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SubjectRe: [PATCH] fasync: Fix deadlock between task-context and interrupt-context kill_fasync()
On Tue, Apr 17, 2018 at 07:01:10AM -0700, Matthew Wilcox wrote:
> 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?
>

I think the following will help lockdep to catch this:


@@ -570,7 +588,14 @@ static inline void print_irqtrace_events(struct task_struct *curr)
#define spin_release(l, n, i) lock_release(l, n, i)

#define rwlock_acquire(l, s, t, i) lock_acquire_exclusive(l, s, t, NULL, i)
-#define rwlock_acquire_read(l, s, t, i) lock_acquire_shared_recursive(l, s, t, NULL, i)
+#define rwlock_acquire_read(l, s, t, i) \
+do { \
+ if (!IS_ENABLED(CONFIG_QUEUED_RWLOCKS) || in_interrupt()) \
+ lock_acquire_shared_recursive(l, s, t, NULL, i); \
+ else \
+ lock_acquire_shared(l, s, t, NULL, i); \
+} while (0)
+

However, this will break several self tests in lib/locking-selftest.c,
because we used to treat read_lock() as recursive read locks for all
callsites from lockdep's viewpoint.

Besides, the above change will bring an interesting challenge for the
recursive read lock deadlock detection work that I'm doing:

https://marc.info/?l=linux-kernel&m=152345444100825

I will explain that in the thread of that patchset and add you and
others Cced in case that you're interested.

Regards,
Boqun

> > - 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 */
> >
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