Re: [RFC PATCH kernel] vfio/spapr_tce: Get rid of possible infinite loop

[Serhii Popovych <spopovyc@redhat.com>]

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Alexey Kardashevskiy wrote:
> As a part of cleanup, the SPAPR TCE IOMMU subdriver releases preregistered
> memory. If there is a bug in memory release, the loop in
> tce_iommu_release() becomes infinite; this actually happened to me.
> 
> This makes the loop finite and prints a warning on every failure to make
> the code more bug prone.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
>  drivers/vfio/vfio_iommu_spapr_tce.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> index b1a8ab3..ece0651 100644
> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> @@ -371,6 +371,7 @@ static void tce_iommu_release(void *iommu_data)
>  {
>  	struct tce_container *container = iommu_data;
>  	struct tce_iommu_group *tcegrp;
> +	struct tce_iommu_prereg *tcemem, *tmtmp;
>  	long i;
>  
>  	while (tce_groups_attached(container)) {
> @@ -393,13 +394,8 @@ static void tce_iommu_release(void *iommu_data)
>  		tce_iommu_free_table(container, tbl);
>  	}
>  
> -	while (!list_empty(&container->prereg_list)) {
> -		struct tce_iommu_prereg *tcemem;
> -
> -		tcemem = list_first_entry(&container->prereg_list,
> -				struct tce_iommu_prereg, next);
> -		WARN_ON_ONCE(tce_iommu_prereg_free(container, tcemem));
> -	}
> +	list_for_each_entry_safe(tcemem, tmtmp, &container->prereg_list, next)
> +		WARN_ON(tce_iommu_prereg_free(container, tcemem));

I'm not sure that tce_iommu_prereg_free() call under WARN_ON() is good
idea because WARN_ON() is a preprocessor macro:

  if CONFIG_WARN=n is added by the analogy with CONFIG_BUG=n defining
  WARN_ON() as empty we will loose call to tce_iommu_prereg_free()
  leaking resources.

There is no problem at the moment: WARN_ON() defined for PPC in
arch/powerpc/include/asm/bug.h unconditionally.

So your first version with intermediate variable looks better to me.

>  
>  	tce_iommu_disable(container);
>  	if (container->mm)
> 


-- 
Thanks,
Serhii


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