Re: [PATCH -next v2] futex: Remove requirement for lock_page in get_futex_key

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 07, 2016 at 03:51:14PM -0800, Davidlohr Bueso wrote:
> +++ b/kernel/futex.c
> @@ -520,7 +520,20 @@ again:
>  	else
>  		err = 0;
>  
> -	lock_page(page);
> +	/*
> +	 * The treatment of mapping from this point on is critical. The page
> +	 * lock protects many things but in this context the page lock
> +	 * stabilises mapping, prevents inode freeing in the shared
> +	 * file-backed region case and guards against movement to swap cache.
> +	 *
> +	 * Strictly speaking the page lock is not needed in all cases being
> +	 * considered here and page lock forces unnecessarily serialisation.
> +	 * From this point on, mapping will be reverified if necessary and
> +	 * page lock will be acquired only if it is unavoiable.

Making up new words eh?

> +	 */
> +	page = compound_head(page);
> +	mapping = READ_ONCE(page->mapping);
> +
>  	/*
>  	 * If page->mapping is NULL, then it cannot be a PageAnon
>  	 * page; but it might be the ZERO_PAGE or in the gate area or


>  	} else {
> +		struct inode *inode;
> +
> +		/*
> +		 * The associtated futex object in this case is the inode and
> +		 * the page->mapping must be traversed. Ordinarily this should
> +		 * be stabilised under page lock but it's not strictly
> +		 * necessary in this case as we just want to pin the inode, not
> +		 * update radix tree or anything like that.
> +		 *
> +		 * The RCU read lock is taken as the inode is finally freed
> +		 * under RCU. If the mapping still matches expectations then the
> +		 * mapping->host can be safely accessed as being a valid inode.
> +		 */
> +		rcu_read_lock();
> +		if (READ_ONCE(page->mapping) != mapping ||
> +		    !mapping->host) {
> +			rcu_read_unlock();
> +			put_page(page);
> +
> +			goto again;
> +		}
> +		inode = mapping->host;

Don't you want a single load of mapping->host? Could it not have become
NULL just between these two loads?

> +
> +		/*
> +		 * Take a reference unless it is about to be freed. Previously
> +		 * this reference was taken by ihold under the page lock
> +		 * pinning the inode in place so i_lock was unnecessary. The
> +		 * only way for this check to fail is if the inode was
> +		 * truncated in parallel so warn for now if this happens.
> +		 *
> +		 * We are not calling into get_futex_key_refs() in file-backed
> +		 * cases, therefore a successful atomic_inc return below will
> +		 * guarantee that get_futex_key() will continue to imply MB (B).
> +		 */
> +		if (WARN_ON(!atomic_inc_not_zero(&inode->i_count))) {

ONCE?

> +			rcu_read_unlock();
> +			put_page(page);
> +
> +			goto again;
> +		}
> +
> +		/* Should be impossible but lets be paranoid for now */
> +		BUG_ON(inode->i_mapping != mapping);
> +
>  		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
> -		key->shared.inode = mapping->host;
> +		key->shared.inode = inode;
>  		key->shared.pgoff = basepage_index(page);
> +		rcu_read_unlock();
>  	}