linux-kernel - Re: [PATCH 0/3] tools/memory-model: Add SRCU support

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Date:   Fri, 16 Nov 2018 10:34:34 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] tools/memory-model: Add SRCU support

On Thu, 15 Nov 2018, Paul E. McKenney wrote:

> On Thu, Nov 15, 2018 at 11:19:24AM -0500, Alan Stern wrote:
> > Paul and other LKMM maintainers:
> > 
> > The following series of patches adds support for SRCU to the Linux
> > Kernel Memory Model.  That is, it adds the srcu_read_lock(),
> > srcu_read_unlock(), and synchronize_srcu() primitives to the model.
> > 
> > 	Patch 1/3 does some renaming of the RCU parts of the
> > 	memory model's existing CAT code, to help distinguish them
> > 	from the upcoming SRCU parts.
> > 
> > 	Patch 2/3 refactors the definitions of some RCU relations
> > 	in the CAT code, in a way that the SRCU portions will need.
> > 
> > 	Patch 3/3 actually adds the SRCU support.
> > 
> > This new code requires herd7 version 7.51+4(dev) or later (now 
> > available in the herdtools7 github repository) to run.  Thanks to Luc 
> > for making the necessary changes to support SRCU.
> 
> These patches pass the tests that I have constructed, and also regression
> tests, very nice!  Applied and pushed, thank you.
> 
> > The code does not check that the index argument passed to 
> > srcu_read_unlock() is the same as the value returned by the 
> > corresponding srcu_read_lock() call.  This is deemed to be a semantic 
> > issue, not directly relevant to the memory model.
> 
> Agreed.
> 
> If I understand correctly, there are in theory some use cases that these
> patches do not support, for example:
> 
> 	r1 = srcu_read_lock(a);
> 	do_1();
> 	r2 = srcu_read_lock(a);
> 	do_2();
> 	srcu_read_unlock(a, r1);
> 	do_3();
> 	srcu_read_unlock(a, r2);

Yes, this sort of thing will be misinterpreted as two nested critical 
sections rather than two overlapping critical sections.

> In practice, I would be more worried about this had I ever managed to
> find a non-bogus use case for this pattern.  ;-)

The example is also a little difficult for humans to follow, at least 
without an explanatory comment.

Alan