Re: [PATCH v2 4/9] iommu/ioasid: Add reference couting functions

[Jean-Philippe Brucker]

On Fri, Aug 21, 2020 at 09:35:13PM -0700, Jacob Pan wrote:
> There can be multiple users of an IOASID, each user could have hardware
> contexts associated with the IOASID. In order to align lifecycles,
> reference counting is introduced in this patch. It is expected that when
> an IOASID is being freed, each user will drop a reference only after its
> context is cleared.
> 
> Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> ---
>  drivers/iommu/ioasid.c | 113 +++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/ioasid.h |   4 ++
>  2 files changed, 117 insertions(+)
> 
> diff --git a/drivers/iommu/ioasid.c b/drivers/iommu/ioasid.c
> index f73b3dbfc37a..5f31d63c75b1 100644
> --- a/drivers/iommu/ioasid.c
> +++ b/drivers/iommu/ioasid.c
> @@ -717,6 +717,119 @@ int ioasid_set_for_each_ioasid(struct ioasid_set *set,
>  EXPORT_SYMBOL_GPL(ioasid_set_for_each_ioasid);
>  
>  /**
> + * IOASID refcounting rules
> + * - ioasid_alloc() set initial refcount to 1
> + *
> + * - ioasid_free() decrement and test refcount.
> + *     If refcount is 0, ioasid will be freed. Deleted from the system-wide
> + *     xarray as well as per set xarray. The IOASID will be returned to the
> + *     pool and available for new allocations.
> + *
> + *     If recount is non-zero, mark IOASID as IOASID_STATE_FREE_PENDING.
> + *     No new reference can be added. The IOASID is not returned to the pool
> + *     for reuse.
> + *     After free, ioasid_get() will return error but ioasid_find() and other
> + *     non refcount adding APIs will continue to work until the last reference
> + *     is dropped
> + *
> + * - ioasid_get() get a reference on an active IOASID
> + *
> + * - ioasid_put() decrement and test refcount of the IOASID.
> + *     If refcount is 0, ioasid will be freed. Deleted from the system-wide
> + *     xarray as well as per set xarray. The IOASID will be returned to the
> + *     pool and available for new allocations.
> + *     Do nothing if refcount is non-zero.
> + *
> + * - ioasid_find() does not take reference, caller must hold reference
> + *
> + * ioasid_free() can be called multiple times without error until all refs are
> + * dropped.
> + */

Since you already document this in ioasid.rst, I'm not sure the comment
is necessary. Maybe the doc for _free/_put would be better in the
function's documentation.

> +
> +int ioasid_get_locked(struct ioasid_set *set, ioasid_t ioasid)
> +{
> +	struct ioasid_data *data;
> +
> +	data = xa_load(&active_allocator->xa, ioasid);
> +	if (!data) {
> +		pr_err("Trying to get unknown IOASID %u\n", ioasid);
> +		return -EINVAL;
> +	}
> +	if (data->state == IOASID_STATE_FREE_PENDING) {
> +		pr_err("Trying to get IOASID being freed%u\n", ioasid);
> +		return -EBUSY;
> +	}
> +
> +	if (set && data->set != set) {
> +		pr_err("Trying to get IOASID not in set%u\n", ioasid);
> +		/* data found but does not belong to the set */
> +		return -EACCES;
> +	}
> +	refcount_inc(&data->users);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_get_locked);

Is it necessary to export the *_locked variant?  Who'd call them and how
would they acquire the lock?

> +
> +/**
> + * ioasid_get - Obtain a reference of an ioasid
> + * @set
> + * @ioasid

Can be dropped. The doc checker will throw a warning, though.

> + *
> + * Check set ownership if @set is non-null.
> + */
> +int ioasid_get(struct ioasid_set *set, ioasid_t ioasid)
> +{
> +	int ret = 0;

No need to initialize ret

> +
> +	spin_lock(&ioasid_allocator_lock);
> +	ret = ioasid_get_locked(set, ioasid);
> +	spin_unlock(&ioasid_allocator_lock);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_get);
> +
> +void ioasid_put_locked(struct ioasid_set *set, ioasid_t ioasid)
> +{
> +	struct ioasid_data *data;
> +
> +	data = xa_load(&active_allocator->xa, ioasid);
> +	if (!data) {
> +		pr_err("Trying to put unknown IOASID %u\n", ioasid);
> +		return;
> +	}
> +
> +	if (set && data->set != set) {
> +		pr_err("Trying to drop IOASID not in the set %u\n", ioasid);
> +		return;
> +	}
> +
> +	if (!refcount_dec_and_test(&data->users)) {
> +		pr_debug("%s: IOASID %d has %d remainning users\n",
> +			__func__, ioasid, refcount_read(&data->users));
> +		return;
> +	}
> +	ioasid_do_free(data);
> +}
> +EXPORT_SYMBOL_GPL(ioasid_put_locked);
> +
> +/**
> + * ioasid_put - Drop a reference of an ioasid
> + * @set
> + * @ioasid
> + *
> + * Check set ownership if @set is non-null.
> + */
> +void ioasid_put(struct ioasid_set *set, ioasid_t ioasid)
> +{
> +	spin_lock(&ioasid_allocator_lock);
> +	ioasid_put_locked(set, ioasid);
> +	spin_unlock(&ioasid_allocator_lock);
> +}
> +EXPORT_SYMBOL_GPL(ioasid_put);
> +
> +/**
>   * ioasid_find - Find IOASID data
>   * @set: the IOASID set
>   * @ioasid: the IOASID to find
> diff --git a/include/linux/ioasid.h b/include/linux/ioasid.h
> index 412d025d440e..310abe4187a3 100644
> --- a/include/linux/ioasid.h
> +++ b/include/linux/ioasid.h
> @@ -76,6 +76,10 @@ int ioasid_attach_data(ioasid_t ioasid, void *data);
>  int ioasid_register_allocator(struct ioasid_allocator_ops *allocator);
>  void ioasid_unregister_allocator(struct ioasid_allocator_ops *allocator);
>  void ioasid_is_in_set(struct ioasid_set *set, ioasid_t ioasid);
> +int ioasid_get(struct ioasid_set *set, ioasid_t ioasid);
> +int ioasid_get_locked(struct ioasid_set *set, ioasid_t ioasid);
> +void ioasid_put(struct ioasid_set *set, ioasid_t ioasid);
> +void ioasid_put_locked(struct ioasid_set *set, ioasid_t ioasid);

Please also add the stubs for !CONFIG_IOASID.

Thanks,
Jean

>  int ioasid_set_for_each_ioasid(struct ioasid_set *sdata,
>  			       void (*fn)(ioasid_t id, void *data),
>  			       void *data);
> -- 
> 2.7.4
>