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SubjectRe: [PATCH] cachefiles: fix multiple-put race.
Date
NeilBrown <neilb@suse.com> 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

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