Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe

[Paul E. McKenney]

On Wed, Dec 04, 2019 at 11:05:50AM +0100, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@xxxxxxxxxx> 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