linux-kernel - Re: [PATCH] fscache: fix race between enablement and dropping of object

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Message-ID: <10285.1543417950@warthog.procyon.org.uk>
Date:   Wed, 28 Nov 2018 15:12:30 +0000
From:   David Howells <dhowells@...hat.com>
To:     NeilBrown <neilb@...e.com>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fscache: fix race between enablement and dropping of object

NeilBrown <neilb@...e.com> wrote:

> It was observed that a process blocked indefintely in
> __fscache_read_or_alloc_page(), waiting for FSCACHE_COOKIE_LOOKING_UP
> to be cleared via fscache_wait_for_deferred_lookup().

Was there a backtrace for that?

> This leaves a window where something else can set FSCACHE_COOKIE_LOOKING_UP

Attempting to enable the cookie again would have that effect.

> There is some uncertainty in this analysis, but it seems to be fit the
> observations.  Adding the wake in this patch will be handled correctly
> by __fscache_read_or_alloc_page(), as it checks if ->backing_objects
> is empty again, after waiting.

That should work.  The read op should just then fail with -ENOBUFS, causing
the netfs to issue the read over the network instead.

I think there's a more elegant solution to be had, but it'll take some thought
as to how to achieve it.

David