From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC, PATCH, RESEND] fs: push rcu_barrier() from
 deactivate_locked_super() to filesystems
Date: Fri, 8 Jun 2012 17:28:42 -0700
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Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Boaz Harrosh <bharrosh@panasas.com>,
	Tao Ma <boyu.mt@taobao.com>, Nick Piggin <npiggin@kernel.dk>,
	"Dmitry V. Levin" <ldv@altlinux.org>,
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To: Linus Torvalds <torvalds@linux-foundation.org>
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On Fri, 8 Jun 2012 16:46:47 -0700 Linus Torvalds <torvalds@linux-foundation.org> wrote:

> Of course, if you just mean having a VFS wrapper that does
> 
>     static void vfs_inode_kmem_cache_destroy(struct kmem_cache *cachep)
>     {
>         rcu_barrier();
>         kmem_cache_destroy(cachep);
>     }
> 
> then we could do that. Not much better than what Kirill's patch did,
> but at least we could have that comment in just one single place.

That's conceptually what I meant.  But it has the problem that new and
out-of-tree filesystems might forget to do it.  Which is why I suggest
adding a kmem_cache* argument to unregister_filesystem() for this.

It's a bit awkward, and the fs can pass in NULL if it knows what it's
doing.  But it's reliable.