On Mon, Jun 26, 2023 at 8:42 AM David Vernet <void@xxxxxxxxxxxxx> wrote:
>
> On Fri, Jun 23, 2023 at 08:13:33PM -0700, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@xxxxxxxxxx>
> >
> > Convert bpf_cpumask to bpf_mem_cache_free_rcu.
> >
> > Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
>
> Acked-by: David Vernet <void@xxxxxxxxxxxxx>
>
> LGTM, thanks for cleaning this up. I left one drive-by comment /
> observation below, but it's not a blocker for this patch / series.
>
> > ---
> > kernel/bpf/cpumask.c | 20 ++++++--------------
> > 1 file changed, 6 insertions(+), 14 deletions(-)
> >
> > diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
> > index 938a60ff4295..6983af8e093c 100644
> > --- a/kernel/bpf/cpumask.c
> > +++ b/kernel/bpf/cpumask.c
> > @@ -9,7 +9,6 @@
> > /**
> > * struct bpf_cpumask - refcounted BPF cpumask wrapper structure
> > * @cpumask: The actual cpumask embedded in the struct.
> > - * @rcu: The RCU head used to free the cpumask with RCU safety.
> > * @usage: Object reference counter. When the refcount goes to 0, the
> > * memory is released back to the BPF allocator, which provides
> > * RCU safety.
> > @@ -25,7 +24,6 @@
> > */
> > struct bpf_cpumask {
> > cpumask_t cpumask;
> > - struct rcu_head rcu;
> > refcount_t usage;
> > };
> >
> > @@ -82,16 +80,6 @@ __bpf_kfunc struct bpf_cpumask *bpf_cpumask_acquire(struct bpf_cpumask *cpumask)
> > return cpumask;
> > }
> >
> > -static void cpumask_free_cb(struct rcu_head *head)
> > -{
> > - struct bpf_cpumask *cpumask;
> > -
> > - cpumask = container_of(head, struct bpf_cpumask, rcu);
> > - migrate_disable();
> > - bpf_mem_cache_free(&bpf_cpumask_ma, cpumask);
> > - migrate_enable();
> > -}
> > -
> > /**
> > * bpf_cpumask_release() - Release a previously acquired BPF cpumask.
> > * @cpumask: The cpumask being released.
> > @@ -102,8 +90,12 @@ static void cpumask_free_cb(struct rcu_head *head)
> > */
> > __bpf_kfunc void bpf_cpumask_release(struct bpf_cpumask *cpumask)
> > {
> > - if (refcount_dec_and_test(&cpumask->usage))
> > - call_rcu(&cpumask->rcu, cpumask_free_cb);
> > + if (!refcount_dec_and_test(&cpumask->usage))
> > + return;
> > +
> > + migrate_disable();
> > + bpf_mem_cache_free_rcu(&bpf_cpumask_ma, cpumask);
> > + migrate_enable();
>
> The fact that callers have to disable migration like this in order to
> safely free the memory feels a bit leaky. Is there any reason we can't
> move this into bpf_mem_{cache_}free_rcu()?
migrate_disable/enable() are actually not necessary here.
We can call bpf_mem_cache_free_rcu() directly from any kfunc.
Explicit migrate_disable() is only necessary from syscall.
I believe rcu callbacks also cannot migrate, so the existing
code probably doesn't need them either.
