Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules

From: Joel Fernandes <joel-QYYGw3jwrUn5owFQY34kdNi2O/JbrIOy@public.gmane.org>
To: Mathieu Desnoyers
	<mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
Cc: David Howells <dhowells-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	amd-gfx
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	Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>,
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	dri-devel
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	Lai Jiangshan
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	Eric Dumazet <edumazet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
	Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>,
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	Andrew Morton
	<akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
	paulmck <paulmck-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>,
	Ingo Molnar <mingo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
Date: Sun, 7 Apr 2019 19:32:02 +0000	[thread overview]
Message-ID: <20190407193202.GA30934@localhost> (raw)
In-Reply-To: <134026717.535.1554665176677.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>

On Sun, Apr 07, 2019 at 03:26:16PM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 7, 2019, at 9:59 AM, paulmck paulmck-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org 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.

thanks.

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