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

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Date:   Thu, 26 Oct 2023 11:47:18 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Abhinav Singh <singhabhinav9051571833@...il.com>
Cc:     Mateusz Guzik <mjguzik@...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 09:07:46PM +0530, Abhinav Singh wrote:
> On 10/26/23 20:47, Michael S. Tsirkin wrote:
> > 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.
> Sparse reported 3 similar dereferencing warning this patch contains 2 fixes
> for 2, but yeah I should fixed all 3 of them.
> > > 
> > > I think it would help if the submitter had shown warnings they see.
> The warning message :- warning: dereference of noderef expression
> > 
> > Yes, and this must be tested under lockdep, which I think would
> > spit out warnings for this patch.
> Not sure, but I tested this with sparse (make C=2) and after the above
> changes I dont get the warning.

sparse is a static analysis tool. You should also actually
test your patch.

> > 
> > What should be used here I'm not sure. IIUC rcu_dereference_protected(p, 1)
> > is discouraged now?
> > 
> Not sure but I read that, rcu_dereference should be prefered when reading
> and rcu_dereference_protected should when writing.