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>

diff --git a/fs/fcntl.c b/fs/fcntl.c
index d737ff0..c421694 100644 (file)
--- a/fs/fcntl.c
+++ b/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);
        }
 }

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 760d8da..d6c24b6 100644 (file)
--- 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 */