linux-kernel - Re: [RFC] tools/memory-model: Rule out OOTA

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Message-ID: <4411cf20-f889-4fd7-9e03-14744deaaf72@paulmck-laptop>
Date: Thu, 16 Jan 2025 11:39:32 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: Alan Stern <stern@...land.harvard.edu>, parri.andrea@...il.com,
	will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
	npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
	luc.maranget@...ia.fr, akiyks@...il.com, dlustig@...dia.com,
	joel@...lfernandes.org, urezki@...il.com, quic_neeraju@...cinc.com,
	frederic@...nel.org, linux-kernel@...r.kernel.org,
	lkmm@...ts.linux.dev, hernan.poncedeleon@...weicloud.com
Subject: Re: [RFC] tools/memory-model: Rule out OOTA

On Thu, Jan 16, 2025 at 08:28:06PM +0100, Jonas Oberhauser wrote:
> 
> 
> Am 1/16/2025 um 7:40 PM schrieb Paul E. McKenney:
> > 	o	If a value is read in the seqlock reader and used
> > 		across a "you need to retry" indication, that
> > 		flags a seqlock data race.
> 
> 
> This too is insufficient, you also need to prevent dereferencing or having
> control dependency inside the seqlock. Otherwise you could derefence a torn
> pointer and...

True, but isn't that prohibition separable from the underlying
implementation?

> At this point your definition of data race becomes pretty much the same as
> we have.
> 
> https://github.com/open-s4c/libvsync/blob/main/vmm/vmm.cat#L150
> 
> 
> (also this rule should only concern reads that are actually "data-racy" - if
> the read is synchronized by some other writes, then you can read & use it
> just fine across the seqlock data race)

Perhaps LKMM should adopt this or something similar, but what do others
think?

> I also noticed that in my previous e-mail I had overlooked the reads inside
> the CS in the failure case, but you are of course right, there needs to be
> some mechanism to prevent them from being data racy unless abused.
> 
> But I am not sure how to formalize that in a way that is simpler than just
> re-defining data races in general, without adding some special support to
> herd7 for it.
> 
> What do you think?

I was thinking in terms of identifying reads in critical sections (sort
of like LKMM does for RCU read-side critical sections), then identifying
any dependencies from those reads that cross a failed reader boundary.
If that set is non-empty, flag it.

But I clearly cannot claim to have thought this through.  ;-)

							Thanx, Paul