Messages in this thread | | | Date | Sat, 7 Dec 2019 19:08:42 -0500 | From | Joel Fernandes <> | Subject | Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe |
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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?
thanks,
- Joel
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