fasync: Fix deadlock between task-context and interrupt-context kill_fasync()

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>
Signed-off-by: Jeff Layton <jlayton@redhat.com>

fs/fcntl.c

@@ -871,9 +871,9 @@ 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);
@@ -918,13 +918,13 @@ 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;
@@ -987,14 +987,13 @@ 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);
 		if (fa->fa_file) {
 			fown = &fa->fa_file->f_owner;
 			/* Don't send SIGURG to processes which have not set a
@@ -1003,7 +1002,7 @@ 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);
 	}
 }

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 */