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Message-ID: <20190407210718.GA6656@localhost> Date: Sun, 7 Apr 2019 21:07:18 +0000 From: Joel Fernandes <joel@...lfernandes.org> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: paulmck <paulmck@...ux.ibm.com>, rcu <rcu@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>, dipankar <dipankar@...ibm.com>, Andrew Morton <akpm@...ux-foundation.org>, Josh Triplett <josh@...htriplett.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, rostedt <rostedt@...dmis.org>, David Howells <dhowells@...hat.com>, Eric Dumazet <edumazet@...gle.com>, fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>, linux-nvdimm <linux-nvdimm@...ts.01.org>, dri-devel <dri-devel@...ts.freedesktop.org>, amd-gfx <amd-gfx@...ts.freedesktop.org> Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules 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@...lfernandes.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@...ux.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. thanks! > > Thanks, > > Mathieu > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com
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