Re: [Xen-devel] [PATCH RFC 1/4] xen-gnt: prevent adding duplicate gnt callbacks

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 08/07/13 14:03, Roger Pau Monne wrote:
> With the current implementation, the callback in the tail of the list
> can be added twice, because the check done in
> gnttab_request_free_callback is bogus, callback->next can be NULL if
> it is the last callback in the list. If we add the same callback twice
> we end up with an infinite loop, were callback == callback->next.
> 
> Replace this check with a proper one that iterates over the list to
> see if the callback has already been added.
> 
> Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
>  drivers/xen/grant-table.c |   13 +++++++++++--
>  1 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index 04c1b2d..d5418c1 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -729,9 +729,18 @@ void gnttab_request_free_callback(struct 
> gnttab_free_callback *callback,
>                                 void (*fn)(void *), void *arg, u16 count)
>  {
>       unsigned long flags;
> +     struct gnttab_free_callback *cb;
> +
>       spin_lock_irqsave(&gnttab_list_lock, flags);
> -     if (callback->next)
> -             goto out;
> +
> +     /* Check if the callback is already on the list */
> +     cb = gnttab_free_callback_list;
> +     while (cb) {
> +             if (cb == callback)
> +                     goto out;
> +             cb = cb->next;
> +     }

O(N)?

Suggest using the standard list infrastructure, or using something other
than NULL for the end of the list (e.g., gnttab_free_callback_list).

gnttab_request_free_callback() is also putting the callbacks in the
wrong order -- it should be FIFO not LIFO.  Might be nice to fix this as
well.

David

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