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Date:   Thu, 15 Nov 2018 22:43:10 -0800
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nicholas Piggin <npiggin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/3] tools/memory-model: Add SRCU support
Reply-To: paulmck@linux.ibm.com
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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);

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

							Thanx, Paul