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

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Message-ID: <20230119190752.GA932071@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 19 Jan 2023 11:07:52 -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 Wed, Jan 18, 2023 at 09:41:38AM -0800, Paul E. McKenney wrote:
> On Wed, Jan 18, 2023 at 11:03:35AM -0500, Alan Stern wrote:
> > On Tue, Jan 17, 2023 at 09:17:04PM -0800, Paul E. McKenney wrote:
> > > On Tue, Jan 17, 2023 at 09:15:15PM -0500, Alan Stern wrote:
> > > > Maybe we don't.  Please test the patch below; I think it will do what 
> > > > you want -- and it doesn't rule out nesting.
> > > 
> > > It works like a champ on manual/kernel/C-srcu*.litmus in the litmus
> > > repository on github, good show and thank you!!!
> > > 
> > > I will make more tests, and am checking this against the rest of the
> > > litmus tests in the repo, but in the meantime would you be willing to
> > > have me add your Signed-off-by?
> > 
> > I'll email a real patch submission in the not-too-distant future, 
> > assuming you don't find any problems with the new code.
> 
> Sounds good!
> 
> The current state is that last night's testing found a difference only
> for C-srcu-nest-5.litmus, in which case your version gives the correct
> answer and mainline is wrong.  There were a couple of broken tests, which
> I fixed and a test involving spin_unlock_wait(), which is at this point
> perma-broken due to the Linux kernel no longer having such a thing.
> (Other than its re-introduction into i915, but they define it as a
> spin_lock_irq() followed by a spin_unlock_irq(), so why worry?)
> There were also a few timeouts.
> 
> I intend to run the longer tests overnight.

Except that I had an episode of pilot error.  :-/

But here are a couple of litmus tests illustrating how SRCU read-side
critical sections do not flatten/fuse the way that RCU read-side critical
sections do.

Why the difference?  Because such a Linux-kernel SRCU implementation
would require additional storage of the order (T * S), where T is
the number of tasks and S is the number of srcu_struct structures.
That just won't be happening.

Here they are, and both behave the way that I would expect given your
unofficial patch:

https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-7.litmus
https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-8.litmus

Whew!  ;-)

							Thanx, Paul