linux-kernel - Re: [RFC][PATCH 1/2] fs: use RCU for free_super() vs. __sb_start

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Date:	Wed, 24 Jun 2015 13:21:26 -0700
From:	Dave Hansen <dave@...1.net>
To:	Jan Kara <jack@...e.cz>
CC:	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	tim.c.chen@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [RFC][PATCH 1/2] fs: use RCU for free_super() vs. __sb_start_write()

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.
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