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Date:   Mon, 8 Apr 2019 07:22:30 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     "Joel Fernandes, Google" <joel@joelfernandes.org>,
        rcu <rcu@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        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>,
        rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>,
        fweisbec <fweisbec@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        amd-gfx <amd-gfx@lists.freedesktop.org>
Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
Reply-To: paulmck@linux.ibm.com
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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