Date:   Tue, 24 Jan 2023 12:22:11 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc:     "paulmck@kernel.org" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "parri.andrea" <parri.andrea@gmail.com>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
        dhowells <dhowells@redhat.com>,
        "j.alglave" <j.alglave@ucl.ac.uk>,
        "luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
        dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
        urezki <urezki@gmail.com>,
        quic_neeraju <quic_neeraju@quicinc.com>,
        frederic <frederic@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
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On Tue, Jan 24, 2023 at 04:54:42PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/19/2023 5:41 PM, Alan Stern wrote:
> > On Thu, Jan 19, 2023 at 12:22:50PM +0100, Jonas Oberhauser wrote:
> > > 
> > > On 1/19/2023 3:28 AM, Alan Stern wrote:
> > > > > This is a permanent error; I've given up. Sorry it didn't
> > > > work out.
> > > [It seems the e-mail still reached me through the mailing list]
> > [For everyone else, Jonas is referring to the fact that the last two
> > emails I sent to his huaweicloud.com address could not be delivered, so
> > I copied them off-list to his huawei.com address.]
> > 
> > > > > I consider that a hack though and don't like it.
> > > > It _is_ a bit of a hack, but not a huge one.  srcu_read_lock() really
> > > > is a lot like a load, in that it returns a value obtained by reading
> > > > something from memory (along with some other operations, though, so it
> > > > isn't a simple straightforward read -- perhaps more like an
> > > > atomic_inc_return_relaxed).
> > > The issue I have with this is that it might create accidental ordering. How
> > > does it behave when you throw fences in the mix?
> > I think this isn't going to be a problem.  Certainly any real
> > implementation of scru_read_lock() is going to involve some actual load
> > operations, so any unintentional ordering caused by fences will also
> > apply to real executions.  Likewise for srcu_read_unlock and store
> > operations.
> 
> Note that there may indeed be reads in the implementation, but most likely
> not from the srcu_read_unlock()s of other threads. Most probably from the
> synchronize_srcu() calls. So the rfe edges being added are probably not
> corresponding to any rfe edges in the implementation.
> 
> That said, I believe there may indeed not be any restrictions in behavior
> caused by this, because any code that relies on the order being a certain
> thing would need to use some other ordering mechanism, and that would
> probably restrict the behavior anyways.
> 
> It does have the negative side-effect of creating an explosion of
> permutations though, by ordering all unlocks() in a total way and also
> sometimes allowing multiple options for each lock() (e.g., lock();unlock()
> || lock();unlock()? has 4 executions instead of 1).

That's true.  It would be nice if there was a class of write-like events 
which couldn't be read from and didn't contribute to the coherence 
ordering.

Alan

> Anyways, not much to be done about it right now.
> 
> best wishes, jonas
>