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Date:   Tue, 9 Apr 2019 10:55:49 -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 Tue, Apr 09, 2019 at 12:45:25PM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 9, 2019, at 12:40 PM, paulmck paulmck@linux.ibm.com wrote:
> 
> > On Tue, Apr 09, 2019 at 11:56:03AM -0400, Mathieu Desnoyers wrote:
> >> ----- On Apr 9, 2019, at 11:40 AM, Joel Fernandes, Google joel@joelfernandes.org
> >> wrote:
> >> 
> >> > On Mon, Apr 08, 2019 at 01:24:47PM -0400, Mathieu Desnoyers wrote:
> >> >> ----- On Apr 8, 2019, at 11:46 AM, paulmck paulmck@linux.ibm.com wrote:
> >> >> 
> >> >> > On Mon, Apr 08, 2019 at 10:49:32AM -0400, Mathieu Desnoyers wrote:
> >> >> >> ----- On Apr 8, 2019, at 10:22 AM, paulmck paulmck@linux.ibm.com wrote:
> >> >> >> 
> >> >> >> > 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.
> >> >> >> 
> >> >> >> Not if we invoke srcu_barrier() for that specific domain. If
> >> >> >> call_srcu was never invoked for a srcu domain, I don't see why
> >> >> >> srcu_barrier() should be more expensive than a simple check that
> >> >> >> the domain does not have any srcu work queued.
> >> >> > 
> >> >> > But that simple check does involve a cache miss for each possible CPU (not
> >> >> > just each online CPU), so it is non-trivial, especially on large systems.
> >> >> > 
> >> >> >> > 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.
> >> >> >> 
> >> >> >> I understand that when users do explicit dynamic allocation/cleanup of
> >> >> >> srcu domains, they indeed need to take care of doing explicit srcu_barrier().
> >> >> >> However, if they do static definition of srcu domains, it would be nice
> >> >> >> if we can handle the barriers under the hood.
> >> >> > 
> >> >> > All else being equal, of course.  But...
> >> >> > 
> >> >> >> > 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.
> >> >> >> 
> >> >> >> Considering that we can do a srcu_barrier() for the specific domain,
> >> >> >> and that it should add no noticeable overhead if there is no queued
> >> >> >> callbacks, I don't see a good reason for leaving the srcu_barrier
> >> >> >> invocation to the user rather than implicitly doing it from the
> >> >> >> module going notifier.
> >> >> > 
> >> >> > Now, I could automatically add an indicator of whether or not a
> >> >> > call_srcu() had happened, but then again, that would either add a
> >> >> > call_srcu() scalability bottleneck or again require a scan of all possible
> >> >> > CPUs...  to figure out if it was necessary to scan all possible CPUs.
> >> >> > 
> >> >> > Or is scanning all possible CPUs down in the noise in this case?  Or
> >> >> > am I missing a trick that would reduce the overhead?
> >> >> 
> >> >> Module unloading implicitly does a synchronize_rcu (for RCU-sched), and
> >> >> a stop_machine. So I would be tempted to say that overhead of iteration
> >> >> over all CPUs might not matter that much considering the rest.
> >> >> 
> >> >> About notifying that a call_srcu has happened for the srcu domain in a
> >> >> scalable fashion, let's see... We could have a flag "call_srcu_used"
> >> >> for each call_srcu domain. Whenever call_srcu is invoked, it would
> >> >> load that flag. It sets it on first use.
> >> >> 
> >> >> The idea here is to only use that flag when srcu_barrier is performed
> >> >> right before the srcu domain cleanup (it could become part of that
> >> >> cleanup). Else, using it in all srcu_barrier() might be tricky, because
> >> >> we may then need to add memory barriers or locking to the call_srcu
> >> >> fast-path, which is an overhead we try to avoid.
> >> >> 
> >> >> However, if we only use that flag as part of the srcu domain cleanup,
> >> >> it's already prohibited to invoke call_srcu concurrently with the
> >> >> cleanup of the same domain, so I don't think we would need any
> >> >> memory barriers in call_srcu.
> >> > 
> >> > About the last part of your email, it seems to that if after call_srcu has
> >> > returned, if the module could be unloaded on some other CPU - then it would
> >> > need to see the flag stored by the preceding call_srcu, so I believe there
> >> > would be a memory barrier between the two opreations (call_srcu and module
> >> > unload).
> >> 
> >> In order for the module unload not to race against module execution, it needs
> >> to happen after the call_srcu in a way that is already ordered by other means,
> >> else module unload races against the module code.
> >> 
> >> > 
> >> > Also about doing the unconditional srcu_barrier, since a module could be
> >> > unloaded at any time - don't all SRCU using modules need to invoke
> >> > srcu_barrier() during their clean up anyway so we are incurring the barrier
> >> > overhead anyway? Or, am I missing a design pattern here? It seems to me
> >> > rcutorture module definitely calls srcu_barrier() before it is unloaded.
> >> 
> >> I think a valid approach which is even simpler might be: if a module statically
> >> defines a SRCU domain, it should be expected to use it. So adding a
> >> srcu_barrier()
> >> to its module going notifier should not hurt. The rare case where a module
> >> defines
> >> a static SRCU domain *and* does not actually use it with call_srcu() does not
> >> seem that usual, and not worth optimizing for.
> >> 
> >> Thoughts ?
> > 
> > Most SRCU users use only synchronize_srcu(), and don't ever use
> > call_srcu().  Which is not too surprising given that call_srcu() showed
> > up late in the game.
> > 
> > But something still bothers me about this, and I am not yet sure
> > what.  One thing that seems to reduce anxiety somewhat is doing the
> > srcu_barrier() on all calls to cleanup_srcu_struct() rather than just
> > those invoked from the modules infrastructure, but I don't see why at
> > the moment.
> 
> Indeed, providing similar guarantees for the dynamic allocation case
> would be nice.
> 
> The one thing that is making me anxious here is use-cases where
> users would decide to chain their call_srcu(). Then they would
> need as many srcu_barrier() as chain hops. This would be a valid
> reason for leaving invocation of srcu_barrier() to the user and
> not hide it under the hood.
> 
> Thoughts ?

The current state is not horrible, so my thought would be to give it
some time to see if better thoughts arise.

Either way, cleanup_srcu_struct() keeps its current checks for callbacks
still being in flight, which is why I believe that the current state is
not horrible.  ;-)

							Thanx, Paul