On Mon, Apr 21, 2014 at 05:37:01PM -0400, Pranith Kumar wrote:
> Hi Paul,
>
> I was trying to see the various uses of rcu_dereference_ functions to
> understand how they were being used. A doubt cropped up while doing this:
>
> * rcu_dereference_raw(): the documentation explicitly mentions that this
> should be minimally used as this does no checking of read critical sections
> and does not implement barriers. But looking at the code, there are various
> places where this is being used and it is being used I think in a buggy
> way. For example, in drivers/net/wireless/iwlwifi/dvm/main.c, there is this:
>
> kfree(rcu_dereference_raw(priv->noa_data));
>
> I can't imagine a scenario in which this is valid. So my question is this:
> do most of the uses of rcu_dereference_raw() need to be changed to use other
> dereference functions or are there cases where its usage is valid?
Well, the first call to it from iwl_op_mode_dvm_start() is valid because
that field has never been exposed to readers, so that no other task or
CPU has access to this field.
The second call from iwl_op_mode_dvm_stop() -might- be valid. For it
to be valid, there must be a grace period between the time that the
field was made inaccessible to readers and the time that iwl_uninit_drv()
was called. Usually something like synchronize_rcu() waits for the
needed grace period.
Please let me know what you find!
Thanx, Paul
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