linux-kernel - Re: [PATCH] fs: synchronize_rcu when unregister

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Date:	Thu, 14 Apr 2011 08:52:43 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Milton Miller <miltonm@....com>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: synchronize_rcu when unregister_filesystem success
 not failure

On Thu, Apr 14, 2011 at 8:41 AM, Milton Miller <miltonm@....com> wrote:
>
> While checking unregister_filesystem for saftey vs extra calls for
> "ext4: register ext2 and ext3 alias after ext4" I realized that
> the synchronize_rcu() was called on the error path but not on
> the success path.

Good catch.

I think this is the bug that then caused us to do commit d863b50ab013
("vfs: call rcu_barrier after ->kill_sb()")

That said, that commit says that "synchronize_rcu()" isn't enough, and
uses rcu_barrier().

Which _should_ mean that there are no actual users that care about RCU
events by the time you actually hit "unregister_filesystem()".

So I think your patch is correct, but won't actually matter. But maybe
I'm missing something.

> Should we call it in both?

No, I think the success path is the one that would matter.

Comments?

                              Linus
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