Re: [PATCH v2 06/11] perf/uprobe: SRCU-ify uprobe->consumer list

Andrii Nakryiko
2024-07-15T17:31:00Z

On Mon, Jul 15, 2024 at 4:25 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Jul 12, 2024 at 02:06:08PM -0700, Andrii Nakryiko wrote:
> > + bpf@vger, please cc bpf ML for the next revision, these changes are
> > very relevant there as well, thanks
> >
> > On Thu, Jul 11, 2024 at 4:07 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > With handle_swbp() hitting concurrently on (all) CPUs the
> > > uprobe->register_rwsem can get very contended. Add an SRCU instance to
> > > cover the consumer list and consumer lifetime.
> > >
> > > Since the consumer are externally embedded structures, unregister will
> > > have to suffer a synchronize_srcu().
> > >
> > > A notably complication is the UPROBE_HANDLER_REMOVE logic which can
> > > race against uprobe_register() such that it might want to remove a
> > > freshly installer handler that didn't get called. In order to close
> > > this hole, a seqcount is added. With that, the removal path can tell
> > > if anything changed and bail out of the removal.
> > >
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > ---
> > > kernel/events/uprobes.c | 60 ++++++++++++++++++++++++++++++++++++++++--------
> > > 1 file changed, 50 insertions(+), 10 deletions(-)
> > >
> >
> > [...]
> >
> > > @@ -800,7 +808,7 @@ static bool consumer_del(struct uprobe *
> > > down_write(&uprobe->consumer_rwsem);
> > > for (con = &uprobe->consumers; *con; con = &(*con)->next) {
> > > if (*con == uc) {
> > > - *con = uc->next;
> > > + WRITE_ONCE(*con, uc->next);
> >
> > I have a dumb and mechanical question.
> >
> > Above in consumer_add() you are doing WRITE_ONCE() for uc->next
> > assignment, but rcu_assign_pointer() for uprobe->consumers. Here, you
> > are doing WRITE_ONCE() for the same operation, if it so happens that
> > uc == *con == uprobe->consumers. So is rcu_assign_pointer() necessary
> > in consumer_addr()? If yes, we should have it here as well, no? And if
> > not, why bother with it in consumer_add()?
>
> add is a publish and needs to ensure all stores to the object are
> ordered before the object is linked in. Note that rcu_assign_pointer()
> is basically a fancy way of writing smp_store_release().
>
> del otoh does not publish, it removes and doesn't need ordering.
>
> It does however need to ensure the pointer write itself isn't torn. That
> is, without the WRITE_ONCE() the compiler is free to do byte stores in
> order to update the 8 byte pointer value (assuming 64bit). This is
> pretty dumb, but very much permitted by C and also utterly fatal in the
> case where an RCU iteration comes by and reads a half-half pointer
> value.
>

Thanks for elaborating, very helpful! It's basically the same idea as
with rb_find_add_rcu(), got it.

> > > ret = true;
> > > break;
> > > }
> > > @@ -1139,9 +1147,13 @@ void uprobe_unregister(struct inode *ino
> > > return;
> > >
> > > down_write(&uprobe->register_rwsem);
> > > + raw_write_seqcount_begin(&uprobe->register_seq);
> > > __uprobe_unregister(uprobe, uc);
> > > + raw_write_seqcount_end(&uprobe->register_seq);
> > > up_write(&uprobe->register_rwsem);
> > > put_uprobe(uprobe);
> > > +
> > > + synchronize_srcu(&uprobes_srcu);
> > > }
> > > EXPORT_SYMBOL_GPL(uprobe_unregister);
> >
> > …