Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/7/19 18:14, Boqun Feng wrote:
> On Mon, Jul 19, 2021 at 03:43:00AM +0100, Matthew Wilcox wrote:
>> On Mon, Jul 19, 2021 at 10:24:18AM +0800, Zhouyi Zhou wrote:
>>> Meanwhile, I examined the 5.12.17 by naked eye, and found a suspicious place
>>> that could possibly trigger that problem:
>>>
>>> struct swap_info_struct *get_swap_device(swp_entry_t entry)
>>> {
>>>      struct swap_info_struct *si;
>>>      unsigned long offset;
>>>
>>>      if (!entry.val)
>>>              goto out;
>>>     si = swp_swap_info(entry);
>>>     if (!si)
>>>        goto bad_nofile;
>>>
>>>    rcu_read_lock();
>>>   if (data_race(!(si->flags & SWP_VALID)))
>>>      goto unlock_out;
>>>   offset = swp_offset(entry);
>>>   if (offset >= si->max)
>>>    goto unlock_out;
>>>
>>>   return si;
>>> bad_nofile:
>>>   pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
>>> out:
>>>   return NULL;
>>> unlock_out:
>>>   rcu_read_unlock();
>>>   return NULL;
>>> }
>>> I guess the function "return si" without a rcu_read_unlock.
>>
>> Yes, but the caller is supposed to call put_swap_device() which
>> calls rcu_read_unlock().  See commit eb085574a752.
> 
> Right, but we need to make sure there is no sleepable function called
> before put_swap_device() called, and the call trace showed the following
> happened:
> 
> 	do_swap_page():
> 	  si = get_swap_device():
> 	    rcu_read_lock();
> 	  lock_page_or_retry():
> 	    might_sleep(); // call a sleepable function inside RCU read-side c.s.
> 	    __lock_page_or_retry():
> 	      wait_on_page_bit_common():
> 	        schedule():
> 		  rcu_note_context_switch();
> 		  // Warn here
> 	  put_swap_device();
> 	    rcu_read_unlock();
> 
> , which introduced by commit 2799e77529c2a

When in the commit 2799e77529c2a, we're using the percpu_ref to serialize against
concurrent swapoff, i.e. there's percpu_ref inside get_swap_device() instead of
rcu_read_lock(). Please see commit 63d8620ecf93 ("mm/swapfile: use percpu_ref to
serialize against concurrent swapoff") for detail.

Thanks.

> 
> [Copy the author]
> 
> Regards,
> Boqun
> 
> .
>