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SubjectRe: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
On Mon, Apr 08, 2019 at 09:05:34AM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 7, 2019, at 10:27 PM, paulmck paulmck@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 joel@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 paulmck@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.

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.

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.

Thanx, Paul

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