linux-kernel - Re: [PATCH 4.4 022/118] IB/uverbs: Fix race between uverbs_close and remove

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Message-ID: <20161003170533.GD6801@obsidianresearch.com>
Date:   Mon, 3 Oct 2016 11:05:33 -0600
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Vegard Nossum <vegard.nossum@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Devesh Sharma <devesh.sharma@...adcom.com>,
        Yishai Hadas <yishaih@...lanox.com>,
        Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>
Subject: Re: [PATCH 4.4 022/118] IB/uverbs: Fix race between uverbs_close and
 remove_one

On Mon, Oct 03, 2016 at 02:28:40PM +0200, Vegard Nossum wrote:

> This lock is literally called "cleanup mutex" and it's not really
> documented what data it protects. Is there a better solution here?

I agree it is very complex and hard to understand. This is why it
needed patching :| The mutex is in fact pretty much locking
code. (ensuring that ib_uverbs_cleanup_ucontext only runs on one
thread during this race)

There are at least three locks involved in this process. I didn't see
any obvious way to extend any of the other locks to handle this case.

The argument against most simple solutions (eg a rw lock rather than
the srcu) has been performance on these paths.

Jason