Date:   Mon, 24 Aug 2020 10:26:55 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jacob Pan <jacob.pan.linux@...il.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>
Cc:     baolu.lu@...ux.intel.com, Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Eric Auger <eric.auger@...hat.com>, Wu Hao <hao.wu@...el.com>
Subject: Re: [PATCH v2 4/9] iommu/ioasid: Add reference couting functions

Hi Jacob,

On 8/22/20 12:35 PM, 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@...ux.intel.com>
> ---
>   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.
> + */
> +
> +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);
> +
> +/**
> + * ioasid_get - Obtain a reference of an ioasid
> + * @set
> + * @ioasid
> + *
> + * Check set ownership if @set is non-null.
> + */
> +int ioasid_get(struct ioasid_set *set, ioasid_t ioasid)
> +{
> +	int ret = 0;
> +
> +	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

Do you need to increase the refcount of the found ioasid and ask the
caller to drop it after use? Otherwise, the ioasid might be freed
elsewhere.

Best regards,
baolu

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

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