On Wed, Dec 07, 2022 at 10:24:55AM +0800, Hou Tao wrote:
> Forget to cc Paul and RCU maillist for more comments.
>
> On 12/6/2022 12:29 PM, Hou Tao wrote:
> > From: Hou Tao <houtao1@xxxxxxxxxx>
> >
> > If there are pending rcu callback, free_mem_alloc() will use
> > rcu_barrier_tasks_trace() and rcu_barrier() to wait for the pending
> > __free_rcu_tasks_trace() and __free_rcu() callback.
> >
> > If rcu_trace_implies_rcu_gp() is true, there will be no pending
> > __free_rcu(), so it will be OK to skip rcu_barrier() as well.
The bit about there being no pending __free_rcu() is guaranteed by
your algorithm, correct? As in you have something like this somewhere
else in the code?
if (!rcu_trace_implies_rcu_gp())
call_rcu(...);
Or am I missing something more subtle?
Thanx, Paul
> > Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
> > ---
> > kernel/bpf/memalloc.c | 10 +++++++++-
> > 1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> > index 7daf147bc8f6..d43991fafc4f 100644
> > --- a/kernel/bpf/memalloc.c
> > +++ b/kernel/bpf/memalloc.c
> > @@ -464,9 +464,17 @@ static void free_mem_alloc(struct bpf_mem_alloc *ma)
> > {
> > /* waiting_for_gp lists was drained, but __free_rcu might
> > * still execute. Wait for it now before we freeing percpu caches.
> > + *
> > + * rcu_barrier_tasks_trace() doesn't imply synchronize_rcu_tasks_trace(),
> > + * but rcu_barrier_tasks_trace() and rcu_barrier() below are only used
> > + * to wait for the pending __free_rcu_tasks_trace() and __free_rcu(),
> > + * so if call_rcu(head, __free_rcu) is skipped due to
> > + * rcu_trace_implies_rcu_gp(), it will be OK to skip rcu_barrier() by
> > + * using rcu_trace_implies_rcu_gp() as well.
> > */
> > rcu_barrier_tasks_trace();
> > - rcu_barrier();
> > + if (!rcu_trace_implies_rcu_gp())
> > + rcu_barrier();
> > free_mem_alloc_no_barrier(ma);
> > }
> >
>
