Hi Stefano,
On 08/06/2019 01:17, Stefano Stabellini wrote:
On Wed, 5 Jun 2019, Julien Grall wrote:
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.
Thanks for the detailed explanation, I am just trying to understand and
double-check the race. Wouldn't vm_alloc guarantee to return a different
va in the two cases (CPU0 and CPU1 above), given that the selection of
the va is done under spin_lock?
Yes vm_alloc() would guarantee you to have a different VA.
But it would be still possible to have X
and Y so that they select the same &xen_second entry, hence, the race
with create_xen_table(). It looks like the race is there.
That's correct.
Reviewed-by: Stefano Stabellini <sstabell...@kernel.org>
Thank you!
Cheers,
--
Julien Grall
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