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Date:   Thu, 21 Dec 2017 12:48:01 +0100
From:   Jan Kara <jack@...e.cz>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk, linux-nfs@...r.kernel.org, neilb@...e.de,
        jack@...e.de, linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, linux-xfs@...r.kernel.org,
        darrick.wong@...cle.com, linux-btrfs@...r.kernel.org, clm@...com,
        jbacik@...com, dsterba@...e.com, linux-integrity@...r.kernel.org,
        zohar@...ux.vnet.ibm.com, dmitry.kasatkin@...il.com,
        linux-afs@...ts.infradead.org, dhowells@...hat.com,
        jaltman@...istor.com
Subject: Re: [PATCH v3 19/19] fs: handle inode->i_version more efficiently

On Thu 21-12-17 06:25:55, Jeff Layton wrote:
> Got it, I think. How about this (sorry for the unrelated deltas here):
> 
> [PATCH] SQUASH: add memory barriers around i_version accesses

Yep, this looks good to me.

								Honza
> 
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> ---
>  include/linux/iversion.h | 60 +++++++++++++++++++++++++++++++-----------------
>  1 file changed, 39 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/iversion.h b/include/linux/iversion.h
> index a9fbf99709df..1b3b5ed7c5b8 100644
> --- a/include/linux/iversion.h
> +++ b/include/linux/iversion.h
> @@ -89,6 +89,23 @@ inode_set_iversion_raw(struct inode *inode, const u64 val)
>  	atomic64_set(&inode->i_version, val);
>  }
>  
> +/**
> + * inode_peek_iversion_raw - grab a "raw" iversion value
> + * @inode: inode from which i_version should be read
> + *
> + * Grab a "raw" inode->i_version value and return it. The i_version is not
> + * flagged or converted in any way. This is mostly used to access a self-managed
> + * i_version.
> + *
> + * With those filesystems, we want to treat the i_version as an entirely
> + * opaque value.
> + */
> +static inline u64
> +inode_peek_iversion_raw(const struct inode *inode)
> +{
> +	return atomic64_read(&inode->i_version);
> +}
> +
>  /**
>   * inode_set_iversion - set i_version to a particular value
>   * @inode: inode to set
> @@ -152,7 +169,18 @@ inode_maybe_inc_iversion(struct inode *inode, bool force)
>  {
>  	u64 cur, old, new;
>  
> -	cur = (u64)atomic64_read(&inode->i_version);
> +	/*
> +	 * The i_version field is not strictly ordered with any other inode
> +	 * information, but the legacy inode_inc_iversion code used a spinlock
> +	 * to serialize increments.
> +	 *
> +	 * Here, we add full memory barriers to ensure that any de-facto
> +	 * ordering with other info is preserved.
> +	 *
> +	 * This barrier pairs with the barrier in inode_query_iversion()
> +	 */
> +	smp_mb();
> +	cur = inode_peek_iversion_raw(inode);
>  	for (;;) {
>  		/* If flag is clear then we needn't do anything */
>  		if (!force && !(cur & I_VERSION_QUERIED))
> @@ -183,23 +211,6 @@ inode_inc_iversion(struct inode *inode)
>  	inode_maybe_inc_iversion(inode, true);
>  }
>  
> -/**
> - * inode_peek_iversion_raw - grab a "raw" iversion value
> - * @inode: inode from which i_version should be read
> - *
> - * Grab a "raw" inode->i_version value and return it. The i_version is not
> - * flagged or converted in any way. This is mostly used to access a self-managed
> - * i_version.
> - *
> - * With those filesystems, we want to treat the i_version as an entirely
> - * opaque value.
> - */
> -static inline u64
> -inode_peek_iversion_raw(const struct inode *inode)
> -{
> -	return atomic64_read(&inode->i_version);
> -}
> -
>  /**
>   * inode_iversion_need_inc - is the i_version in need of being incremented?
>   * @inode: inode to check
> @@ -248,15 +259,22 @@ inode_query_iversion(struct inode *inode)
>  {
>  	u64 cur, old, new;
>  
> -	cur = atomic64_read(&inode->i_version);
> +	cur = inode_peek_iversion_raw(inode);
>  	for (;;) {
>  		/* If flag is already set, then no need to swap */
> -		if (cur & I_VERSION_QUERIED)
> +		if (cur & I_VERSION_QUERIED) {
> +			/*
> +			 * This barrier (and the implicit barrier in the
> +			 * cmpxchg below) pairs with the barrier in
> +			 * inode_maybe_inc_iversion().
> +			 */
> +			smp_mb();
>  			break;
> +		}
>  
>  		new = cur | I_VERSION_QUERIED;
>  		old = atomic64_cmpxchg(&inode->i_version, cur, new);
> -		if (old == cur)
> +		if (likely(old == cur))
>  			break;
>  		cur = old;
>  	}
> -- 
> 2.14.3
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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