Re: [PATCH] fasync: Fix deadlock between task-context and interrupt-context kill_fasync()

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 17.04.2018 17:01, Matthew Wilcox wrote:
> On Thu, Apr 05, 2018 at 02:58:06PM +0300, Kirill Tkhai wrote:
>> I observed the following deadlock between them:
>>
>> [task 1]                          [task 2]                         [task 3]
>> kill_fasync()                     mm_update_next_owner()           copy_process()
>>  spin_lock_irqsave(&fa->fa_lock)   read_lock(&tasklist_lock)        write_lock_irq(&tasklist_lock)
>>   send_sigio()                    <IRQ>                             ...
>>    read_lock(&fown->lock)         kill_fasync()                     ...
>>     read_lock(&tasklist_lock)      spin_lock_irqsave(&fa->fa_lock)  ...
>>
>> Task 1 can't acquire read locked tasklist_lock, since there is
>> already task 3 expressed its wish to take the lock exclusive.
>> Task 2 holds the read locked lock, but it can't take the spin lock.
> 
> I think the important question is to Peter ... why didn't lockdep catch this?
> 
>> -		spin_lock_irq(&fa->fa_lock);
>> +		write_lock_irq(&fa->fa_lock);
>>  		fa->fa_file = NULL;
>> -		spin_unlock_irq(&fa->fa_lock);
>> +		write_unlock_irq(&fa->fa_lock);
> ...
>> -		spin_lock_irq(&fa->fa_lock);
>> +		write_lock_irq(&fa->fa_lock);
>>  		fa->fa_fd = fd;
>> -		spin_unlock_irq(&fa->fa_lock);
>> +		write_unlock_irq(&fa->fa_lock);
> 
> Do we really need a lock here?  If we convert each of these into WRITE_ONCE,

We want to pass specific fd to send_sigio(), not a random one. Also, we do want
to dereference specific file in kill_fasync_rcu() without a danger it will be freed
in parallel. So, since there is no rcu_read_lock() or another protection in readers
of this data, we *can't* drop the lock.

> then 
> 
> ...
>> -		spin_lock_irqsave(&fa->fa_lock, flags);
>> +		read_lock(&fa->fa_lock);
>>  		if (fa->fa_file) {
> 
> file = READ_ONCE(fa->fa_file)
> 
> then we're not opening any new races, are we?
> 
>>  			fown = &fa->fa_file->f_owner;
>>  			/* Don't send SIGURG to processes which have not set a
>> @@ -997,7 +996,7 @@ static void kill_fasync_rcu(struct fasync_struct *fa, int sig, int band)
>>  			if (!(sig == SIGURG && fown->signum == 0))
>>  				send_sigio(fown, fa->fa_fd, band);
>>  		}
>> -		spin_unlock_irqrestore(&fa->fa_lock, flags);
>> +		read_unlock(&fa->fa_lock);
>>  		fa = rcu_dereference(fa->fa_next);
>>  	}
>>  }
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index c6baf767619e..297e2dcd9dd2 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -1250,7 +1250,7 @@ static inline int locks_lock_file_wait(struct file *filp, struct file_lock *fl)
>>  }
>>  
>>  struct fasync_struct {
>> -	spinlock_t		fa_lock;
>> +	rwlock_t		fa_lock;
>>  	int			magic;
>>  	int			fa_fd;
>>  	struct fasync_struct	*fa_next; /* singly linked list */

Kirill