On Tue, Apr 09, 2019 at 12:45:25PM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 9, 2019, at 12:40 PM, paulmck paul...@linux.ibm.com wrote:
>
> > On Tue, Apr 09, 2019 at 11:56:03AM -0400, Mathieu Desnoyers wrote:
> >> ----- On Apr 9, 2019, at 11:40 AM, Joel Fernandes, Google
> >> j...@joelfernandes.org
> >> wrote:
> >>
> >> > On Mon, Apr 08, 2019 at 01:24:47PM -0400, Mathieu Desnoyers wrote:
> >> >> ----- On Apr 8, 2019, at 11:46 AM, paulmck paul...@linux.ibm.com wrote:
> >> >>
> >> >> > On Mon, Apr 08, 2019 at 10:49:32AM -0400, Mathieu Desnoyers wrote:
> >> >> >> ----- On Apr 8, 2019, at 10:22 AM, paulmck paul...@linux.ibm.com
> >> >> >> wrote:
> >> >> >>
> >> >> >> > On Mon, Apr 08, 2019 at 09:05:34AM -0400, Mathieu Desnoyers wrote:
> >> >> >> >> ----- On Apr 7, 2019, at 10:27 PM, paulmck paul...@linux.ibm.com
> >> >> >> >> wrote:
> >> >> >> >>
> >> >> >> >> > On Sun, Apr 07, 2019 at 09:07:18PM +0000, Joel Fernandes wrote:
> >> >> >> >> >> On Sun, Apr 07, 2019 at 04:41:36PM -0400, Mathieu Desnoyers
> >> >> >> >> >> wrote:
> >> >> >> >> >> >
> >> >> >> >> >> > ----- On Apr 7, 2019, at 3:32 PM, Joel Fernandes, Google
> >> >> >> >> >> > j...@joelfernandes.org
> >> >> >> >> >> > wrote:
> >> >> >> >> >> >
> >> >> >> >> >> > > On Sun, Apr 07, 2019 at 03:26:16PM -0400, Mathieu
> >> >> >> >> >> > > Desnoyers wrote:
> >> >> >> >> >> > >> ----- On Apr 7, 2019, at 9:59 AM, paulmck
> >> >> >> >> >> > >> paul...@linux.ibm.com wrote:
> >> >> >> >> >> > >>
> >> >> >> >> >> > >> > On Sun, Apr 07, 2019 at 06:39:41AM -0700, Paul E.
> >> >> >> >> >> > >> > McKenney wrote:
> >> >> >> >> >> > >> >> On Sat, Apr 06, 2019 at 07:06:13PM -0400, Joel
> >> >> >> >> >> > >> >> Fernandes wrote:
> >> >> >> >> >> > >> >
> >> >> >> >> >> > >> > [ . . . ]
> >> >> >> >> >> > >> >
> >> >> >> >> >> > >> >> > > diff --git a/include/asm-generic/vmlinux.lds.h
> >> >> >> >> >> > >> >> > > b/include/asm-generic/vmlinux.lds.h
> >> >> >> >> >> > >> >> > > index f8f6f04c4453..c2d919a1566e 100644
> >> >> >> >> >> > >> >> > > --- a/include/asm-generic/vmlinux.lds.h
> >> >> >> >> >> > >> >> > > +++ b/include/asm-generic/vmlinux.lds.h
> >> >> >> >> >> > >> >> > > @@ -338,6 +338,10 @@
> >> >> >> >> >> > >> >> > > KEEP(*(__tracepoints_ptrs)) /*
> >> >> >> >> >> > >> >> > > Tracepoints: pointer array */ \
> >> >> >> >> >> > >> >> > > __stop___tracepoints_ptrs = .;
> >> >> >> >> >> > >> >> > > \
> >> >> >> >> >> > >> >> > > *(__tracepoints_strings)/* Tracepoints:
> >> >> >> >> >> > >> >> > > strings */ \
> >> >> >> >> >> > >> >> > > + . = ALIGN(8);
> >> >> >> >> >> > >> >> > > \
> >> >> >> >> >> > >> >> > > + __start___srcu_struct = .;
> >> >> >> >> >> > >> >> > > \
> >> >> >> >> >> > >> >> > > + *(___srcu_struct_ptrs)
> >> >> >> >> >> > >> >> > > \
> >> >> >> >> >> > >> >> > > + __end___srcu_struct = .;
> >> >> >> >> >> > >> >> > > \
> >> >> >> >> >> > >> >> > > }
> >> >> >> >> >> > >> >> > > \
> >> >> >> >> >> > >> >> >
> >> >> >> >> >> > >> >> > This vmlinux linker modification is not needed. I
> >> >> >> >> >> > >> >> > tested without it and srcu
> >> >> >> >> >> > >> >> > torture works fine with rcutorture built as a
> >> >> >> >> >> > >> >> > module. Putting further prints
> >> >> >> >> >> > >> >> > in kernel/module.c verified that the kernel is able
> >> >> >> >> >> > >> >> > to find the srcu structs
> >> >> >> >> >> > >> >> > just fine. You could squash the below patch into
> >> >> >> >> >> > >> >> > this one or apply it on top
> >> >> >> >> >> > >> >> > of the dev branch.
> >> >> >> >> >> > >> >>
> >> >> >> >> >> > >> >> Good point, given that otherwise FORTRAN named common
> >> >> >> >> >> > >> >> blocks would not
> >> >> >> >> >> > >> >> work.
> >> >> >> >> >> > >> >>
> >> >> >> >> >> > >> >> But isn't one advantage of leaving that stuff in the
> >> >> >> >> >> > >> >> RO_DATA_SECTION()
> >> >> >> >> >> > >> >> macro that it can be mapped read-only? Or am I
> >> >> >> >> >> > >> >> suffering from excessive
> >> >> >> >> >> > >> >> optimism?
> >> >> >> >> >> > >> >
> >> >> >> >> >> > >> > And to answer the other question, in the case where I
> >> >> >> >> >> > >> > am suffering from
> >> >> >> >> >> > >> > excessive optimism, it should be a separate commit.
> >> >> >> >> >> > >> > Please see below
> >> >> >> >> >> > >> > for the updated original commit thus far.
> >> >> >> >> >> > >> >
> >> >> >> >> >> > >> > And may I have your Tested-by?
> >> >> >> >> >> > >>
> >> >> >> >> >> > >> Just to confirm: does the cleanup performed in the
> >> >> >> >> >> > >> modules going
> >> >> >> >> >> > >> notifier end up acting as a barrier first before freeing
> >> >> >> >> >> > >> the memory ?
> >> >> >> >> >> > >> If not, is it explicitly stated that a barrier must be
> >> >> >> >> >> > >> issued before
> >> >> >> >> >> > >> module unload ?
> >> >> >> >> >> > >>
> >> >> >> >> >> > >
> >> >> >> >> >> > > You mean rcu_barrier? It is mentioned in the documentation
> >> >> >> >> >> > > that this is the
> >> >> >> >> >> > > responsibility of the module writer to prevent delays for
> >> >> >> >> >> > > all modules.
> >> >> >> >> >> >
> >> >> >> >> >> > It's a srcu barrier yes. Considering it would be a barrier
> >> >> >> >> >> > specific to the
> >> >> >> >> >> > srcu domain within that module, I don't see how it would
> >> >> >> >> >> > cause delays for
> >> >> >> >> >> > "all" modules if we implicitly issue the barrier on module
> >> >> >> >> >> > unload. What
> >> >> >> >> >> > am I missing ?
> >> >> >> >> >>
> >> >> >> >> >> Yes you are right. I thought of this after I just sent my
> >> >> >> >> >> email. I think it
> >> >> >> >> >> makes sense for srcu case to do and could avoid a class of
> >> >> >> >> >> bugs.
> >> >> >> >> >
> >> >> >> >> > If there are call_srcu() callbacks outstanding, the module
> >> >> >> >> > writer still
> >> >> >> >> > needs the srcu_barrier() because otherwise callbacks arrive
> >> >> >> >> > after
> >> >> >> >> > the module text has gone, which will be disappoint the CPU when
> >> >> >> >> > it
> >> >> >> >> > tries fetching instructions that are no longer mapped. If
> >> >> >> >> > there are
> >> >> >> >> > no call_srcu() callbacks from that module, then there is no
> >> >> >> >> > need for
> >> >> >> >> > srcu_barrier() either way.
> >> >> >> >> >
> >> >> >> >> > So if an srcu_barrier() is needed, the module developer needs to
> >> >> >> >> > supply it.
> >> >> >> >>
> >> >> >> >> When you say "callbacks arrive after the module text has gone",
> >> >> >> >> I think you assume that free_module() is invoked before the
> >> >> >> >> MODULE_STATE_GOING notifiers are called. But it's done in the
> >> >> >> >> opposite order: going notifiers are called first, and then
> >> >> >> >> free_module() is invoked.
> >> >> >> >>
> >> >> >> >> So AFAIU it would be safe to issue the srcu_barrier() from the
> >> >> >> >> module
> >> >> >> >> going notifier.
> >> >> >> >>
> >> >> >> >> Or am I missing something ?
> >> >> >> >
> >> >> >> > We do seem to be talking past each other. ;-)
> >> >> >> >
> >> >> >> > This has nothing to do with the order of events at module-unload
> >> >> >> > time.
> >> >> >> >
> >> >> >> > So please let me try again.
> >> >> >> >
> >> >> >> > If a given srcu_struct in a module never has call_srcu() invoked,
> >> >> >> > there
> >> >> >> > is no need to invoke rcu_barrier() at any time, whether at
> >> >> >> > module-unload
> >> >> >> > time or not. Adding rcu_barrier() in this case adds overhead and
> >> >> >> > latency
> >> >> >> > for no good reason.
> >> >> >>
> >> >> >> Not if we invoke srcu_barrier() for that specific domain. If
> >> >> >> call_srcu was never invoked for a srcu domain, I don't see why
> >> >> >> srcu_barrier() should be more expensive than a simple check that
> >> >> >> the domain does not have any srcu work queued.
> >> >> >
> >> >> > But that simple check does involve a cache miss for each possible CPU
> >> >> > (not
> >> >> > just each online CPU), so it is non-trivial, especially on large
> >> >> > systems.
> >> >> >
> >> >> >> > If a given srcu_struct in a module does have at least one
> >> >> >> > call_srcu()
> >> >> >> > invoked, it is already that module's responsibility to make sure
> >> >> >> > that
> >> >> >> > the code sticks around long enough for the callback to be invoked.
> >> >> >>
> >> >> >> I understand that when users do explicit dynamic allocation/cleanup
> >> >> >> of
> >> >> >> srcu domains, they indeed need to take care of doing explicit
> >> >> >> srcu_barrier().
> >> >> >> However, if they do static definition of srcu domains, it would be
> >> >> >> nice
> >> >> >> if we can handle the barriers under the hood.
> >> >> >
> >> >> > All else being equal, of course. But...
> >> >> >
> >> >> >> > This means that correct SRCU users that invoke call_srcu() already
> >> >> >> > have srcu_barrier() at module-unload time. Incorrect SRCU users,
> >> >> >> > with
> >> >> >> > reasonable probability, now get a WARN_ON() at module-unload time,
> >> >> >> > with
> >> >> >> > the per-CPU state getting leaked. Before this change, they would
> >> >> >> > (also
> >> >> >> > with reasonable probability) instead get an instruction-fetch
> >> >> >> > fault when
> >> >> >> > the SRCU callback was invoked after the completion of the module
> >> >> >> > unload.
> >> >> >> > Furthermore, in all cases where they would previously have gotten
> >> >> >> > the
> >> >> >> > instruction-fetch fault, they now get the WARN_ON(), like this:
> >> >> >> >
> >> >> >> > if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
> >> >> >> > return; /* Forgot srcu_barrier(), so just leak it! */
> >> >> >> >
> >> >> >> > So this change already represents an improvement in usability.
> >> >> >>
> >> >> >> Considering that we can do a srcu_barrier() for the specific domain,
> >> >> >> and that it should add no noticeable overhead if there is no queued
> >> >> >> callbacks, I don't see a good reason for leaving the srcu_barrier
> >> >> >> invocation to the user rather than implicitly doing it from the
> >> >> >> module going notifier.
> >> >> >
> >> >> > Now, I could automatically add an indicator of whether or not a
> >> >> > call_srcu() had happened, but then again, that would either add a
> >> >> > call_srcu() scalability bottleneck or again require a scan of all
> >> >> > possible
> >> >> > CPUs... to figure out if it was necessary to scan all possible CPUs.
> >> >> >
> >> >> > Or is scanning all possible CPUs down in the noise in this case? Or
> >> >> > am I missing a trick that would reduce the overhead?
> >> >>
> >> >> Module unloading implicitly does a synchronize_rcu (for RCU-sched), and
> >> >> a stop_machine. So I would be tempted to say that overhead of iteration
> >> >> over all CPUs might not matter that much considering the rest.
> >> >>
> >> >> About notifying that a call_srcu has happened for the srcu domain in a
> >> >> scalable fashion, let's see... We could have a flag "call_srcu_used"
> >> >> for each call_srcu domain. Whenever call_srcu is invoked, it would
> >> >> load that flag. It sets it on first use.
> >> >>
> >> >> The idea here is to only use that flag when srcu_barrier is performed
> >> >> right before the srcu domain cleanup (it could become part of that
> >> >> cleanup). Else, using it in all srcu_barrier() might be tricky, because
> >> >> we may then need to add memory barriers or locking to the call_srcu
> >> >> fast-path, which is an overhead we try to avoid.
> >> >>
> >> >> However, if we only use that flag as part of the srcu domain cleanup,
> >> >> it's already prohibited to invoke call_srcu concurrently with the
> >> >> cleanup of the same domain, so I don't think we would need any
> >> >> memory barriers in call_srcu.
> >> >
> >> > About the last part of your email, it seems to that if after call_srcu
> >> > has
> >> > returned, if the module could be unloaded on some other CPU - then it
> >> > would
> >> > need to see the flag stored by the preceding call_srcu, so I believe
> >> > there
> >> > would be a memory barrier between the two opreations (call_srcu and
> >> > module
> >> > unload).
> >>
> >> In order for the module unload not to race against module execution, it
> >> needs
> >> to happen after the call_srcu in a way that is already ordered by other
> >> means,
> >> else module unload races against the module code.
> >>
> >> >
> >> > Also about doing the unconditional srcu_barrier, since a module could be
> >> > unloaded at any time - don't all SRCU using modules need to invoke
> >> > srcu_barrier() during their clean up anyway so we are incurring the
> >> > barrier
> >> > overhead anyway? Or, am I missing a design pattern here? It seems to me
> >> > rcutorture module definitely calls srcu_barrier() before it is unloaded.
> >>
> >> I think a valid approach which is even simpler might be: if a module
> >> statically
> >> defines a SRCU domain, it should be expected to use it. So adding a
> >> srcu_barrier()
> >> to its module going notifier should not hurt. The rare case where a module
> >> defines
> >> a static SRCU domain *and* does not actually use it with call_srcu() does
> >> not
> >> seem that usual, and not worth optimizing for.
> >>
> >> Thoughts ?
> >
> > Most SRCU users use only synchronize_srcu(), and don't ever use
> > call_srcu(). Which is not too surprising given that call_srcu() showed
> > up late in the game.
> >
> > But something still bothers me about this, and I am not yet sure
> > what. One thing that seems to reduce anxiety somewhat is doing the
> > srcu_barrier() on all calls to cleanup_srcu_struct() rather than just
> > those invoked from the modules infrastructure, but I don't see why at
> > the moment.
>
> Indeed, providing similar guarantees for the dynamic allocation case
> would be nice.
>
> The one thing that is making me anxious here is use-cases where
> users would decide to chain their call_srcu(). Then they would
> need as many srcu_barrier() as chain hops. This would be a valid
> reason for leaving invocation of srcu_barrier() to the user and
> not hide it under the hood.
>
> Thoughts ?
The current state is not horrible, so my thought would be to give it
some time to see if better thoughts arise.
Either way, cleanup_srcu_struct() keeps its current checks for callbacks
still being in flight, which is why I believe that the current state is
not horrible. ;-)
Thanx, Paul
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