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SubjectRe: [PATCH kernel] srcu: Fix static initialization
On Fri, Sep 11, 2020 at 06:52:08AM -0700, Paul E. McKenney wrote:
> On Fri, Sep 11, 2020 at 03:09:41PM +1000, Alexey Kardashevskiy wrote:
> > On 11/09/2020 04:53, Paul E. McKenney wrote:
> > > On Wed, Sep 09, 2020 at 10:31:03PM +1000, Alexey Kardashevskiy wrote:

[ . . . ]

> > >> init_srcu_struct_nodes() assumes ssp->sda!=NULL but alloc_percpu() fails
> > >> here:
> > >>
> > >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/percpu.c#n1734
> > >> ===
> > >> } else if (mutex_lock_killable(&pcpu_alloc_mutex)) {
> > >> pcpu_memcg_post_alloc_hook(objcg, NULL, 0, size);
> > >> return NULL;
> > >> ===
> > >>
> > >> I am still up to reading that osr-rcuusage.pdf to provide better
> > >> analysis :) Thanks,
> > >
> > > Ah, got it! Does the following patch help?
> > >
> > > There will likely also need to be changes to cleanup_srcu_struct(),
> > > but first let's see if I understand the problem. ;-)
> > >
> > > Thanx, Paul
> > >
> > > ------------------------------------------------------------------------
> > >
> > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > index c13348e..6f7880a 100644
> > > --- a/kernel/rcu/srcutree.c
> > > +++ b/kernel/rcu/srcutree.c
> > > @@ -177,11 +177,13 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
> > > INIT_DELAYED_WORK(&ssp->work, process_srcu);
> > > if (!is_static)
> > > ssp->sda = alloc_percpu(struct srcu_data);
> > > + if (!ssp->sda)
> > > + return -ENOMEM;
> > > init_srcu_struct_nodes(ssp, is_static);
> > > ssp->srcu_gp_seq_needed_exp = 0;
> > > ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
> > > smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
> >
> > The line above confuses me a bit. What you propose returns without
> > smp_store_release() called which should not matter I suppose.
>
> The idea is that if init_srcu_struct() returns -ENOMEM, the structure
> has not been initialized and had better not be used. If the calling code
> cannot handle that outcome, then the calling code needs to do something
> to insulate init_srcu_struct() from signals. One thing that it could
> do would be to invoke init_srcu_struct() from a workqueue handler and
> wait for this handler to complete.
>
> Please keep in mind that there is nothing init_srcu_struct() can do
> about this: The srcu_struct is useless unless alloc_percpu() succeeds.
>
> And yes, I do need to update the header comments to make this clear.
>
> > Otherwise it should work, although I cannot verify right now as my box
> > went down and since it is across Pacific - it may take time to power
> > cycle it :) Thanks,
>
> I know that feeling! And here is hoping that the box is out of reach
> of the local hot spots. ;-)

Just following up... Did that patch help?

Thanx, Paul

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