Re: [PATCH] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant <code@siddh.me>]

On Wed, 27 Jul 2022 20:16:40 +0530  David Howells <dhowells@redhat.com> wrote:
> Siddh Raman Pant <code@siddh.me> wrote:
> 
> > Greg KH <gregkh@linuxfoundation.org> wrote:
> 
> > > > - spin_unlock_bh(&wqueue->lock);
> > > >   rcu_read_unlock();
> > >
> > > Also you now have a spinlock held when calling rcu_read_unlock(), are
> > > you sure that's ok?
> 
> Worse, we have softirqs disabled still, which might cause problems for
> rcu_read_unlock()?
> 
> > We logically should not do write operations in a read critical section, so the
> > nulling of `wqueue->pipe->watch_queue` should happen after rcu_read_unlock().
> > Also, since we already have a spinlock, we can use it to ensure the nulling.
> > So I think it is okay.
> 
> Read/write locks are perhaps misnamed in this sense; they perhaps should be
> shared/exclusive.  But, yes, we *can* do certain write operations with the
> lock held - if we're careful.  Locks are required if we need to pairs of
> related memory accesses; if we're only making a single non-dependent write,
> then we don't necessarily need a write lock.
> 
> However, you're referring to RCU read lock.  That's a very special lock that
> has to do with maintenance of persistence of objects without taking any other
> lock.  The moment you drop that lock, anything you accessed under RCU protocol
> rules should be considered to have evaporated.
> 
> Think of it more as a way to have a deferred destructor/deallocator.
> 
> So I would do:
> 
> +
> +       /* Clearing the watch queue, so we should clean the associated pipe. */
> +       if (wqueue->pipe) {
> +               wqueue->pipe->watch_queue = NULL;
> +               wqueue->pipe = NULL;
> +       }
> +
>         spin_unlock_bh(&wqueue->lock);
>         rcu_read_unlock();
>  }
> 
> However, since you're now changing wqueue->pipe whilst a notification is being
> posted, you need a barrier in post_one_notification() to prevent the compiler
> from reloading the value:
> 
>         struct pipe_inode_info *pipe = READ_ONCE(wqueue->pipe);
> 
> David
> 

Thank you for explaining it!

I will send a v3. Should I add a Suggested-by tag mentioning you?

Thanks,
Siddh