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

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 17.04.2018 16:31, Jeff Layton wrote:
> On Tue, 2018-04-17 at 14:53 +0300, Kirill Tkhai wrote:
>> Hi, Jeff,
>>
>> On 17.04.2018 14:42, Jeff Layton wrote:
>>> On Thu, 2018-04-05 at 14:58 +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.
>>>>
>>>> Also, there is possible another deadlock (which I haven't observed):
>>>>
>>>> [task 1]                            [task 2]
>>>> f_getown()                          kill_fasync()
>>>>  read_lock(&f_own->lock)             spin_lock_irqsave(&fa->fa_lock,)
>>>>  <IRQ>                               send_sigio()                     write_lock_irq(&f_own->lock)
>>>>   kill_fasync()                       read_lock(&fown->lock)
>>>>    spin_lock_irqsave(&fa->fa_lock,)
>>>>
>>>> Actually, we do not need exclusive fa->fa_lock in kill_fasync_rcu(),
>>>> as it guarantees fa->fa_file->f_owner integrity only. It may seem,
>>>> that it used to give a task a small possibility to receive two sequential
>>>> signals, if there are two parallel kill_fasync() callers, and task
>>>> handles the first signal fastly, but the behaviour won't become
>>>> different, since there is exclusive sighand lock in do_send_sig_info().
>>>>
>>>> The patch converts fa_lock into rwlock_t, and this fixes two above
>>>> deadlocks, as rwlock is allowed to be taken from interrupt handler
>>>> by qrwlock design.
>>>>
>>>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>>>>
>>>> I used the following program for testing:
>>>>
>>>> #include <unistd.h>
>>>> #include <stdlib.h>
>>>> #include <signal.h>
>>>> #include <fcntl.h>
>>>> #include <errno.h>
>>>> #include <stdio.h>
>>>>
>>>> #ifndef F_SETSIG
>>>> #define F_SETSIG 10
>>>> #endif
>>>>
>>>> void handler(int sig)
>>>> {
>>>> }
>>>>
>>>> main()
>>>> {
>>>> 	unsigned int flags;
>>>> 	int fd;
>>>>
>>>> 	system("echo 8 > /proc/sys/kernel/random/read_wakeup_threshold");
>>>> 	system("while :; do ls -R / > /dev/random 2>&1 ; echo 3 > /proc/sys/vm/drop_caches; done &");
>>>>
>>>> 	if (signal(SIGINT, handler) < 0) {
>>>> 		perror("Signal");
>>>> 		exit(1);
>>>> 	}
>>>>
>>>> 	fd = open("/dev/random", O_RDWR);
>>>> 	if (fd < 0) {
>>>> 		perror("Can't open");
>>>> 		exit(1);
>>>> 	}
>>>>
>>>> 	flags = FASYNC | fcntl(fd, F_GETFL);
>>>> 	if (fcntl(fd, F_SETFL, flags) < 0) {
>>>> 		perror("Setfl");
>>>> 		exit(1);
>>>> 	}
>>>> 	if (fcntl(fd, F_SETOWN, getpid())) {
>>>> 		perror("Setown");
>>>> 		exit(1);
>>>> 	}
>>>> 	if (fcntl(fd, F_SETSIG, SIGINT)) {
>>>> 		perror("Setsig");
>>>> 		exit(1);
>>>> 	}
>>>>
>>>> 	while (1)
>>>> 		sleep(100);
>>>> }
>>>> ---
>>>>  fs/fcntl.c         |   15 +++++++--------
>>>>  include/linux/fs.h |    2 +-
>>>>  2 files changed, 8 insertions(+), 9 deletions(-)
>>>>
>>>> diff --git a/fs/fcntl.c b/fs/fcntl.c
>>>> index 1e97f1fda90c..780161a11f9d 100644
>>>> --- a/fs/fcntl.c
>>>> +++ b/fs/fcntl.c
>>>> @@ -865,9 +865,9 @@ int fasync_remove_entry(struct file *filp, struct fasync_struct **fapp)
>>>>  		if (fa->fa_file != filp)
>>>>  			continue;
>>>>  
>>>> -		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);
>>>>  
>>>>  		*fp = fa->fa_next;
>>>>  		call_rcu(&fa->fa_rcu, fasync_free_rcu);
>>>> @@ -912,13 +912,13 @@ struct fasync_struct *fasync_insert_entry(int fd, struct file *filp, struct fasy
>>>>  		if (fa->fa_file != filp)
>>>>  			continue;
>>>>  
>>>> -		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);
>>>>  		goto out;
>>>>  	}
>>>>  
>>>> -	spin_lock_init(&new->fa_lock);
>>>> +	rwlock_init(&new->fa_lock);
>>>>  	new->magic = FASYNC_MAGIC;
>>>>  	new->fa_file = filp;
>>>>  	new->fa_fd = fd;
>>>> @@ -981,14 +981,13 @@ static void kill_fasync_rcu(struct fasync_struct *fa, int sig, int band)
>>>>  {
>>>>  	while (fa) {
>>>>  		struct fown_struct *fown;
>>>> -		unsigned long flags;
>>>>  
>>>>  		if (fa->magic != FASYNC_MAGIC) {
>>>>  			printk(KERN_ERR "kill_fasync: bad magic number in "
>>>>  			       "fasync_struct!\n");
>>>>  			return;
>>>>  		}
>>>> -		spin_lock_irqsave(&fa->fa_lock, flags);
>>>> +		read_lock(&fa->fa_lock);
>>>
>>> Does this need to be read_lock_irq? Why is it ok to allow interrupts
>>> here?
>>
>> Read locked rwlock can be taken for reading from IRQ once again even
>> if there is a writer pending, while spin lock can't:
>>
>> void queued_read_lock_slowpath(struct qrwlock *lock)
>> {
>>         /*
>>          * Readers come here when they cannot get the lock without waiting
>>          */
>>         if (unlikely(in_interrupt())) {
>>                 /*
>>                  * Readers in interrupt context will get the lock immediately
>>                  * if the writer is just waiting (not holding the lock yet),
>>                  * so spin with ACQUIRE semantics until the lock is available
>>                  * without waiting in the queue.
>>                  */
>>                 atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
>>                 return;
>>         }
>>
>> So, when we replace spinlock with read_lock(), we don't need disable IRQs anymore.
>> All we need is to make write_lock always disable IRQs.
> 
> Got it, thanks.
> 
> read_lock_irq is still used in several (rather obscure) places. Does
> this mean that we should do a global s/read_lock_irq/read_lock/ and
> remove it? Or is it still useful to disable irqs for some read_lock
> acquisitions?

I haven't analyzed them, but it seems it's possible to introduce a situation,
when rwlock nests with exclusive lock and require to disable IRQ. Let's see
at fasync example. The deadlock also was in:

	https://lkml.org/lkml/2018/4/5/125

and we could fixed it in another way by disabling IRQ during read_lock(). But
in case of fasync we are successful as exclusive lock is not need, and we replaced
spin lock with rwlock.

If the rest of places nest read_lock() with spin lock, they are need in irq disable.
 
>>
>>>>  		if (fa->fa_file) {
>>>>  			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 */
>>>>
>>>
>>> I've no objection to the patch in principle, but I'm not as familiar
>>> with the fasync code as others here.
>>
>> I took the reviewers list from MAINTAINERS and ./scripts/get_maintainer.pl,
>> don't have an ideas what else should be CCed.

Oh, my English. I.e., "who else". 

> 
> 
> No worries. The patch seems sane enough to me. You can add:
> 
> Acked-by: Jeff Layton <jlayton@kernel.org>

Thanks! Should I resend this with some more CC or you are going to take the patch
via your tree?

Kirill