From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Hansen <dave@sr71.net>
Subject: Re: [RFC][PATCH 1/2] fs: use RCU for free_super() vs. __sb_start_write()
Date: Wed, 24 Jun 2015 13:21:26 -0700
Message-ID: <558B1146.5050106@sr71.net>
References: <20150619223223.10B658AD@viggo.jf.intel.com> <20150623110933.GE2427@quack.suse.cz>
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Cc: viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com,
	tim.c.chen@linux.intel.com, ak@linux.intel.com
To: Jan Kara <jack@suse.cz>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20150623110933.GE2427@quack.suse.cz>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

On 06/23/2015 04:09 AM, Jan Kara wrote:
>> @@ -1340,7 +1344,7 @@ int freeze_super(struct super_block *sb)
>>  			printk(KERN_ERR
>>  				"VFS:Filesystem freeze failed\n");
>>  			sb->s_writers.frozen = SB_UNFROZEN;
>> -			smp_wmb();
>> +			synchronize_rcu();
> 
> Do we really need synchronize_rcu() here? We just need to make sure write
> to sb->s_writers.frozen happens before we start waking processes...

I don't think it is necessary.  We only need to be concerned in practice
if someone could be inside a critical section when we are executing
this.  I *think* the only case that we have that really matters will be
taken care of by the _first_ synchronize_rcu().

It's definitely worth adding a comment.