Re: [Xen-devel] [PATCH 1/2] fix locking in offline_page()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/11/13 08:25, Jan Beulich wrote:
> Coverity ID 1055655
>
> Apart from the Coverity-detected lock order reversal (a domain's
> page_alloc_lock taken with the heap lock already held), calling
> put_page() with heap_lock is a bad idea too (as a possible descendant
> from put_page() is free_heap_pages(), which wants to take this very
> lock).
>
> From all I can tell the region over which heap_lock was held was far
> too large: All we need to protect are the call to mark_page_offline()
> and reserve_heap_page() (and I'd even put under question the need for
> the former). Hence by slightly re-arranging the if/else-if chain we
> can drop the lock much earlier, at once no longer covering the two
> put_page() invocations.
>
> Once at it, do a little bit of other cleanup: Put the "pod_replace"
> code path inline rather than at its own label, and drop the effectively
> unused variable "ret".
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

>
> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -957,7 +957,6 @@ int offline_page(unsigned long mfn, int 
>  {
>      unsigned long old_info = 0;
>      struct domain *owner;
> -    int ret = 0;
>      struct page_info *pg;
>  
>      if ( !mfn_valid(mfn) )
> @@ -1007,16 +1006,28 @@ int offline_page(unsigned long mfn, int 
>      if ( page_state_is(pg, offlined) )
>      {
>          reserve_heap_page(pg);
> -        *status = PG_OFFLINE_OFFLINED;
> +
> +        spin_unlock(&heap_lock);
> +
> +        *status = broken ? PG_OFFLINE_OFFLINED | PG_OFFLINE_BROKEN
> +                         : PG_OFFLINE_OFFLINED;
> +        return 0;
>      }
> -    else if ( (owner = page_get_owner_and_reference(pg)) )
> +
> +    spin_unlock(&heap_lock);
> +
> +    if ( (owner = page_get_owner_and_reference(pg)) )
>      {
>          if ( p2m_pod_offline_or_broken_hit(pg) )
> -            goto pod_replace;
> +        {
> +            put_page(pg);
> +            p2m_pod_offline_or_broken_replace(pg);
> +            *status = PG_OFFLINE_OFFLINED;
> +        }
>          else
>          {
>              *status = PG_OFFLINE_OWNED | PG_OFFLINE_PENDING |
> -              (owner->domain_id << PG_OFFLINE_OWNER_SHIFT);
> +                      (owner->domain_id << PG_OFFLINE_OWNER_SHIFT);

domain_id will be promoted from a uint16_t to an int32_t, then shifted
left by 16 bits which is undefined if the top of domain_id bit was set.

Do we care about the likelyhood of a domain_id with the top bit set? I
certainly cant see how one would get into that state.

~Andrew

>              /* Release the reference since it will not be allocated anymore 
> */
>              put_page(pg);
>          }
> @@ -1024,7 +1035,7 @@ int offline_page(unsigned long mfn, int 
>      else if ( old_info & PGC_xen_heap )
>      {
>          *status = PG_OFFLINE_XENPAGE | PG_OFFLINE_PENDING |
> -          (DOMID_XEN << PG_OFFLINE_OWNER_SHIFT);
> +                  (DOMID_XEN << PG_OFFLINE_OWNER_SHIFT);
>      }
>      else
>      {
> @@ -1043,21 +1054,7 @@ int offline_page(unsigned long mfn, int 
>      if ( broken )
>          *status |= PG_OFFLINE_BROKEN;
>  
> -    spin_unlock(&heap_lock);
> -
> -    return ret;
> -
> -pod_replace:
> -    put_page(pg);
> -    spin_unlock(&heap_lock);
> -
> -    p2m_pod_offline_or_broken_replace(pg);
> -    *status = PG_OFFLINE_OFFLINED;
> -
> -    if ( broken )
> -        *status |= PG_OFFLINE_BROKEN;
> -
> -    return ret;
> +    return 0;
>  }
>  
>  /*
>
>
>


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