Re: [RFCv2][PATCH 2/7] fs: use RCU for free_super() vs. __sb_start_write()

[Jan Kara]

On Wed 24-06-15 17:16:05, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> 
> 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.

I believe yes. Because all accesses (be it reads or writes) must finish
before the current RCU period finishes. And synchronize_rcu() must make
sure that any code (loads / stores) after it execute only after the RCU
period has finished...

The patch looks good to me. You can add:
Reviewed-by: Jan Kara <jack@xxxxxxxx>

								Honza

> 
> This patch increases the number of writes/second that I can do
> by 5.6%.
> 
> Does anybody see any holes with this?
> 
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> ---
> 
>  b/fs/super.c |   63 ++++++++++++++++++++++++++++++++++++-----------------------
>  1 file changed, 39 insertions(+), 24 deletions(-)
> 
> diff -puN fs/super.c~rcu-__sb_start_write fs/super.c
> --- a/fs/super.c~rcu-__sb_start_write	2015-06-24 17:14:34.939125713 -0700
> +++ b/fs/super.c	2015-06-24 17:14:34.942125847 -0700
> @@ -1190,27 +1190,21 @@ static void acquire_freeze_lock(struct s
>   */
>  int __sb_start_write(struct super_block *sb, int level, bool wait)
>  {
> -retry:
> -	if (unlikely(sb->s_writers.frozen >= level)) {
> +	rcu_read_lock();
> +	while (unlikely(sb->s_writers.frozen >= level)) {
> +		rcu_read_unlock();
>  		if (!wait)
>  			return 0;
>  		wait_event(sb->s_writers.wait_unfrozen,
>  			   sb->s_writers.frozen < level);
> +		rcu_read_lock();
>  	}
>  
>  #ifdef CONFIG_LOCKDEP
>  	acquire_freeze_lock(sb, level, !wait, _RET_IP_);
>  #endif
>  	percpu_counter_inc(&sb->s_writers.counter[level-1]);
> -	/*
> -	 * Make sure counter is updated before we check for frozen.
> -	 * freeze_super() first sets frozen and then checks the counter.
> -	 */
> -	smp_mb();
> -	if (unlikely(sb->s_writers.frozen >= level)) {
> -		__sb_end_write(sb, level);
> -		goto retry;
> -	}
> +	rcu_read_unlock();
>  	return 1;
>  }
>  EXPORT_SYMBOL(__sb_start_write);
> @@ -1254,6 +1248,29 @@ static void sb_wait_write(struct super_b
>  	} while (writers);
>  }
>  
> +static void __thaw_super(struct super_block *sb)
> +{
> +	sb->s_writers.frozen = SB_UNFROZEN;
> +	/*
> +	 * RCU protects us against races where we are taking
> +	 * s_writers.frozen in to a less permissive state.  When
> +	 * that happens, __sb_start_write() might not yet have
> +	 * seen our write and might still increment
> +	 * s_writers.counter.
> +	 *
> +	 * Here, however, we are transitioning to a _more_
> +	 * permissive state.  The filesystem is frozen and no
> +	 * writes to s_writers.counter are being permitted.
> +	 *
> +	 * A smp_wmb() is sufficient here because we just need
> +	 * to ensure that new calls __sb_start_write() are
> +	 * allowed, not that _concurrent_ calls have finished.
> +	 */
> +	smp_wmb();
> +	wake_up(&sb->s_writers.wait_unfrozen);
> +	deactivate_locked_super(sb);
> +}
> +
>  /**
>   * freeze_super - lock the filesystem and force it into a consistent state
>   * @sb: the super to lock
> @@ -1312,7 +1329,13 @@ int freeze_super(struct super_block *sb)
>  
>  	/* From now on, no new normal writers can start */
>  	sb->s_writers.frozen = SB_FREEZE_WRITE;
> -	smp_wmb();
> +	/*
> +	 * After we synchronize_rcu(), we have ensured that everyone
> +	 * who reads sb->s_writers.frozen under rcu_read_lock() can
> +	 * now see our update.  This pretty much means that
> +	 * __sb_start_write() will not allow any new writers.
> +	 */
> +	synchronize_rcu();
>  
>  	/* Release s_umount to preserve sb_start_write -> s_umount ordering */
>  	up_write(&sb->s_umount);
> @@ -1322,7 +1345,7 @@ int freeze_super(struct super_block *sb)
>  	/* Now we go and block page faults... */
>  	down_write(&sb->s_umount);
>  	sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
> -	smp_wmb();
> +	synchronize_rcu();
>  
>  	sb_wait_write(sb, SB_FREEZE_PAGEFAULT);
>  
> @@ -1331,7 +1354,7 @@ int freeze_super(struct super_block *sb)
>  
>  	/* Now wait for internal filesystem counter */
>  	sb->s_writers.frozen = SB_FREEZE_FS;
> -	smp_wmb();
> +	synchronize_rcu();
>  	sb_wait_write(sb, SB_FREEZE_FS);
>  
>  	if (sb->s_op->freeze_fs) {
> @@ -1339,11 +1362,7 @@ int freeze_super(struct super_block *sb)
>  		if (ret) {
>  			printk(KERN_ERR
>  				"VFS:Filesystem freeze failed\n");
> -			sb->s_writers.frozen = SB_UNFROZEN;
> -			smp_wmb();
> -			wake_up(&sb->s_writers.wait_unfrozen);
> -			deactivate_locked_super(sb);
> -			return ret;
> +			__thaw_super(sb);
>  		}
>  	}
>  	/*
> @@ -1386,11 +1405,7 @@ int thaw_super(struct super_block *sb)
>  	}
>  
>  out:
> -	sb->s_writers.frozen = SB_UNFROZEN;
> -	smp_wmb();
> -	wake_up(&sb->s_writers.wait_unfrozen);
> -	deactivate_locked_super(sb);
> -
> +	__thaw_super(sb);
>  	return 0;
>  }
>  EXPORT_SYMBOL(thaw_super);
> _
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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