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

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Message-ID: <20181127002642.GA4087@andrea>
Date:   Tue, 27 Nov 2018 01:26:42 +0100
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.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

> commit 72f61917f12236514a70017d1ebafb9b8d34a9b6
> Author: Paul E. McKenney <paulmck@...ux.ibm.com>
> Date:   Mon Nov 26 14:26:43 2018 -0800
> 
>     tools/memory-model: Update README for addition of SRCU
>     
>     This commit updates the section on LKMM limitations to no longer say
>     that SRCU is not modeled, but instead describe how LKMM's modeling of
>     SRCU departs from the Linux-kernel implementation.
>     
>     TL;DR:  There is no known valid use case that cares about the Linux
>     kernel's ability to have partially overlapping SRCU read-side critical
>     sections.
>     
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>

Indeed!,

Acked-by: Andrea Parri <andrea.parri@...rulasolutions.com>

  Andrea


> 
> diff --git a/tools/memory-model/README b/tools/memory-model/README
> index 0f2c366518c6..9d7d4f23503f 100644
> --- a/tools/memory-model/README
> +++ b/tools/memory-model/README
> @@ -221,8 +221,29 @@ The Linux-kernel memory model has the following limitations:
>  		additional call_rcu() process to the site of the
>  		emulated rcu-barrier().
>  
> -	e.	Sleepable RCU (SRCU) is not modeled.  It can be
> -		emulated, but perhaps not simply.
> +	e.	Although sleepable RCU (SRCU) is now modeled, there
> +		are some subtle differences between its semantics and
> +		those in the Linux kernel.  For example, the kernel
> +		might interpret the following sequence as two partially
> +		overlapping SRCU read-side critical sections:
> +
> +			 1  r1 = srcu_read_lock(&my_srcu);
> +			 2  do_something_1();
> +			 3  r2 = srcu_read_lock(&my_srcu);
> +			 4  do_something_2();
> +			 5  srcu_read_unlock(&my_srcu, r1);
> +			 6  do_something_3();
> +			 7  srcu_read_unlock(&my_srcu, r2);
> +
> +		In contrast, LKMM will interpret this as a nested pair of
> +		SRCU read-side critical sections, with the outer critical
> +		section spanning lines 1-7 and the inner critical section
> +		spanning lines 3-5.
> +
> +		This difference would be more of a concern had anyone
> +		identified a reasonable use case for partially overlapping
> +		SRCU read-side critical sections.  For more information,
> +		please see: https://paulmck.livejournal.com/40593.html
>  
>  	f.	Reader-writer locking is not modeled.  It can be
>  		emulated in litmus tests using atomic read-modify-write
>