Re: [RFC][PATCH 1/2] fs: use RCU for free_super() vs. __sb_start_write()

[Jan Kara]

On Fri 19-06-15 15:32:23, Dave Hansen wrote:
> 
> Currently, __sb_start_write() and freeze_super() can race with
> each other.  __sb_start_write() uses a smp_mb() to ensure that
> freeze_super() can see its write to sb->s_writers.counter and
> that it can see freeze_super()'s update to sb->s_writers.frozen.
> This all seems to work fine.
> 
> But, this smp_mb() makes __sb_start_write() the single hottest
> function in the kernel if I sit in a loop and do tiny write()s to
> tmpfs over and over.  This is on a very small 2-core system, so
> it will only get worse on larger systems.
> 
> This _seems_ like an ideal case for RCU.  __sb_start_write() is
> the RCU read-side and is in a very fast, performance-sensitive
> path.  freeze_super() is the RCU writer and is in an extremely
> rare non-performance-sensitive path.
> 
> Instead of doing and smp_wmb() in __sb_start_write(), we do
> rcu_read_lock().  This ensures that a CPU doing freeze_super()
> can not proceed past its synchronize_rcu() until the grace
> period has ended and the 's_writers.frozen = SB_FREEZE_WRITE'
> is visible to __sb_start_write().
> 
> One question here: Does the work that __sb_start_write() does in
> a previous grace period becomes visible to freeze_super() after
> its call to synchronize_rcu()?  It _seems_ like it should, but it
> seems backwards to me since __sb_start_write() is the "reader" in
> this case.
> 
> This patch increases the number of writes/second that I can do
> by 10.4%.
> 
> Does anybody see any holes with this?

Nice speed up and looks good to me. Just one question below.

> @@ -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...

>  			wake_up(&sb->s_writers.wait_unfrozen);
>  			deactivate_locked_super(sb);
>  			return ret;
> @@ -1387,7 +1391,7 @@ int thaw_super(struct super_block *sb)
>  
>  out:
>  	sb->s_writers.frozen = SB_UNFROZEN;
> -	smp_wmb();
> +	synchronize_rcu();
>  	wake_up(&sb->s_writers.wait_unfrozen);

And here as well...

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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