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SubjectRe: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe
On Wed, Dec 04, 2019 at 11:05:50AM +0100, Ingo Molnar wrote:
>
> * Paul E. McKenney <paulmck@kernel.org> wrote:
>
> > > * This list-traversal primitive may safely run concurrently with
> > > * the _rcu list-mutation primitives such as hlist_add_head_rcu()
> > > * as long as the traversal is guarded by rcu_read_lock().
> > > */
> > > #define hlist_for_each_entry_rcu(pos, head, member, cond...) \
> > >
> > > is actively harmful. Why is it there?
> >
> > For cases where common code might be invoked both from the reader
> > (with RCU protection) and from the updater (protected by some
> > lock). This common code can then use the optional argument to
> > hlist_for_each_entry_rcu() to truthfully tell lockdep that it might be
> > called with either form of protection in place.
> >
> > This also combines with the __rcu tag used to mark RCU-protected
> > pointers, in which case sparse complains when a non-RCU API is applied
> > to these pointers, to get back to your earlier question about use of
> > hlist_for_each_entry_rcu() within the update-side lock.
> >
> > But what are you seeing as actively harmful about all of this?
> > What should we be doing instead?
>
> Yeah, so basically in the write-locked path hlist_for_each_entry()
> generates (slightly) more efficient code than hlist_for_each_entry_rcu(),
> correct?

Potentially yes, if the READ_ONCE() constrains the compiler. Or not,
depending of course on the compiler and the surrounding code.

> Also, the principle of passing warning flags around is problematic - but
> I can see the point in this specific case.

Would it help to add an hlist_for_each_entry_protected() that expected
RCU-protected pointers and write-side protection, analogous to
rcu_dereference_protected()? Or would that expansion of the RCU API
outweigh any benefits?

Thanx, Paul

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