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Date:   Thu, 19 Jan 2023 10:43:38 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        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 Thu, Jan 19, 2023 at 11:41:01AM -0500, 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.
> 
> > It really does not work like an increment at all, I think srcu_read_lock()
> > only reads the currently active index, but the index is changed by
> > srcu_sync. But even that is an implementation detail of sorts. I think the
> > best way to think of it would be for srcu_read_lock to just return an
> > arbitrary value.
> 
> I think I'll stick to it always returning the initial value.  Paul said 
> that would be okay.

Just confirming.

> > The user can not rely on any kind of "accidental" rfe edges between these
> > events for ordering.
> > 
> > Perhaps if you flag any use of these values in address or control
> > dependencies, as well as any event which depends on more than one of these
> > values, you could prove that it's impossible to contrain the behavior
> > through these rfe(and/or co) edges because you can anyways never inspect the
> > value returned by the operation (except to pass it into srcu_unlock).
> > 
> > Or you might be able to explicitly eliminate the events everywhere, just
> > like you have done for carry-dep in your patch.
> 
> On second thought, I'll make it impossible to read from the 
> srcu_read_unlock events by removing them from the rf (and rfi/rfe) 
> relation.  Then it won't be necessary to change carry-dep or anything 
> else.

Although that works very well for srcu_read_lock() and srcu_read_unlock(),
it would be an issue for srcu_down_read() and srcu_up_read().  But one
thing at a time!  ;-)

							Thanx, Paul