Re: [PATCH 1/5] lib: logic_pio: Fix RCU usage

[Bjorn Helgaas]

On Thu, Jun 20, 2019 at 06:31:52PM +0800, John Garry wrote:
> The traversing of io_range_list with list_for_each_entry_rcu()
> is not properly protected by rcu_read_lock(), so add it.
> 
> In addition, the list traversing used in logic_pio_register_range()
> does not need to use the rcu variant.

Not being an RCU expert myself, a few words here about why one path
needs protection but the other doesn't would be helpful.  This
basically restates what the patch *does*, which is obvious from the
diff, but not *why*.

> Fixes: 031e3601869c ("lib: Add generic PIO mapping method")
> Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> ---
>  lib/logic_pio.c | 49 +++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 35 insertions(+), 14 deletions(-)
> 
> diff --git a/lib/logic_pio.c b/lib/logic_pio.c
> index feea48fd1a0d..761296376fbc 100644
> --- a/lib/logic_pio.c
> +++ b/lib/logic_pio.c
> @@ -46,7 +46,7 @@ int logic_pio_register_range(struct logic_pio_hwaddr *new_range)
>  	end = new_range->hw_start + new_range->size;
>  
>  	mutex_lock(&io_range_mutex);
> -	list_for_each_entry_rcu(range, &io_range_list, list) {
> +	list_for_each_entry(range, &io_range_list, list) {
>  		if (range->fwnode == new_range->fwnode) {
>  			/* range already there */
>  			goto end_register;
> @@ -108,26 +108,38 @@ int logic_pio_register_range(struct logic_pio_hwaddr *new_range)
>   */
>  struct logic_pio_hwaddr *find_io_range_by_fwnode(struct fwnode_handle *fwnode)
>  {
> -	struct logic_pio_hwaddr *range;
> +	struct logic_pio_hwaddr *range, *found_range = NULL;
>  
> +	rcu_read_lock();
>  	list_for_each_entry_rcu(range, &io_range_list, list) {
> -		if (range->fwnode == fwnode)
> -			return range;
> +		if (range->fwnode == fwnode) {
> +			found_range = range;
> +			break;
> +		}
>  	}
> -	return NULL;
> +	rcu_read_unlock();
> +
> +	return found_range;
>  }
>  
>  /* Return a registered range given an input PIO token */
>  static struct logic_pio_hwaddr *find_io_range(unsigned long pio)
>  {
> -	struct logic_pio_hwaddr *range;
> +	struct logic_pio_hwaddr *range, *found_range = NULL;
>  
> +	rcu_read_lock();
>  	list_for_each_entry_rcu(range, &io_range_list, list) {
> -		if (in_range(pio, range->io_start, range->size))
> -			return range;
> +		if (in_range(pio, range->io_start, range->size)) {
> +			found_range = range;
> +			break;
> +		}
>  	}
> -	pr_err("PIO entry token %lx invalid\n", pio);
> -	return NULL;
> +	rcu_read_unlock();
> +
> +	if (!found_range)
> +		pr_err("PIO entry token 0x%lx invalid\n", pio);
> +
> +	return found_range;
>  }
>  
>  /**
> @@ -180,14 +192,23 @@ unsigned long logic_pio_trans_cpuaddr(resource_size_t addr)
>  {
>  	struct logic_pio_hwaddr *range;
>  
> +	rcu_read_lock();
>  	list_for_each_entry_rcu(range, &io_range_list, list) {
>  		if (range->flags != LOGIC_PIO_CPU_MMIO)
>  			continue;
> -		if (in_range(addr, range->hw_start, range->size))
> -			return addr - range->hw_start + range->io_start;
> +		if (in_range(addr, range->hw_start, range->size)) {
> +			unsigned long cpuaddr;
> +
> +			cpuaddr = addr - range->hw_start + range->io_start;
> +
> +			rcu_read_unlock();
> +			return cpuaddr;
> +		}
>  	}
> -	pr_err("addr %llx not registered in io_range_list\n",
> -	       (unsigned long long) addr);
> +	rcu_read_unlock();
> +
> +	pr_err("addr %pa not registered in io_range_list\n", &addr);
> +
>  	return ~0UL;
>  }
>  
> -- 
> 2.17.1
>