linux-kernel - Re: [PATCH 07/35] UBI: Fastmap: Fix races in ubi_wl_get

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Date:	Wed, 05 Nov 2014 17:51:47 +0200
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Richard Weinberger <richard@....at>
Cc:	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	tlinder@...eaurora.org
Subject: Re: [PATCH 07/35] UBI: Fastmap: Fix races in ubi_wl_get_peb()

On Wed, 2014-10-29 at 13:45 +0100, Richard Weinberger wrote:
> ubi_wl_get_peb() has two problems, it reads the pool
> size and usage counters without any protection.
> While reading one value would be perfectly fine it reads multiple
> values and compares them. This is racy and can lead to incorrect
> pool handling.
> Furthermore ubi_update_fastmap() is called without wl_lock held,
> before incrementing the used counter it needs to be checked again.
> It could happen that another thread consumed all PEBs from the
> pool and the counter goes beyond ->size.

So wl_lock protects the 'pool->*' variables? Could you please add this
information to ubi.h. Namely, in the huge comment above 'struct device'
we document each lock, and we list the variables the lock protects.

> -	if (!pool->size || !wl_pool->size || pool->used == pool->size ||
> -	    wl_pool->used == wl_pool->size)
> +again:
> +	spin_lock(&ubi->wl_lock);

Is it possible to add a little comment here which translates the
condition below into English?

> +	if (!pool->size || !wl_pool->size || pool->used >= pool->size ||
> +	    wl_pool->used >= wl_pool->size) {
> +		spin_unlock(&ubi->wl_lock);
>  		ubi_update_fastmap(ubi);
> -


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