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

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Dec 17, 2019 at 11:59:21PM +0900, Masami Hiramatsu wrote:
> Hi,
> 
> On Sun, 8 Dec 2019 19:39:11 -0800
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
> 
> > On Sat, Dec 07, 2019 at 07:08:42PM -0500, Joel Fernandes wrote:
> > > On Thu, Dec 05, 2019 at 07:11:51PM -0800, Paul E. McKenney wrote:
> > > > On Thu, Dec 05, 2019 at 08:11:37PM -0500, Joel Fernandes wrote:
> > > > > On Wed, Dec 04, 2019 at 08:12:39AM -0800, Paul E. McKenney wrote:
> > > > > > 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?
> > > > > 
> > > > > Personally, I like keeping the same API and using the optional argument like
> > > > > we did thus preventing too many APIs / new APIs.
> > > > 
> > > > Would you be willing to put together a prototype patch so that people
> > > > can see exactly how it would look?
> > > 
> > > Hi Paul,
> > > 
> > > I was referring to the same API we have at the moment (that is
> > > hlist_for_each_entry_rcu() with the additional cond parameter). I was saying
> > > let us keep that and not add a hlist_for_each_entry_protected() instead, so
> > > as to not proliferate the number of APIs.
> > > 
> > > Or did I miss the point?
> > 
> > This would work for me.  The only concern would be inefficiency, but we
> > have heard from people saying that the unnecessary inefficiency is only
> > on code paths that they do not care about, so we should be good.
> 
> So, what will be the conclusion here, Ingo?
> 
> I faced other warnings in tracing subsystem, so I need to add more
> lockdep_is_held()s there to suppress warnings.

Please don't add a new:
hlist_for_each_entry_rcu_protected(..., lockdep_is_held(...))

Instead use the existing one in mainline:
 hlist_for_each_entry_rcu(..., lockdep_is_held(...)).

How many warnings are you facing? I think it is a good idea to add
lockdep_is_held() wherever needed so as to prevent false-positive warnings as
it would harden your code and prevent fireworks.

thanks,

 - Joel

> 
> Thank you,
> 
> -- 
> Masami Hiramatsu <mhiramat@kernel.org>