Messages in this thread | | | Date | Thu, 26 Oct 2023 11:47:18 -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 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@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. > 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.
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