linux-kernel - Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

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Date:   Tue, 17 Jan 2023 09:43:08 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Andrea Parri <parri.andrea@...il.com>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)

On Tue, Jan 17, 2023 at 10:56:34AM -0500, Alan Stern wrote:
> On Tue, Jan 17, 2023 at 07:14:16AM -0800, Paul E. McKenney wrote:
> > On Tue, Jan 17, 2023 at 12:46:28PM +0100, Andrea Parri wrote:
> > > This was reminiscent of old discussions, in fact, we do have:
> > > 
> > > [tools/memory-model/Documentation/litmus-tests.txt]
> > > 
> > > e.	Although sleepable RCU (SRCU) is now modeled, there
> > > 	are some subtle differences between its semantics and
> > > 	those in the Linux kernel.  For example, the kernel
> > > 	might interpret the following sequence as two partially
> > > 	overlapping SRCU read-side critical sections:
> > > 
> > > 		 1  r1 = srcu_read_lock(&my_srcu);
> > > 		 2  do_something_1();
> > > 		 3  r2 = srcu_read_lock(&my_srcu);
> > > 		 4  do_something_2();
> > > 		 5  srcu_read_unlock(&my_srcu, r1);
> > > 		 6  do_something_3();
> > > 		 7  srcu_read_unlock(&my_srcu, r2);
> > > 
> > > 	In contrast, LKMM will interpret this as a nested pair of
> > > 	SRCU read-side critical sections, with the outer critical
> > > 	section spanning lines 1-7 and the inner critical section
> > > 	spanning lines 3-5.
> > > 
> > > 	This difference would be more of a concern had anyone
> > > 	identified a reasonable use case for partially overlapping
> > > 	SRCU read-side critical sections.  For more information
> > > 	on the trickiness of such overlapping, please see:
> > > 	https://paulmck.livejournal.com/40593.html
> > 
> > Good point, if we do change the definition, we also need to update
> > this documentation.
> > 
> > > More recently/related,
> > > 
> > >   https://lore.kernel.org/lkml/20220421230848.GA194034@paulmck-ThinkPad-P17-Gen-1/T/#m2a8701c7c377ccb27190a6679e58b0929b0b0ad9
> > 
> > It would not be a bad thing for LKMM to be able to show people the
> > error of their ways when they try non-nested partially overlapping SRCU
> > read-side critical sections.  Or, should they find some valid use case,
> > to help them prove their point.  ;-)
> 
> Isn't it true that the current code will flag srcu-bad-nesting if a 
> litmus test has non-nested overlapping SRCU read-side critical sections?

Now that you mention it, it does indeed, flagging srcu-bad-nesting.

Just to see if I understand, different-values yields true if the set
contains multiple elements with the same value mapping to different
values.  Or, to put it another way, if the relation does not correspond
to a function.

Or am I still missing something?

> And if it is true, is there any need to change the memory model at this 
> point?
> 
> (And if it's not true, that's most likely due to a bug in herd7.)

Agreed, changes must wait for SRCU support in herd7.

At which point something roughly similar to this might work?

let srcu-rscs = return_value(Srcu-lock) ; (dep | rfi)* ;
		parameter(Srcu-unlock, 2)

Given an Srcu-down and an Srcu-up:

let srcu-rscs = ( return_value(Srcu-lock) ; (dep | rfi)* ;
		  parameter(Srcu-unlock, 2) ) |
		( return_value(Srcu-down) ; (dep | rf)* ;
		  parameter(Srcu-up, 2) )

Seem reasonable, or am I missing yet something else?

							Thanx, Paul