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

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Date:	Thu, 14 Apr 2011 18:59:05 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Milton Miller <miltonm@....com>, 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

Il 14/04/2011 17:52, Linus Torvalds ha scritto:
> 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?
>

If I well remember the rcu_barrier was needed for the fs module 
unloading problem. In that case synchronize_rcu() wasn't enough. That 
said, I agree with you, it won't have any impact.

Marco
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