Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value

[Linus Torvalds]

On Mon, Sep 2, 2019 at 10:04 AM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>
> I like using the storage we will later use for the rcu_head.
>
> Is the intention your new variable xxx start as 0, and the only
> on the second write it becomes 1 and we take action?
>
> That should work but it is a funny way to encode a decrement.  I think
> it would be more straight forward to use refcount_dec_and_test.
>
> So something like this:

I like how this patch looks. It makes more sense to me than some of
the ad-hoc cases, and I wonder if this might be a pattern in general.

We have a very different "some users don't need RCU" in the dentry
code, and recently in the credential handling code. So I wonder if
this is a larger pattern, but I think your patch looks good
independently on its own.

But this is all based on "that patch _feels_ conceptually right",
rather than any deep thinking or (God forbid) any actual testing.

                Linus