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From:   Mateusz Guzik <mjguzik@gmail.com>
Date:   Thu, 26 Oct 2023 16:06:24 +0200
Message-ID: <CAGudoHFXH_FDgKRaJvVgQ3W8wD2TC=8yhiNm1NECApnQ-CNAZQ@mail.gmail.com>
Subject: Re: [PATCH v2] Fixing directly deferencing a __rcu pointer warning
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Abhinav Singh <singhabhinav9051571833@gmail.com>,
        akpm@linux-foundation.org, brauner@kernel.org, surenb@google.com,
        michael.christie@oracle.com, mathieu.desnoyers@efficios.com,
        npiggin@gmail.com, shakeelb@google.com, peterz@infradead.org,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org
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On 10/26/23, Michael S. Tsirkin <mst@redhat.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@gmail.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.

-- 
Mateusz Guzik <mjguzik gmail.com>