Messages in this thread | | | Date | Tue, 14 Jan 2020 20:49:28 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH -tip V2 0/2] kprobes: Fix RCU warning and cleanup |
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On Mon, 13 Jan 2020 11:23:31 -0800 "Paul E. McKenney" <paulmck@kernel.org> wrote:
> On Mon, Jan 13, 2020 at 10:09:53PM +0900, Masami Hiramatsu wrote: > > Hi Joel, > > > > On Mon, 13 Jan 2020 12:16:40 +0900 > > Masami Hiramatsu <mhiramat@kernel.org> wrote: > > > > > > > > > > > > Hi Masami, > > > > > > > > > > > > I believe I had commented before that I don't agree with this patch: > > > > > > https://lore.kernel.org/lkml/157535318870.16485.6366477974356032624.stgit@devnote2/ > > > > > > > > > > > > The rationale you used is to replace RCU-api with non-RCU api just to avoid > > > > > > warnings. I think a better approach is to use RCU api and pass the optional > > > > > > expression to silence the false-positive warnings by informing the RCU API > > > > > > about the fact that locks are held (similar to what we do for > > > > > > rcu_dereference_protected()). The RCU API will do additional checking > > > > > > (such as making sure preemption is disabled for safe RCU usage etc) as well. > > > > > > > > > > Yes, that is what I did in [1/2] for get_kprobe(). > > > > > Let me clarify the RCU list usage in [2/2]. > > > > > > > > > > With the careful check, other list traversals never be done in non-sleepable > > > > > context, those are always runs with kprobe_mutex held. > > > > > If I correctly understand the Documentation/RCU/listRCU.rst, we should/can use > > > > > non-RCU api for those cases, or do I miss something? > > > > > > > > Yes, that is fine. However personally I prefer not to mix usage of > > > > list_for_each_entry_rcu() and list_for_each_entry() on the same pointer > > > > (kprobe_table). I think it is more confusing and error prone. Just use > > > > list_for_each_entry_rcu() everywhere and pass the appropriate lockdep > > > > expression, instead of calling lockdep_assert_held() independently. Is this > > > > not doable? > > > > > > Hmm, but isn't it more confusing that user just take a mutex but > > > no rcu_read_lock() with list_for_each_entry_rcu()? In that case, > > > sometimes it might sleep inside list_for_each_entry_rcu(), I thought > > > that might be more confusing mind model for users... > > The correct answer will be different in different situations. > For example, code that might be called either with the mutex held or > within an RCU read-side critical section will definitely need the _rcu() > and the lockdep_is_held(). Code that looks OK to call from within > RCU readers, but must not be (e.g., because it sleeps), will just as > definitely need to avoid _rcu().
I see. So the patch [2/2] is just removing useless rcu_read_lock() and use non RCU api for kprobe_table, because those code never be called from rcu read-side critical section. (It makes a critical section only for using RCU list operation)
> (If the lack of _rcu() proves confusing, > maybe list_for_each_entry() needs to grow an optional lockdep expression?)
That is OK for me, anyway the [2/2] also introduces some lockdep_assert_held() instead of rcu_read_lock() so that lockdep can check sanity.
> > I am therefore personally OK with either approach, though in confusing > cases a comment might help. > > > I meant, do we always need to do something like below? > > > > { > > mutex_lock(&lock); > > list_for_each_entry_rcu(list, ..., lockdep_is_held(&lock)) { > > ... > > } > > mutex_unlock(&lock); > > } > > > > BTW, I found another problem on this policy, since we don't have > > list_for_each_*_safe() equivalents for RCU, we can not do a safe > > loop on it. Should we call a find function for each time? > > Good point. > > RCU readers don't need _safe() because RCU grace periods provide this > for free within RCU read-side critical sections. > > So agreed, if you need _safe() on the update side, you would need to > call list_for_each_entry_safe(). If this proves confusing due to RCU > readers, maybe it should grow a lockdep expression? In the meantime, > lockdep_assert_held() could be used if needed to let people know that > this should not be used in an RCU reader.
I think lockdep_assert_held() is enough.
> > Does that work, or am I missing part of the problem? > > Thanx, Paul
Thank you,
-- Masami Hiramatsu <mhiramat@kernel.org>
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