Re: [PATCH] cachefiles: fix multiple-put race.

[David Howells]

NeilBrown <neilb@xxxxxxxx> wrote:

> +	fscache_enqueue_retrieval(monitor->op);
> +
>  	spin_lock(&object->work_lock);
>  	list_add_tail(&monitor->op_link, &monitor->op->to_do);
>  	spin_unlock(&object->work_lock);
>  
> -	fscache_enqueue_retrieval(monitor->op);

That won't necessarily work because the work processor can then happen before
you've added the work to the to_do list.

I'm thinking that KiranKumar's solution might be the best one.

The problem is that cachefiles_read_waiter() doesn't have a ref on the monitor
object but is entirely dependent on the waitqueue lock for safety.  I think
KiranKumar's patch is correct to take a ref before doing the queuing.  It
might be possible to then pass this along to the work processor, but that
might be too fiddly.

Actually, I want to get rid of the page monitoring stuff entirely as it's
quite fragile and use an iterator and direct-IO instead, but we have to fix
this for now.

David