lkml.org 
[lkml]   [2019]   [Dec]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe
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.

thanks,

- Joel

\
 
 \ /
  Last update: 2019-12-06 02:12    [W:0.065 / U:0.544 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site