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>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 760d8da1b6c7..d6c24b68a092 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 */