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Date:   Wed, 27 Jul 2022 21:50:52 +0530
From:   Siddh Raman Pant <code@siddh.me>
To:     "David Howells" <dhowells@redhat.com>
Cc:     "Greg KH" <gregkh@linuxfoundation.org>,
        "Christophe JAILLET" <christophe.jaillet@wanadoo.fr>,
        "Eric Dumazet" <edumazet@google.com>,
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Message-ID: <182407602ce.190e58816827.7904364186178466266@siddh.me>
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References: <1822b768504.1d4e377e236061.5518350412857967240@siddh.me> <20220723135447.199557-1-code@siddh.me> <Ytv/4Tljvlt0PJ2r@kroah.com> <3558070.1658933200@warthog.procyon.org.uk>
Subject: Re: [PATCH] kernel/watch_queue: Make pipe NULL while clearing
 watch_queue
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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