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,
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 */
>
