Messages in this thread | | | Date | Thu, 26 Oct 2023 11:17:16 -0400 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH v2] Fixing directly deferencing a __rcu pointer warning |
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On Thu, Oct 26, 2023 at 04:06:24PM +0200, Mateusz Guzik wrote: > 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.
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
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