Re: [Xen-devel] [for-4.7 2/2] xen/arm: p2m: Release the p2m lock before undoing the mappings

[Wei Chen <wei.chen@xxxxxxx>]

Hi Julien,

This code looks good to me, and I have tested that
the deadlock is fixed by this patch.

Reviewed-and-Tested-by: Wei Chen <Wei.Chen@xxxxxxx>

Original:
(XEN) smmu: /smb/smmu@e0800000: P2M IPA size not supported (P2M=44 SMMU=40)!
(XEN) I/O virtualisation disabled
(XEN) Request p2m Spinlock...
(XEN) Request p2m Spinlock...OK
(XEN) apply_p2m_changes failed! rollback! call apply_p2m_changes
recursively!
(XEN) Request p2m Spinlock...

Patched:
(XEN) smmu: /smb/smmu@e0800000: P2M IPA size not supported (P2M=44 SMMU=40)!
(XEN) I/O virtualisation disabled
(XEN) Request p2m Spinlock...
(XEN) Request p2m Spinlock...OK
(XEN) apply_p2m_changes failed! rollback! call apply_p2m_changes
recursively!
(XEN) Request p2m Spinlock...
(XEN) Request p2m Spinlock...OK


On 2016/5/16 22:08, Julien Grall wrote:
> Since commit 4b25423a "arch/arm: unmap partially-mapped memory regions",
> Xen has been undoing the P2M mappings when an error occurred during
> insertion or memory allocation.
>
> This is done by calling recursively apply_p2m_changes, however the
> second call is done with the p2m lock taken which will result in a
> deadlock for the current processor.
>
> Fix it by moving the recursive call after the lock has been released.
>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
>
> ---
>     I think we could unlock the p2m lock before freeing the temporary
>     mapping. Although, I played safe as this is a bug fix for Xen 4.7
>     and to be backported up to Xen 4.5.
> ---
>  xen/arch/arm/p2m.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 68c67b0..838d004 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1184,6 +1184,14 @@ out:
>      while ( (pg = page_list_remove_head(&free_pages)) )
>          free_domheap_page(pg);
>
> +    for ( level = P2M_ROOT_LEVEL; level < 4; level ++ )
> +    {
> +        if ( mappings[level] )
> +            unmap_domain_page(mappings[level]);
> +    }
> +
> +    spin_unlock(&p2m->lock);
> +
>      if ( rc < 0 && ( op == INSERT || op == ALLOCATE ) &&
>           addr != start_gpaddr )
>      {
> @@ -1196,14 +1204,6 @@ out:
>                            mattr, 0, p2m_invalid, d->arch.p2m.default_access);
>      }
>
> -    for ( level = P2M_ROOT_LEVEL; level < 4; level ++ )
> -    {
> -        if ( mappings[level] )
> -            unmap_domain_page(mappings[level]);
> -    }
> -
> -    spin_unlock(&p2m->lock);
> -
>      return rc;
>  }
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