Re: [PATCH v3] futex: Remove requirement for lock_page in

[Davidlohr Bueso]

On Tue, 19 Jan 2016, Bueso wrote:
> 	/*
> 	 * Private mappings are handled in a simple way.
> 	 *
> +	 * If the futex key is stored on an anonymous page, then the associated
> +	 * object is the mm which is implicitly pinned by the calling process.
> +	 *
> 	 * NOTE: When userspace waits on a MAP_SHARED mapping, even if
> 	 * it's a read-only handle, it's expected that futexes attach to
> 	 * the object not the particular process.
> @@ -566,16 +592,61 @@ again:
> 		key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
> 		key->private.mm = mm;
> 		key->private.address = address;
> +
> +		get_futex_key_refs(key); /* implies MB (B) */
> +
> 	} 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 = READ_ONCE(mapping->host);
> +
> +		/*
> +		 * 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_ONCE(!atomic_inc_not_zero(&inode->i_count))) {
> +			rcu_read_unlock();
> +			put_page(page);
> +
> +			goto again;
> +		}
> +
> +		/* Should be impossible but lets be paranoid for now */
> +		BUG_ON(inode->i_mapping != mapping);

Hmm, do we want to transform this into an if and do rcu unlock and then just
call BUG()? I't doesn't matter at this point _anyway_, but it would be the right
thing to do, no?

> +
> 		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();
> 	}
>
> -	get_futex_key_refs(key); /* implies MB (B) */
> -
> out:
> -	unlock_page(page);
> 	put_page(page);
> 	return err;
> }