Re: [XEN][PATCH v7 14/19] common/device_tree: Add rwlock for dt_host

[Julien Grall <julien@xen.org>]

Hi,

On 02/06/2023 01:48, Vikram Garhwal wrote:
>   Dynamic programming ops will modify the dt_host and there might be other
>   function which are browsing the dt_host at the same time. To avoid the race
>   conditions, adding rwlock for browsing the dt_host during runtime.
Please explain that writer will be added in a follow-up patch.

> 
>   Reason behind adding rwlock instead of spinlock:
>      For now, dynamic programming is the sole modifier of dt_host in Xen during
>          run time. All other access functions like iommu_release_dt_device() are
>          just reading the dt_host during run-time. So, there is a need to protect
>          others from browsing the dt_host while dynamic programming is modifying
>          it. rwlock is better suitable for this task as spinlock won't be able to
>          differentiate between read and write access.
> 
> Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
> 
> ---
> Changes from v6:
>      Remove redundant "read_unlock(&dt_host->lock);" in the following case:
>           XEN_DOMCTL_deassign_device
> ---
>   xen/common/device_tree.c              |  4 ++++
>   xen/drivers/passthrough/device_tree.c | 15 +++++++++++++++
>   xen/include/xen/device_tree.h         |  6 ++++++
>   3 files changed, 25 insertions(+)
> 
> diff --git a/xen/common/device_tree.c b/xen/common/device_tree.c
> index c5250a1644..c8fcdf8fa1 100644
> --- a/xen/common/device_tree.c
> +++ b/xen/common/device_tree.c
> @@ -2146,7 +2146,11 @@ int unflatten_device_tree(const void *fdt, struct dt_device_node **mynodes)
>   
>       dt_dprintk(" <- unflatten_device_tree()\n");
>   
> +    /* Init r/w lock for host device tree. */
> +    rwlock_init(&dt_host->lock);
> +
>       return 0;
> +
>   }
>   
>   static void dt_alias_add(struct dt_alias_prop *ap,
> diff --git a/xen/drivers/passthrough/device_tree.c b/xen/drivers/passthrough/device_tree.c
> index 301a5bcd97..f4d9deb624 100644
> --- a/xen/drivers/passthrough/device_tree.c
> +++ b/xen/drivers/passthrough/device_tree.c
> @@ -112,6 +112,8 @@ int iommu_release_dt_devices(struct domain *d)
>       if ( !is_iommu_enabled(d) )
>           return 0;
>   
> +    read_lock(&dt_host->lock);
> +
>       list_for_each_entry_safe(dev, _dev, &hd->dt_devices, domain_list)
>       {
>           rc = iommu_deassign_dt_device(d, dev);
> @@ -119,10 +121,14 @@ int iommu_release_dt_devices(struct domain *d)
>           {
>               dprintk(XENLOG_ERR, "Failed to deassign %s in domain %u\n",
>                       dt_node_full_name(dev), d->domain_id);
> +
> +            read_unlock(&dt_host->lock);
>               return rc;
>           }
>       }
>   
> +    read_unlock(&dt_host->lock);
> +
>       return 0;
>   }
>   
> @@ -246,6 +252,8 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct domain *d,
>       int ret;
>       struct dt_device_node *dev;
>   
> +    read_lock(&dt_host->lock);
> +
>       switch ( domctl->cmd )
>       {
>       case XEN_DOMCTL_assign_device:
> @@ -295,7 +303,10 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct domain *d,
>           spin_unlock(&dtdevs_lock);
>   
>           if ( d == dom_io )
> +        {
> +            read_unlock(&dt_host->lock);
>               return -EINVAL;
> +        }
>   
>           ret = iommu_add_dt_device(dev);
>           if ( ret < 0 )
> @@ -333,7 +344,10 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct domain *d,
>               break;
>   
>           if ( d == dom_io )
> +        {
> +            read_unlock(&dt_host->lock);
>               return -EINVAL;
> +        }
>   
>           ret = iommu_deassign_dt_device(d, dev);
>   
> @@ -348,5 +362,6 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct domain *d,
>           break;
>       }
>   
> +    read_unlock(&dt_host->lock);
>       return ret;
>   }
> diff --git a/xen/include/xen/device_tree.h b/xen/include/xen/device_tree.h
> index e239f7de26..dee40d2ea3 100644
> --- a/xen/include/xen/device_tree.h
> +++ b/xen/include/xen/device_tree.h
> @@ -18,6 +18,7 @@
>   #include <xen/string.h>
>   #include <xen/types.h>
>   #include <xen/list.h>
> +#include <xen/rwlock.h>
>   
>   #define DEVICE_TREE_MAX_DEPTH 16
>   
> @@ -106,6 +107,11 @@ struct dt_device_node {
>       struct list_head domain_list;
>   
>       struct device dev;
> +
> +    /*
> +     * Lock that protects r/w updates to unflattened device tree i.e. dt_host.
> +     */

 From the description, it sounds like the rwlock will only be used to 
protect the entire device-tree rather than a single node. So it doesn't 
seem to be sensible to increase each node structure (there are a lot) by 
12 bytes.

Can you outline your plan?

> +    rwlock_t lock;
>   };
>   
>   #define dt_to_dev(dt_node)  (&(dt_node)->dev)

Cheers,

-- 
Julien Grall