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Date:   Sun, 7 Apr 2019 16:41:36 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     "Joel Fernandes, Google" <joel@joelfernandes.org>
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        linux-kernel <linux-kernel@vger.kernel.org>,
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        dipankar <dipankar@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
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        Eric Dumazet <edumazet@google.com>,
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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 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 ?

Thanks,

Mathieu

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