linux-kernel - Re: [PATCH v2] Fixing directly deferencing a _

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Date:   Thu, 26 Oct 2023 11:17:16 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Abhinav Singh <singhabhinav9051571833@...il.com>,
        akpm@...ux-foundation.org, brauner@...nel.org, surenb@...gle.com,
        michael.christie@...cle.com, mathieu.desnoyers@...icios.com,
        npiggin@...il.com, shakeelb@...gle.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v2] Fixing directly deferencing a __rcu pointer warning

On Thu, Oct 26, 2023 at 04:06:24PM +0200, Mateusz Guzik wrote:
> On 10/26/23, Michael S. Tsirkin <mst@...hat.com> wrote:
> > On Thu, Oct 26, 2023 at 05:46:21PM +0530, Abhinav Singh wrote:
> >> This patch fixes the warning about directly dereferencing a pointer
> >> tagged with __rcu annotation.
> >>
> >> Dereferencing the pointers tagged with __rcu directly should
> >> always be avoided according to the docs. There is a rcu helper
> >> functions rcu_dereference(...) to use when dereferencing a __rcu
> >> pointer. This functions returns the non __rcu tagged pointer which
> >> can be dereferenced just like a normal pointers.
> >>
> >> Signed-off-by: Abhinav Singh <singhabhinav9051571833@...il.com>
> >
> > Well yes but these need to be called under rcu_read_lock.
> > Who does it here?
> > If no one then maybe you found an actual bug and we need to
> > fix it not paper over it.
> >
> 
> There is no bug here.
> 
> p is the newly created thread, ->real_cred was initialized just prior
> to this code and there is nobody to whack the creds from under it.
> 
> Second bit in the patch changes one real_parent deref, but leaves 2
> others just above it. Once more no bug since the entire thing happens
> under tasklist_lock, but the patch should either sort all these cases
> or none.
> 
> I think it would help if the submitter had shown warnings they see.

Yes, and this must be tested under lockdep, which I think would
spit out warnings for this patch.

What should be used here I'm not sure. IIUC rcu_dereference_protected(p, 1)
is discouraged now?

-- 
MST