Re: [Xen-devel] [PATCH MM-PART2 RESEND v2 16/19] xen/arm: mm: Protect Xen page-table update with a spinlock

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 05/06/2019 00:11, Stefano Stabellini wrote:
> On Tue, 14 May 2019, Julien Grall wrote:
> > The function create_xen_entries() may be called concurrently. For
> > instance, while the vmap allocation is protected by a spinlock, the
> > mapping is not.
> Do you have an example of potential concurrent calls of
> create_xen_entries() which doesn't involve concurrent vmaps (because
> vmaps are already protected by their spinlock)? vmap + something_else
> for instance?
Well, I gave an example here. The vmap allocation (i.e vm_alloc) is protected by 
a spinlock. However, when the mapping is done there are no spinlock to protected 
against concurrent one.
So the following scenario could happen:

CPU0                                  | CPU1
                                      |
vmap()                                | vmap()
  vm_alloc()                          |   vm_alloc()
    spin_lock()                       |
    ...                               |
    spin_unlock()                     |
                                      |     spin_lock()
    * interrupt *                     |     ...
                                      |     spin_unlock()
                                      |
    map_pages_to_xen()                |     map_pages_to_xen()
        entry = &xen_second[X]            |         entry = &xen_second[Y]
        * entry invalid *             |         * entry invalid *
        create_xen_table()            |         create_xen_table()
        

Assuming X == Y (i.e we the xen second entry is the same), then one will win the 
race and therefore one mapping will be inexistent.
But as I wrote, the chance it is happening is very limited.

I can add that in the commit message.

Cheers,

--
Julien Grall

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