linux-kernel - Re: [PATCH 1/4] fs, afs: convert afs_cell.usage from atomic_t to refcount

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Message-ID: <12908.1487780160@warthog.procyon.org.uk>
Date:   Wed, 22 Feb 2017 16:16:00 +0000
From:   David Howells <dhowells@...hat.com>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     dhowells@...hat.com, linux-kernel@...r.kernel.org,
        linux-afs@...ts.infradead.org, peterz@...radead.org,
        gregkh@...uxfoundation.org, Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 1/4] fs, afs: convert afs_cell.usage from atomic_t to refcount_t

Elena Reshetova <elena.reshetova@...el.com> wrote:

> refcount_t type and corresponding API should be
> used instead of atomic_t when the variable is used as
> a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free
> situations.

This causes an assertion failure because cells aren't immediately destroyed
when their refcount reaches 0, but may be resurrected provided the cache lock
is held.  However, attempting to increment the 0 refcount does nothing, not
even giving a warning.

So please place a hold on this patch.  I will check the other AFS patches also.

David