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Date:   Mon, 8 Apr 2019 09:05:34 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     paulmck <paulmck@linux.ibm.com>
Cc:     "Joel Fernandes, Google" <joel@joelfernandes.org>,
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Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
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Thread-Topic: Forbid static SRCU use in modules
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----- 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 ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com