linux-kernel - Re: [PATCH 1/3] zpool: add zpool_has

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Message-Id: <20150805130836.16c42cd0a9fe6f4050cf0620@linux-foundation.org>
Date:	Wed, 5 Aug 2015 13:08:36 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dan Streetman <ddstreet@...e.org>
Cc:	Seth Jennings <sjennings@...iantweb.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] zpool: add zpool_has_pool()

On Wed,  5 Aug 2015 09:46:41 -0400 Dan Streetman <ddstreet@...e.org> wrote:

> Add zpool_has_pool() function, indicating if the specified type of zpool
> is available (i.e. zsmalloc or zbud).  This allows checking if a pool is
> available, without actually trying to allocate it, similar to
> crypto_has_alg().
> 
> This is used by a following patch to zswap that enables the dynamic
> runtime creation of zswap zpools.
> 
> ...
>
>  /**
> + * zpool_has_pool() - Check if the pool driver is available
> + * @type	The type of the zpool to check (e.g. zbud, zsmalloc)
> + *
> + * This checks if the @type pool driver is available.
> + *
> + * Returns: true if @type pool is available, false if not
> + */
> +bool zpool_has_pool(char *type)
> +{
> +	struct zpool_driver *driver = zpool_get_driver(type);
> +
> +	if (!driver) {
> +		request_module("zpool-%s", type);
> +		driver = zpool_get_driver(type);
> +	}
> +
> +	if (!driver)
> +		return false;
> +
> +	zpool_put_driver(driver);
> +	return true;
> +}

This looks racy: after that zpool_put_driver() has completed, an rmmod
will invalidate zpool_has_pool()'s return value.

If there's some reason why this can't happen, can we please have a code
comment which reveals that reason?

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