Date:   Tue, 7 Jul 2020 08:05:22 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Dave Jiang <dave.jiang@...el.com>, vkoul@...nel.org
Cc:     Swathi Kovvuri <swathi.kovvuri@...el.com>,
        dmaengine@...r.kernel.org,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] dmaengine: check device and channel list for empty

On 26. 06. 20, 20:09, Dave Jiang wrote:
> Check dma device list and channel list for empty before iterate as the
> iteration function assume the list to be not empty. With devices and
> channels now being hot pluggable this is a condition that needs to be
> checked. Otherwise it can cause the iterator to spin forever.

Could you be a little bit more specific how this can spin forever? I.e.
can you attach a stacktrace of such a behaviour?

As in the empty case, "&pos->member" is "head" (look into
list_for_each_entry) and the for loop should loop exactly zero times.

> Fixes: e81274cd6b52 ("dmaengine: add support to dynamic register/unregister of channels")
> Reported-by: Swathi Kovvuri <swathi.kovvuri@...el.com>
> Signed-off-by: Dave Jiang <dave.jiang@...el.com>
> Tested-by: Swathi Kovvuri <swathi.kovvuri@...el.com>
> ---
> 
> Rebased to dmaengine next tree
> 
>  drivers/dma/dmaengine.c |  119 +++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 94 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 2b06a7a8629d..0d6529eff66f 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -85,6 +85,9 @@ static void dmaengine_dbg_summary_show(struct seq_file *s,
>  {
>  	struct dma_chan *chan;
>  
> +	if (list_empty(&dma_dev->channels))
> +		return;
> +
>  	list_for_each_entry(chan, &dma_dev->channels, device_node) {
>  		if (chan->client_count) {
>  			seq_printf(s, " %-13s| %s", dma_chan_name(chan),
> @@ -104,6 +107,11 @@ static int dmaengine_summary_show(struct seq_file *s, void *data)
>  	struct dma_device *dma_dev = NULL;
>  
>  	mutex_lock(&dma_list_mutex);
> +	if (list_empty(&dma_device_list)) {
> +		mutex_unlock(&dma_list_mutex);
> +		return 0;
> +	}
> +
>  	list_for_each_entry(dma_dev, &dma_device_list, global_node) {
>  		seq_printf(s, "dma%d (%s): number of channels: %u\n",
>  			   dma_dev->dev_id, dev_name(dma_dev->dev),
> @@ -324,10 +332,15 @@ static struct dma_chan *min_chan(enum dma_transaction_type cap, int cpu)
>  	struct dma_chan *min = NULL;
>  	struct dma_chan *localmin = NULL;
>  
> +	if (list_empty(&dma_device_list))
> +		return NULL;
> +
>  	list_for_each_entry(device, &dma_device_list, global_node) {
>  		if (!dma_has_cap(cap, device->cap_mask) ||
>  		    dma_has_cap(DMA_PRIVATE, device->cap_mask))
>  			continue;
> +		if (list_empty(&device->channels))
> +			continue;
>  		list_for_each_entry(chan, &device->channels, device_node) {
>  			if (!chan->client_count)
>  				continue;
> @@ -365,6 +378,9 @@ static void dma_channel_rebalance(void)
>  	int cpu;
>  	int cap;
>  
> +	if (list_empty(&dma_device_list))
> +		return;
> +
>  	/* undo the last distribution */
>  	for_each_dma_cap_mask(cap, dma_cap_mask_all)
>  		for_each_possible_cpu(cpu)
> @@ -373,6 +389,8 @@ static void dma_channel_rebalance(void)
>  	list_for_each_entry(device, &dma_device_list, global_node) {
>  		if (dma_has_cap(DMA_PRIVATE, device->cap_mask))
>  			continue;
> +		if (list_empty(&device->channels))
> +			continue;
>  		list_for_each_entry(chan, &device->channels, device_node)
>  			chan->table_count = 0;
>  	}
> @@ -556,6 +574,10 @@ void dma_issue_pending_all(void)
>  	struct dma_chan *chan;
>  
>  	rcu_read_lock();
> +	if (list_empty(&dma_device_list)) {
> +		rcu_read_unlock();
> +		return;
> +	}
>  	list_for_each_entry_rcu(device, &dma_device_list, global_node) {
>  		if (dma_has_cap(DMA_PRIVATE, device->cap_mask))
>  			continue;
> @@ -613,6 +635,10 @@ static struct dma_chan *private_candidate(const dma_cap_mask_t *mask,
>  		dev_dbg(dev->dev, "%s: wrong capabilities\n", __func__);
>  		return NULL;
>  	}
> +
> +	if (list_empty(&dev->channels))
> +		return NULL;
> +
>  	/* devices with multiple channels need special handling as we need to
>  	 * ensure that all channels are either private or public.
>  	 */
> @@ -749,6 +775,11 @@ struct dma_chan *__dma_request_channel(const dma_cap_mask_t *mask,
>  
>  	/* Find a channel */
>  	mutex_lock(&dma_list_mutex);
> +	if (list_empty(&dma_device_list)) {
> +		mutex_unlock(&dma_list_mutex);
> +		return NULL;
> +	}
> +
>  	list_for_each_entry_safe(device, _d, &dma_device_list, global_node) {
>  		/* Finds a DMA controller with matching device node */
>  		if (np && device->dev->of_node && np != device->dev->of_node)
> @@ -819,6 +850,11 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
>  
>  	/* Try to find the channel via the DMA filter map(s) */
>  	mutex_lock(&dma_list_mutex);
> +	if (list_empty(&dma_device_list)) {
> +		mutex_unlock(&dma_list_mutex);
> +		return NULL;
> +	}
> +
>  	list_for_each_entry_safe(d, _d, &dma_device_list, global_node) {
>  		dma_cap_mask_t mask;
>  		const struct dma_slave_map *map = dma_filter_match(d, name, dev);
> @@ -942,10 +978,17 @@ void dmaengine_get(void)
>  	mutex_lock(&dma_list_mutex);
>  	dmaengine_ref_count++;
>  
> +	if (list_empty(&dma_device_list)) {
> +		mutex_unlock(&dma_list_mutex);
> +		return;
> +	}
> +
>  	/* try to grab channels */
>  	list_for_each_entry_safe(device, _d, &dma_device_list, global_node) {
>  		if (dma_has_cap(DMA_PRIVATE, device->cap_mask))
>  			continue;
> +		if (list_empty(&device->channels))
> +			continue;
>  		list_for_each_entry(chan, &device->channels, device_node) {
>  			err = dma_chan_get(chan);
>  			if (err == -ENODEV) {
> @@ -980,10 +1023,17 @@ void dmaengine_put(void)
>  	mutex_lock(&dma_list_mutex);
>  	dmaengine_ref_count--;
>  	BUG_ON(dmaengine_ref_count < 0);
> +	if (list_empty(&dma_device_list)) {
> +		mutex_unlock(&dma_list_mutex);
> +		return;
> +	}
> +
>  	/* drop channel references */
>  	list_for_each_entry_safe(device, _d, &dma_device_list, global_node) {
>  		if (dma_has_cap(DMA_PRIVATE, device->cap_mask))
>  			continue;
> +		if (list_empty(&device->channels))
> +			continue;
>  		list_for_each_entry(chan, &device->channels, device_node)
>  			dma_chan_put(chan);
>  	}
> @@ -1132,6 +1182,39 @@ void dma_async_device_channel_unregister(struct dma_device *device,
>  }
>  EXPORT_SYMBOL_GPL(dma_async_device_channel_unregister);
>  
> +static int dma_channel_enumeration(struct dma_device *device)
> +{
> +	struct dma_chan *chan;
> +	int rc;
> +
> +	if (list_empty(&device->channels))
> +		return 0;
> +
> +	/* represent channels in sysfs. Probably want devs too */
> +	list_for_each_entry(chan, &device->channels, device_node) {
> +		rc = __dma_async_device_channel_register(device, chan);
> +		if (rc < 0)
> +			return rc;
> +	}
> +
> +	/* take references on public channels */
> +	if (dmaengine_ref_count && !dma_has_cap(DMA_PRIVATE, device->cap_mask))
> +		list_for_each_entry(chan, &device->channels, device_node) {
> +			/* if clients are already waiting for channels we need
> +			 * to take references on their behalf
> +			 */
> +			if (dma_chan_get(chan) == -ENODEV) {
> +				/* note we can only get here for the first
> +				 * channel as the remaining channels are
> +				 * guaranteed to get a reference
> +				 */
> +				return -ENODEV;
> +			}
> +		}
> +
> +	return 0;
> +}
> +
>  /**
>   * dma_async_device_register - registers DMA devices found
>   * @device:	pointer to &struct dma_device
> @@ -1247,33 +1330,15 @@ int dma_async_device_register(struct dma_device *device)
>  	if (rc != 0)
>  		return rc;
>  
> +	mutex_lock(&dma_list_mutex);
>  	mutex_init(&device->chan_mutex);
>  	ida_init(&device->chan_ida);
> -
> -	/* represent channels in sysfs. Probably want devs too */
> -	list_for_each_entry(chan, &device->channels, device_node) {
> -		rc = __dma_async_device_channel_register(device, chan);
> -		if (rc < 0)
> -			goto err_out;
> +	rc = dma_channel_enumeration(device);
> +	if (rc < 0) {
> +		mutex_unlock(&dma_list_mutex);
> +		goto err_out;
>  	}
>  
> -	mutex_lock(&dma_list_mutex);
> -	/* take references on public channels */
> -	if (dmaengine_ref_count && !dma_has_cap(DMA_PRIVATE, device->cap_mask))
> -		list_for_each_entry(chan, &device->channels, device_node) {
> -			/* if clients are already waiting for channels we need
> -			 * to take references on their behalf
> -			 */
> -			if (dma_chan_get(chan) == -ENODEV) {
> -				/* note we can only get here for the first
> -				 * channel as the remaining channels are
> -				 * guaranteed to get a reference
> -				 */
> -				rc = -ENODEV;
> -				mutex_unlock(&dma_list_mutex);
> -				goto err_out;
> -			}
> -		}
>  	list_add_tail_rcu(&device->global_node, &dma_device_list);
>  	if (dma_has_cap(DMA_PRIVATE, device->cap_mask))
>  		device->privatecnt++;	/* Always private */
> @@ -1291,6 +1356,9 @@ int dma_async_device_register(struct dma_device *device)
>  		return rc;
>  	}
>  
> +	if (list_empty(&device->channels))
> +		return rc;
> +
>  	list_for_each_entry(chan, &device->channels, device_node) {
>  		if (chan->local == NULL)
>  			continue;
> @@ -1317,8 +1385,9 @@ void dma_async_device_unregister(struct dma_device *device)
>  
>  	dmaengine_debug_unregister(device);
>  
> -	list_for_each_entry_safe(chan, n, &device->channels, device_node)
> -		__dma_async_device_channel_unregister(device, chan);
> +	if (!list_empty(&device->channels))
> +		list_for_each_entry_safe(chan, n, &device->channels, device_node)
> +			__dma_async_device_channel_unregister(device, chan);
>  
>  	mutex_lock(&dma_list_mutex);
>  	/*



-- 
js
suse labs

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