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

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Message-ID: <24655.1530695695@warthog.procyon.org.uk>
Date:   Wed, 04 Jul 2018 10:14:55 +0100
From:   David Howells <dhowells@...hat.com>
To:     NeilBrown <neilb@...e.com>
Cc:     dhowells@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
        Anthony DeRobertis <aderobertis@...rics.net>,
        linux-cachefs@...hat.com, linux-kernel@...r.kernel.org,
        Lei Xue <carmark.dlut@...il.com>,
        Vegard Nossum <vegard.nossum@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        KiranKumar Modukuri <kiran.modukuri@...il.com>
Subject: Re: [PATCH] cachefiles: fix multiple-put race.

NeilBrown <neilb@...e.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