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Date:   Thu, 7 Feb 2019 04:14:55 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Salman Qazi <sqazi@google.com>
Cc:     Eric Biederman <ebiederm@xmission.com>,
        Eric Dumazet <edumazet@google.com>,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] fs, ipc: Use an asynchronous version of kern_unmount in
 IPC
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References: <20190206195354.40576-1-sqazi@google.com>
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On Wed, Feb 06, 2019 at 11:53:54AM -0800, Salman Qazi wrote:

> This patch solves the issue by removing synchronize_rcu from mq_put_mnt.
> This is done by implementing an asynchronous version of kern_unmount.
> 
> Since mntput() sleeps, it needs to be deferred to a work queue.
> 
> Additionally, the callers of mq_put_mnt appear to be safe having
> it behave asynchronously.  In particular, put_ipc_ns calls
> mq_clear_sbinfo which renders the inode inaccessible for the purposes of
> mqueue_create by making s_fs_info NULL.  This appears
> to be the thing that prevents access while free_ipc_ns is taking place.
> So, the unmount should be able to proceed lazily.

Ugh...  I really doubt that it's correct.  The caller is
                mq_put_mnt(ns);
                free_ipc_ns(ns);
and we have
static void mqueue_evict_inode(struct inode *inode)
{

...

        ipc_ns = get_ns_from_inode(inode);

with

static struct ipc_namespace *get_ns_from_inode(struct inode *inode)
{
        struct ipc_namespace *ns;

        spin_lock(&mq_lock);
        ns = __get_ns_from_inode(inode);
        spin_unlock(&mq_lock);
        return ns;
}

and

static inline struct ipc_namespace *__get_ns_from_inode(struct inode *inode)
{
        return get_ipc_ns(inode->i_sb->s_fs_info);
}

with ->s_fs_info being the ipc_namespace we are freeing after mq_put_ns()

Are you saying that get_ipc_ns() after free_ipc_ns() is safe?  Because
->evict_inode() *IS* called on umount.  What happens to your patch if
there was a regular file left on that filesystem?

Smells like a memory corruptor...