.
So the code in those functions need to be protected against concurrent 
access.
Regards,

--
Julien Grall

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