linux-kernel - Re: [PATCH] xen/grant-table: remove unnecessary BUG_ON on gnttab

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Date:   Mon, 16 Dec 2019 08:38:19 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Aditya Pakki <pakki001@....edu>
Cc:     kjlu@....edu, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xen/grant-table: remove unnecessary BUG_ON on
 gnttab_interface

On 15.12.19 21:13, Aditya Pakki wrote:
> grow_gnttab_list() checks for NULL on gnttab_interface immediately
> after gnttab_expand() check. The patch removes the redundant assertion
> and replaces the BUG_ON call with recovery code.
> 
> Signed-off-by: Aditya Pakki <pakki001@....edu>
> ---
>   drivers/xen/grant-table.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index 49b381e104ef..f59694c352be 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -664,7 +664,6 @@ static int grow_gnttab_list(unsigned int more_frames)
>   	unsigned int nr_glist_frames, new_nr_glist_frames;
>   	unsigned int grefs_per_frame;
>   
> -	BUG_ON(gnttab_interface == NULL);
>   	grefs_per_frame = gnttab_interface->grefs_per_grant_frame;
>   
>   	new_nr_grant_frames = nr_grant_frames + more_frames;
> @@ -1388,7 +1387,9 @@ static int gnttab_expand(unsigned int req_entries)
>   	int rc;
>   	unsigned int cur, extra;
>   
> -	BUG_ON(gnttab_interface == NULL);
> +	if (!gnttab_interface)
> +		return -EINVAL;
> +
>   	cur = nr_grant_frames;
>   	extra = ((req_entries + gnttab_interface->grefs_per_grant_frame - 1) /
>   		 gnttab_interface->grefs_per_grant_frame);
> @@ -1423,7 +1424,9 @@ int gnttab_init(void)
>   	/* Determine the maximum number of frames required for the
>   	 * grant reference free list on the current hypervisor.
>   	 */
> -	BUG_ON(gnttab_interface == NULL);
> +	if (!gnttab_interface)
> +		return -EINVAL;
> +

I'd just remove the BUG_ON().

gnttab_request_version() called some lines up always sets
gnttab_interface.

The BUG_ON() in nr_status_frames() can be removed, too. It is either
called by v2 specific functions (for those to be reached
gnttab_interface must be set) or by gnttab_init() (reasoning see
above).


Juergen