Messages in this thread | | | Date | Thu, 5 Dec 2019 13:19:56 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe |
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Hi Ingo,
On Wed, 4 Dec 2019 08:12:39 -0800 "Paul E. McKenney" <paulmck@kernel.org> 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.
For this kprobes case, I can introduce get_kprobe_locked() which uses hlist_for_each_entry() instead of hlist_for_each_entry_rcu().
However, this sounds like a bit strange choice, because get_kprobe (RCU version) should be used on "hot" paths (because it is lock-free), and get_kprobe_locked() is used on slow paths. If hlist_for_each_entry() can be more efficient, we will keep unefficient API for hot paths, but use the efficient one for slow paths.
Thank you,
-- Masami Hiramatsu <mhiramat@kernel.org>
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